DC coupled 2.7 GHz Active Probe Project - Now Available! - Page 1
Forum section: electronics / testgear

author: lasmux
author_rank: Regular Contributor
timestamp: July 22, 2023, 07:59:21 am
content: UPDATE, 17/04/2024: These are now available athttp://www.lasmux.com/product/single-ended-active-probes/. 2.7GHz probe.I've been working on an active probe design for around a year. The goal started off as creating a DC-coupled active probe to support a photon counting sensor I am also working on, but it was a very fun project and I spent so much time on it that now the plan is to sell it. Could I have some feedback on the probe/performance, and on the contents of the datasheet, before I start buying the first batch of parts... which will be quite expensive.I have another post that I'm putting together where I'll go into the development process a bit more.I'm making two versions, a 1GHz version, and a 2GHz version.The datasheet is here:https://www.lasmux.com/wp-content/uploads/2023/07/LD-ASP-1G_2G.pdfQuick specs:Bandwidth: DC-1GHz, DC-2GHzInput capacitance (measured at 1GHz): 0.7pFAttenuation: 20xDC input resistance: 1Mohm1GHz version frequency response (linear and log axis):2GHz version frequency response (linear and log axis):Tip input impedance of both probes, depending on which ground lead is fitted:The resistive ground lead can be used to stop a resonance developing on the ground connection, which reduced the input impedance above 1.5GHz. I talk about this a bit more in the datasheet.In terms of step response for the system, I've 'only' got a 500MHz oscilloscope, so can't properly test the rising edge speed unfortunately. This is the probe measuring a 50 ohm terminated 100MHz signal, with a <100ps rise time. This greatly exceeds the bandwidth of the scope so there's some ringing. The trace looks basically identical if I measure the signal directly by the oscilloscope.Currently I'm aiming for around £150 for the 1GHz version, and £185 for the 2GHz version.

author: lasmux
author_rank: Regular Contributor
timestamp: July 22, 2023, 10:15:44 am
content: There have been... quite a lot of iterations of the hardware design...I started off with a BNC output on the probe itself, and the batteries mounted to the probe also, but then went for an SMA connector and an external 9V battery box. It does mean more cables, and the coax cable needs to be custom to go from SMA-BNC, but it is a much easier device to use because of it.I went through a lot of iterations on the amplifier and signal conditioning circuitry, and got very used to soldering 0402's, which made my life miserable. Also given the extremely small capacitances on the board, every single time I made a change to the passive networks, I'd have to clean the board in IPA to remove flux residues.

author: djsb
author_rank: Super Contributor
timestamp: July 22, 2023, 11:40:02 am
content: Hi,When will this be back in stock? Or is it initially for evaluation purposes only (as you mention hardware changes)? Thanks.

author: lasmux
author_rank: Regular Contributor
timestamp: July 22, 2023, 11:55:49 am
content: I've not made the first batch yet. I'd expect maybe six to eight weeks, assuming my PCB assembler doesn't mess me about.If the response from this thread is positive, I'll make an initial batch of 25 and start the process immediately.

author: lasmux
author_rank: Regular Contributor
timestamp: July 22, 2023, 10:05:36 pm
content: Here is a simplified schematic of the probe input to give you a gist of how it works.C1 balances the parasitic capacitance across R2, to get a resistor/capacitor divider between R2/R3. As you could imagine, the parasitic capacitance across R2 is extremely low, given it's an 0402 passive. By varying C1 and observing the probe frequency response, I measured it at less than 0.1pF! This is not a lot of capacitance.The capacitance of the probe tip is a lot more than 0.1pF (0.7pF) because there is some coupling between the probe tip and ground itself. This is not so difficult to avoid by just isolating that resistor from ground completely. Unfortunately, the more you pull ground away from R2, the worse the noise pick-up on the probe is. The net which connects R2 to the non-inverting amp input is high impedance, and if you pull the ground away from it, it picks up a lot of noise. One of my design iterations removed a lot of the ground planes around this net to really try to keep the tip capacitance down as low as possible, and the noise pickup was really bad. So you kinda have to choose your demon. The noise level is quite a lot better than other active probes (only 650uV), but because it's a x20 probe, I needed to keep the noise level to a minimum.One thing which I could do in future is reduce the R2 to 200k or something, so I would have to correspondingly reduce R3, which would make that node much lower impedance and less succeptible to noise. I could then pull the ground a bit further back to try to further reduce the tip capacitance. Not sure.The 1Mohm resistor R2 also protects the non-inverting input of the high speed FET input op amp from ESD damage. It does have ESD protection diodes built in, but the resistor helps keep them safe. A lot safer than just having discrete FETs on the inputs.

author: nctnico
author_rank: Super Contributor
timestamp: July 22, 2023, 10:12:22 pm
content: Quote from: lasmux on July 22, 2023, 10:15:44 amYou can buy these cables off-the-shelve from China.

author: lasmux
author_rank: Regular Contributor
timestamp: July 22, 2023, 10:18:39 pm
content: Quote from: nctnico on July 22, 2023, 10:12:22 pmlol. I've just looked and they're everywhere on ebay etc. I have no idea how I missed that. Thanks.

author: Weston
author_rank: Regular Contributor
timestamp: July 22, 2023, 11:36:49 pm
content: Really cool work! It looks like the plastic for the probe tip is injection molded? Thats a lot of commitment.Have you considered selling this through crowd supply? There might be a bit of product testing required (CE certification), but Crowd Supply provides a direct path to distribution though Mouser. It's also a lot easier to not have to deal with fulfillment yourself and the crowdfunding model removes any monetary risks with fronting money for parts. Its now discontinued, but I sold the Little Bee current probe through Crowd Supply and it was a great experiencehttps://www.crowdsupply.com/weston-braun/little-beeIs the probe stable with an AC coupled 50 ohm termination? A lot of equipment, such as VNAs/Spectrum Analyzers have an AC coupled 50 ohm termination. You obviously don't need a probe that works down to DC for that, but this would be a cost effective high impedance probe.

author: lasmux
author_rank: Regular Contributor
timestamp: July 23, 2023, 10:17:01 am
content: Quote from: Weston on July 22, 2023, 11:36:49 pmIt's not injection moulded, it's MJF 3D printed, and then vibro polished to get a smooth finish. Really quite remarkable how comparable it is to injection moulding. I should do a post on the mechanical side of things also.Quote from: Weston on July 22, 2023, 11:36:49 pmI'd not known about this. Very cool that they distribute through Mouser. I'll have to look into this. Did you have to promote the crowd funding event? Getting 30k in funding is very impressive! I wonder if they'd accept a self certification CE process.Quote from: Weston on July 22, 2023, 11:36:49 pmYes it works with AC coupling on the input or output. So long as the instrument it's plugged into can operate with small DC offsets on the input, as obviously the output is not AC coupled. My VNA didn't have any trouble.

author: lasmux
author_rank: Regular Contributor
timestamp: July 23, 2023, 08:26:29 pm
content: Modelling was completed in the venerable Fusion 360. I'm not the best at mechanical design so this was quite a learning curve to create an ergonomic (ish) design.I then went through a number of different 3D printing technologies till I found one I was happy with. The PCB is a very close fit with the enclosure so I needed really good dimensional accuracy, and I didn't want lots of layer lines visible either, which would be more apparent on such a small part.From left to right we have:MJF nylon 12, vibro-polished. This was the best result which combined a smooth finish with excellent dimensional accuracy.SLS nylon 12. Good enough dimensional accuracy, but the finish wasn't smooth enough. I tried melting/smoothing the surface by passing a hot air gun over it lightly. This did work, and ended up with a nice gloss, but was quite risky as a little too much heat and the thin walls of the part would start to distort.SLA resin "tough 2k". Blotchy finish and not enough dimensional accuracy. Also the support structure was more apparent where it was broken off.SLS nylon 12. In grey, same issues as before.SLA resin "rigid 10k". A glass filled resin which gives a really smooth finish, unfortunately the dimensional accuracy wasn't quite good enough. The support structures here weren't as visible.SLA resin "clear". Blotchy finish, and not enough dimensional accuracy. Had quite a lot of issues with the support structures not having broken off cleanly with this one.

author: KungFuJosh
author_rank: Super Contributor
timestamp: July 23, 2023, 11:17:42 pm
content: I want one. I don't ever measure anything that I really need an active probe for, but that looks cool. 😉

author: lasmux
author_rank: Regular Contributor
timestamp: July 24, 2023, 12:47:27 pm
content: ThanksIt's quite good as a general purpose high-speed probe, of if you want to measure a moderately fast or high impedance signal... but yes, it's not for everyone.I was also wondering if it could be used as a sensor preamp, perhaps for a high-speed photodiode or something.

author: mawyatt
author_rank: Super Contributor
timestamp: July 24, 2023, 02:01:10 pm
content: Quote from: Weston on July 22, 2023, 11:36:49 pmWe have the Little Bee B1, don't use it much, but when we need to measure smaller currents it's served wellThanks for the productBest,

author: mawyatt
author_rank: Super Contributor
timestamp: July 24, 2023, 02:05:48 pm
content: @ lasmux,Nice project, well doneWe didn't know about the MJF printing, that case looks very nice indeed!!You certainly picked up on the Fusion 360, and produced a nice rendering as well!!A question, why the selection of 26dB rather than 20dB attenuation?? Guessing limits of the +-2.5V supply rails?Best,

author: nctnico
author_rank: Super Contributor
timestamp: July 24, 2023, 02:21:20 pm
content: 1:10 for the probe and another 1:2 for the 50 Ohm termination makes 26 dB.

author: lasmux
author_rank: Regular Contributor
timestamp: July 24, 2023, 02:30:15 pm
content: Quote from: mawyatt on July 24, 2023, 02:05:48 pmThanks for the encouraging wordsYes, MJF 3D printing is amazing. Combined with vibro-polishing I feel it sets a new standard for low-volume projects.I could have made it x10 into the 50 ohm, but I would have had to double the gain of the op amp which would have reduced the bandwidth somewhat. Also it would have reduced the measurable voltage range to +/-7V due to the limited rails, as you alluded to.

author: lasmux
author_rank: Regular Contributor
timestamp: July 24, 2023, 02:40:15 pm
content: To add. I also couldn't reduce the input voltage divider as it would have also meant reducing C1 by half (see schematic above) to maintain the capacitor divider ratio. C1 is a combination of an 0402 capacitor, and parasitic capacitance to ground. The 0402 capacitor part of C1 would have likely needed to be a smaller value than the smallest 0402 capacitor available.

author: lasmux
author_rank: Regular Contributor
timestamp: July 25, 2023, 12:53:15 pm
content: Yes, theoretically, it's a 10:1 probe if you put it into a 1Mohm oscilloscope, but then with a 1 metre coax cable, you're going to get reflection artefacts causing issues over 20MHz or so. Typically for 50 ohm terminated active probes, they say the attenuation as measured across a 50 ohm terminated scope.

author: JohnG
author_rank: Frequent Contributor
timestamp: July 25, 2023, 02:16:29 pm
content: This looks like a really nice and affordable probe.I have a question regarding large signal performance. The datasheet alludes to something that sounds like slew rate limiting that may affect the large-signal bandwidth. This is important to me because my primary application would be a scope probe. Have you done anything to look as this, or can you find someone with a high-bandwidth scope and source to look at it? I'm happy to volunteer, but I bet there are folks who would do the same and are much closer to you.Thanks,John

author: lasmux
author_rank: Regular Contributor
timestamp: July 25, 2023, 04:36:49 pm
content: Quote from: JohnG on July 25, 2023, 02:16:29 pmThanks!The slew rate of the output is limited to 2V/ns, so for example a 10V rising edge input would be divided by 10x due to the probe attenuation to 1V (before 50 ohm termination), so due to the slew rate limitation would havean additional 500ps rise time (I think)(edit. around a 600ps rise, see post below). You'd have to have quite the unusual signal to be generating this kind of signal though. In my spice simulations this kind of input starts to make step responses/square waves look a little trapezoid. I don't have a high amplitidue pulse generator (or fast enough oscilloscope) to test this properly. In general, I don't know what the rise time is for more sensible signals as my oscilloscope just isn't fast enough (500MHz).Note though that other active probes such as the Keysight N2796A 2GHz probe also limit their dynamic range at higher frequencies. This is a screenshot from their datasheet:

author: lasmux
author_rank: Regular Contributor
timestamp: July 25, 2023, 04:48:31 pm
content: Also if you (or anyone else) do want to verify the probes performance, I'd be happy to loan you my prototype. Especially if you have a decent VNA to check the bandwidth, and fast scope to check the rise time.I don't mind shipping it internationally.

author: lasmux
author_rank: Regular Contributor
timestamp: July 25, 2023, 06:08:10 pm
content: I just ran this through my spice simulation again, and the rise time went from around 250ps to around 610ps, for a 1V rising edge or a 10V rising edge. These are just indications as my spice model isn't that similar to the actual performance of the probe. The spice version only has a bandwidth of 1.4GHz.Edited with updated numbers.

author: hpw
author_rank: Frequent Contributor
timestamp: July 25, 2023, 07:55:16 pm
content: Well,to measure the slew rate, consider Bodnar Fast Pulse and a DSO may with 10Gs and RIS...I started with an active Probe from CalTest as CT4121 (now recognized that the ground to the probe tip is missing),have also a 2.5G HPF and now a differential AP034.All active probe have a certain connection pins, not fully side by side as yours, but love now my single AP034While you may simple connect an 2.54mm dual pin header as they are parallel and an additional GND pin on the side.So easy to use 2..4 of them (as for I2S), leave them on the table along, without having the hands full of them. And turn with 2 hands on the DSO.So my 2 cents on real measurement taskshp

author: lasmux
author_rank: Regular Contributor
timestamp: July 26, 2023, 09:00:34 am
content: Quote from: hpw on July 25, 2023, 07:55:16 pmThat's a very fast rising edge from that pulse generator! If I manage to find a faster scope, I'll have a look at that, thanks.

author: tszaboo
author_rank: Super Contributor
timestamp: July 26, 2023, 09:26:00 am
content: Looks cool. Those socket connectors at the end, are they the same size as the Agilent active probes?For me it's a bit strange that you choose to have the 1M to be in series with the signal, those probes have a relatively small series resistance, and the 1M is used to shunt the 1pF. Any reason for that? Not judging, I'm genuinely curious, as I haven't built high speed scope probe yet, only high gain ones for shunt resistors.

author: lasmux
author_rank: Regular Contributor
timestamp: July 26, 2023, 10:38:38 am
content: Quote from: tszaboo on July 26, 2023, 09:26:00 amI'm not sure the size of the keysight/agilent probes. I think they might be 25mil (0.635mm), the ones I selected are 1mm, so a bit larger maybe. Would that be a problem for you? I will include solderable adapters that you could maybe connect up to those probes, but that's not as seamless.I could theoretically replace the socket with this which I think would be compatible if you think it would be a deal breaker, although I'd have to change the enclosure which would be a bit of a faffhttps://www.mill-max.com/products/pin-receptacle/wire-termination-receptacle-soldercup-type/1134The 1M is to make the probe a bit more general purpose for low speed signals. Above a few MHz the input resistor divider between R2 and R3 has almost no bearing on the signal attenuation. It's mostly down to the capacitor divider between the parasitic capacitance across the 1M, and C1. To get a linear frequency response from low frequencies to high frequencies, you need to closely match the resistor divider and the capacitor divider attenuation. So in that sense it doesn't matter whether it's 1M or 10k, it just pushes the frequency out further at which point the capacitor divider starts to dominate. But the capacitor divider still needs to be correct due to the high bandwidth of the measurement.

author: tszaboo
author_rank: Super Contributor
timestamp: July 26, 2023, 12:33:13 pm
content: Quote from: lasmux on July 26, 2023, 10:38:38 amProblem? No, I have those 1GHz active probes at work (N2795), and saw how versatile the accessory package is. According to my caliper, the pins are 0.7mm. I don't know if they sell the probe pins separate.The equivalent input on these are 120Ohm series, 1pF 1Mohm parallel to ground.

author: JohnG
author_rank: Frequent Contributor
timestamp: July 26, 2023, 01:12:55 pm
content: Quote from: lasmux on July 25, 2023, 04:36:49 pmI measure such signals on a routine basis, so it doesn't seem so unusual to me (maybe I am an outlier, though).Regarding the Agilent/Keysight probe example, usually such a graph shows probe derating due to some reliability or safety limitation. The same rolloff is typical for completely passive probes which will not exhibit slew rate limiting. The data sheet was not very clear on this, but from the impedance plot, an 8V AC amplitude signal at 1 GHz will give you about 50ish mA AC RMS of probe tip current, which is not insignificant. These probes have an adjustable offset of +/-12V, which is how they arrive at the 20V max input. This is not slew rate limiting, this is a way to get around the AC voltage limitations of the probe tip amplifier. It's useful for looking at noise on a voltage bus, for example, by adjusting the offset to the bus voltage so you have the full AC dynamic range centered around the bus voltage.As I mentioned, I'd be happy to test the probe. I have a Bodnar pulser and an HP8131 pulse generator, and an old, but very functional, 6 GHz scope. I should warn you ahead of time that I'm not real quick about getting to these things, so if you find someone locally, you are likely to get a quicker turnaround. Also, if it is really slew rate limited to 1V/ns at the output, it's not something I can use.John

author: lasmux
author_rank: Regular Contributor
timestamp: July 26, 2023, 01:18:43 pm
content: Quote from: tszaboo on July 26, 2023, 12:33:13 pmMy probe divides the signal prior to the amplifier, and then re-amplifies it. The N2795 doesn't attenuate at all to begin with, and then I guess has the amplifier gain configured to divide by 10 perhaps. I'd guess that that could also be why their ESD damage tolerance is so low also, the input is only protected by the 120R resistor.

author: lasmux
author_rank: Regular Contributor
timestamp: July 26, 2023, 02:36:39 pm
content: Quote from: JohnG on July 26, 2023, 01:12:55 pmI think because of how I divide the voltage prior to the amplifier, I am less succeptible to damage induced by high amplitude/high frequency signals. Last time I checked (edit. in spice) for a 28V p-p signal at 2GHz into the probe, there would be less than 1mA RMS going into the op amp, with most of the power dissipated within the input resistor network, which can handle it. I do need to run some more simulations on this though. But thanks for the clarification, I had misunderstood that the dynamic range line on that graph was not referring to the probe dynamic range in general, but more focusing on the voltage derating.The slew rate is limited to 2V/ns at the output (remember the 10x output attenuation). For example, your HP8131 has a maximum p-p voltage of 5V, with a 200ps transition time. The slew rate limited rise time on that signal would be just slew rate limited to 250ps on my probe (theoretically). So only slightly longer, maybe here the bandwidth is reduced to around 1.3-1.4GHz for such signals. For the Bodnar pulser, the maximum p-p voltage is only 1.2V, so the slew rate limitation of the probe wouldn't affect the reading, it would be bandwidth limited here and wouldnt achieve the 40ps rise time.If no-one else volunteers, I would be very grateful if you'd have a look at the probe. It'll be really interesting to see how it performs on a fast oscilloscope!

author: joeqsmith
author_rank: Super Contributor
timestamp: July 26, 2023, 05:40:36 pm
content: Looks like you have done a nice job with it. Any idea on the selling cost yet? With the 50 ohm drive, I can see using it for applications outside of a scope probe. You mentioned using it with your VNA. There was a thread some time back about someone wanting to make a buffer to drive their 50 ohm inputs. I had posted a commercial one and some of my own but they are capped at a bit over 100MHz.If you are looking for people to evaluate them, I may be able to help as well.Thanks for posting and keep us updated.

author: lasmux
author_rank: Regular Contributor
timestamp: July 26, 2023, 07:08:30 pm
content: Hi Joe,I've appreciated a lot of your forum posts on VNAs this past year I've been working on this project!Currently I'm aiming for around £150 for the 1GHz version, and £180 for the 2GHz version on ebay. I've not gone through my BOM thoroughly yet though, so things may shift, potentially down. I can do it a bit cheaper for the forumites as 12% ebay fees wouldn't apply, I just need to figure out how to set up a store correctly on my website.And yeah, I think some kind of sensor preamp, or an input buffer for a VNA are very valid use cases.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 26, 2023, 08:28:00 pm
content: Very reasonable. Just looking at used prices for the LeCroy 2.5GHz single ended HFP2500, about $400 - $500 USD. Even then, you don't know if they work or how they were treated. It's a total crap shoot.For the most part, when looking at single ended low voltage signals, I tend to stay with resistive dividers. Cheap and minimalish loading. I have a few LeCroy 4ish GHz diff probes for my scope. I thought about trying to build a DC - >>GHz differential probe for logic signals but at the time, I wasn't able to source popcorn parts that would do it. I haven't checked in several years and would guess there are much higher performance parts available today.Having that DC-100MHz buffer around has been helpful. I've used it to drive my spectrum analyzers as well. Nice thing I can use what ever 1X // 10X probe with it. Downside, 100MHz.Thanks. Yes, I've had a lot of fun playing with these low cost VNAs. I'm very impressed with what they have been able to achieve in such a small package. Especially with that LiteVNA and Dislords firmware. Hopefully your scope probes will be as successful.

author: KungFuJosh
author_rank: Super Contributor
timestamp: July 26, 2023, 10:08:12 pm
content: Quote from: lasmux on July 26, 2023, 07:08:30 pmYou already have woocommerce installed, you just need to make sure it's setup correctly, and setup the products, payments, and shipping options. I can help in exchange for a couple 2G probes. 😉

author: lasmux
author_rank: Regular Contributor
timestamp: July 27, 2023, 12:20:34 am
content: Quote from: joeqsmith on July 26, 2023, 08:28:00 pmI'm glad you think the pricing is reasonable. In general, probes are in a weird space in terms of pricing. They don't feel like they should be so expensive, but they are quite difficult to get right, and contain expensive and low volume custom components. That said, I feel commercial active probes are unreasonably expensive. High speed passive probes aren't much better.Yeah, DIY Zo probes are definitely a reasonable (and dirt cheap) alternative. That said, their linearity can be a bit questionable, and I think if you measured the input impedance on one of them (I've not done this), I suspect it may be a bit variable. The inductance of the ground connection really screws up everything, which is why the expensive active probes often provide ground blade/leaf connections. In my probe I can counter the issue with a resistive ground connection. I don't think that would work in a Zo probe.I'll send you a PM tomorrow regarding sending you a probe for you to have a play with. Thanks for offering to help outQuote from: KungFuJosh on July 26, 2023, 10:08:12 pmI really hated setting up that website lol. The woocommerce plugin is just rammed full of features that make figuring out how to do something simple, very difficult. I really just need to watch a few tutorials on youtube and I'm sure it'll all be easy.

author: KungFuJosh
author_rank: Super Contributor
timestamp: July 27, 2023, 01:44:41 am
content: Quote from: lasmux on July 27, 2023, 12:20:34 amIf you're only selling 2 products, you can skip woocommerce and just use PayPal buttons. If you plan on expanding later, that's a different story.

author: lasmux
author_rank: Regular Contributor
timestamp: July 27, 2023, 06:48:17 am
content: I had another look around, there's a little bit of testing of Zo probes done by... you!https://www.eevblog.com/forum/testgear/fifty-ohm-probes/msg607659/#msg607659And some others:https://www.eevblog.com/forum/projects/lo-z-probe/msg801477/#msg801477There's also this one:http://jahonen.kapsi.fi/Electronics/DIY%201k%20probe/Everyone seems to get very different results. Not quite sure what's going on. I think my active probes are probably better performing up to their stated bandwidth, but lower bandwidth potential than a well built Zo probe. Although getting a linear response across the bandwidth seems quite tricky on a Zo probe.Quote from: KungFuJosh on July 27, 2023, 01:44:41 amI might go with this. Thanks for the suggestion.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 27, 2023, 12:06:10 pm
content: Quote from: lasmux on July 27, 2023, 06:48:17 amIndeed, I have tried various configurations when making resistive probes. Some better than others. You can image that component selection and construction is going to have a major effect as you move beyond a GHz.Another from 2017 where I was playing with a cheap demo board. The resistive elements were placed onto a board.https://www.eevblog.com/forum/microcontrollers/typical-speed-of-fpgas/msg1279898/#msg1279898Even my LeCroy PP061 probes I mentioned are not good for these higher frequencies. The diff probes I have are good for about 4GHz. I also have the PP066 which are a 7.5GHz resistive probe.https://cdn.teledynelecroy.com/files/manuals/pp066-rp4030-user-manual.pdf

author: joeqsmith
author_rank: Super Contributor
timestamp: July 30, 2023, 08:55:08 pm
content: It would be nice if probes did not load down the circuit you are wanting to test. I started out building one of Bob Pease's active FET probes. From memory, maybe 100MHz BW. It saved me a few times as a young engineer when the 10X probes we had were presenting too much load for my circuits. For home, I eventually bought a Tektronix P6202 active probe. This served me well until I started playing with faster circuits.Capture1 lists the higher BW probes I have for my hobby use along with the basic specs. I included yours as a reference.I have a LeCroy coplanar waveguide test board and used this as the thru for the LiteVNA. I then attached both a LeCroy PP061 and PP005 probe to a scope and used them to probe the board. I was never able to locate a manual or data for the PP061.Capture2 is looking at S21. The old 10x probe presents less of a load than the resistive probe until the cross over point at 70MHz.Capture3 is looking at S11. The impedance the VNA sees with the probe attached. This is not the probe, but the probe in parallel with port 2. Typically I would have some driver and load and want to probe between them.I think to run this sort of test I would want to use my old PNA. Some of my probes are limited to about -4dBm. And I would want to use the unknown thru model for that waveguide.***Added photo of my old Tektronix active probe along with with the probes for my old WaveMaster.

author: lasmux
author_rank: Regular Contributor
timestamp: August 01, 2023, 12:03:55 am
content: You have a lot of probes!Thanks for this data. Interesting. I guess with a lot of the active probes, their manufacturer specific interface makes it very difficult to actually measure their response. Obviously you're able to measure the impedance. Regarding capture 2... it looks like it's true that passive probes don't help signal integrityThe same is also true of certain Ukrainian active probes that can be found on Ebay these days. I bought one because it was cheap, and the signal loading was not pretty. Response linearity was apalling too. Not bad for $25, but not that useful either.I personally have used three methods to measure the impedance.1. I use a SMA T piece on the VNA port 1, with a 50 ohm termination. I then place the probe in the other side of the T piece. This is better than using the port 2 as termination as I think there will be worse reflection artefacts by not measuring near the termination point.2. I use a custom built 50 ohm load which has had it's resistors exposed, and measure directly across those.3. A custom 'open' SMA termination which I can measure directly across.1 and 2 tend to give fairly close results. 3 tends to give lower impedance values at lower frequencies for some reason. I'm not sure if this is because it's trying to measure impedances much much higher than 50 ohms. Maybe it gets less accurate. I'm not sure.For bandwidth measurements, I use the same setup method 1/2, but then have the probe output measured by the VNA port 2.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 01, 2023, 04:10:07 am
content: LeCroy does or did offer an accessory that allows their probes to be used with a standard scope. Keysight and Tektronix also offers this for some of their probes. I have yet to find a used one for mine.I would think to measure a probes impedance you want to have a fixture that you can remove all the errors right to where the probe attaches. No added 50 ohm termination, no Ts, no extra length... Your option 3 with an open that had the reference plane matched to the SOL standards, then just measure S11. For low frequencies (GHz ish) I would just use those coplanar waveguide boards. Then attach the probe in place of where the two 100 ohm resistors attach. Beyond about 1.5GHz, I wouldn't trust it.Where my low cost VNAs don't have good return loss for port 2, the old PNA is another story. Still just to see the loading effects between those two probes, good enough to use the Lite.***I should add that when you measure the impedance, you need to choose the proper method for best accuracy. Depending on the range, you would use the shunt, shunt thru, or series method. In your case, you may want to use two different methods to cover the range.For BW, I would still stick with the terminated coplanar waveguide, but in this case, I have one from LeCroy. The T adds error. Same with the custom terminator. Consider cal'ing at the end of the waveguide, then leaving the cal standard attached when making the measurement. You just want to know the probe. Hope this makes some sense.The other test would be to just measure some basic digital signals and compare results.I had told a friend about your project and projected selling price. I looked up the current cost of the PP005 on Digikey and even that probe would cost more. So yes, I think your target price is still very fair, assuming the probe works. Let me know once you have these first set built up.***Reminds me of my attempt to replicate someones experiment where they attempted to measure the capacitance of some resistors. Far away from 50 ohms....https://www.eevblog.com/forum/rf-microwave/shunt-capacitance-of-1206-smd-resistors-jeroen-belleman-december-2010/

author: joeqsmith
author_rank: Super Contributor
timestamp: August 01, 2023, 06:03:38 pm
content: The PP005A attached to the home made coplanar waveguide with the terminators not populated. The probe wasn't terminated. Tip capacitance seems to measure around 6p rather than 11. Note too that rather than 10M, it measures 100k (300 shown). If I have time tonight, I will try and measure it in series with my low frequency network analyzer to give you some idea of the difference. That NA can go up to 150MHz.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 08, 2023, 02:08:50 am
content: Using a PECL buffer that feeds a splitter. One leg which goes to one of the scopes inputs (50ohms). The other goes to the LeCroy coplanar waveguide test board which was then terminated to 50 ohms. Scope probe when then attached to second channel on scope and then used to probe the waveguide.No load: No second probe attached. This scope is only spec'ed for 600MHz (approx 600ps), and that PECL driver is about 350ps. Cable adds a fair bit of loading. Idea is to show the effects of the probe loading.PP002: This is a 350MHz 10X probe. Causes a fair bit of loading. Think about that PECL signal going off to some other part of the circuit rather than the scope. Also, with the splitter, things are somewhat isolated.PP061: The resistive probe still has some loading (not a great probe) but much better than the 10X probe.Once you get a bit closer, I'll bust out my faster scope and active probes.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 08, 2023, 03:58:33 am
content: Next, I removed the splitter and connected the LeCroy waveguide directly to the PECL driver. The opposite end of the waveguide connected to the scopes 50 ohm input. Basically, how you would probe a typical signal on a board but the scope is again the target load.145: The PP061 resistive probe pink. M1 is the unloaded waveguide (nothing attached). C1 is the loaded waveguide. Again, some loading. Scopes BW is limiting the edge to about 600ps.146: The PP005 500MHz 10X probe. It really messes with the signal. I did not clear the traces so ignore the averaged data.147: The P6202A 500MHz active probe. Has very little loading effects compared with the 10X passive probe. According to the link below, the probe cost about $1300 in 1998. The cables are thick and heavy making the probe difficult to use. You can start to see why the resistive probes were a better choice in most cases.http://www.barrytech.com/tektronix/probes/tekp6202a.html

author: lasmux
author_rank: Regular Contributor
timestamp: August 08, 2023, 10:52:05 am
content: I think your second set of measurements will be better? With the splitter you will have two 50 ohm terminations (to ground, not VCC-2V) on the PECL driver, which it might not like? I've not used PECL before though to be honest, only a bit in simulation.Very interesting how the PP005 really does a lot of damage to the signal. The PP061 lowers the PECL voltage levels slightly with it's loading? But the shape of the loaded waveform is more similar. The P6202A waveform has almost no loading (at these frequencies), but the output waveform maybe has slightly more peaking on the rising edge? It's still qualitatively very similar.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 08, 2023, 12:22:20 pm
content: Quote from: lasmux on August 08, 2023, 10:52:05 amI had used a resistive splitter, so with the two ports loaded with 50 ohms, the PECL driver sees 50 ohms. The signal the scope sees is 6dB lower than the original. The reason I thought about running this test was to somewhat isolate the two ports.https://www.microwaves101.com/encyclopedias/resistive-power-splittersQuoteFrom my previous posts where I provided some basic metrics for my probes, you can see the PP061 is a 10X probe, so the DCR was 500 ohms (which includes the scope's input). The waveguide was loaded with 50 ohms by the scopes other channel. Now we added another 500 ohms. Your comment about the P6202A "at these frequencies" is key. It's not a perfect 500 ohms and the response is not going to be flat as we start to look at faster signals.Currently the scope is doing a good job of masking what the actual signal looks like.***Thinking about your previous comment, I am guessing you thought I was using a "T" which is not the same thing as a splitter.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 08, 2023, 01:44:01 pm
content: They claim 1GHz, for $250 and it is not DC coupled. I would have no use for something like this.https://www.ebay.com/itm/254162548019?hash=item3b2d44b533$29 and again, not DC coupled.https://www.ebay.com/itm/175751864133?hash=item28eba09f45There doesn't appear to be a low cost 2GHz product that would compete with you. The only choice for the hobbyist is the used market...

author: joeqsmith
author_rank: Super Contributor
timestamp: August 08, 2023, 05:48:50 pm
content: QuoteBecause much of what I had shown here by attempting to embed resistors in the cable and such, isn't really how I typically would make a probe, I wanted to show what something a bit more common. This is a 20X (953 ohm) probe. Zoomed in 1ns/div. Again, fairly low frequencies and the scope is hiding the details but hopefully it helps show that the results shouldn't vary by much. The cable (Teflon) and connector came from Pasternak. Surface mount resistor is stabilized with a bit of shrink tube. Not a whole lot invested. Maybe a half hour labor and $15.Maybe we can run some of these on the VNA and my faster scope later on to help paint a clearer picture.

author: lasmux
author_rank: Regular Contributor
timestamp: August 08, 2023, 08:07:41 pm
content: Quote from: joeqsmith on August 08, 2023, 12:22:20 pmAh thanks about clarifying about the splitter. I had indeed misunderstood, and yes, I have the same problem with my oscilloscope. The true signal form is hidden behind the insufficient bandwidth.With regards to those other probes, I have seen those. They're not great tbh. The first ones construction looks quite good, but the input capacitance is 3.5pF. This is really quite bad, not far off a good passive probe. As you said, not DC coupled either. The second one looks very similar to other active probes based on the BF-998 that are on ebay. I have a similar version from Ukraine. The frequency response linearity is awful (swings +/- 8db or so across the bandwidth), and the input capacitance is in reality much higher than the quoted 0.5pF. I measured the tip loading drop to 20 ohms at around 500MHz, which again, is really bad.Quote from: joeqsmith on August 08, 2023, 05:48:50 pmIndeed, there is a very low loading here! Would definitely be interesting to see how it performs on the VNA/faster scope.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 08, 2023, 09:37:40 pm
content: Before doing this, another option may be to make a new one that uses a high grade SMA rather than the BNC. I should have some cable and connectors on-hand. The tip loop is also fairly large on this one. I wouldn't want to give you the wrong impression about these homemade probes once we start speeding things up.To look at the BW with the VNA, I am thinking we stay with the same waveguide. Terminate the backside to 50. Then attach the probe to the second port. Using my homemade probe, we should see 26dB. I would need to use my old Agilent for this. Maybe then check the tip capacitance as well. I could run your probe as well as my resistive probes but none of my active or standard 10X probes.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 09, 2023, 12:25:00 am
content: M2 is no load, C2 is the loaded waveguide. PP061 resistive probe blue. Rise time is roughly 500ps for the probe, or around 700MHz. Edge without the probe attached measures about 200ps.With a better scope and lower transitions the probe doesn't look so impressive.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 09, 2023, 12:31:09 am
content: But if you think that's bad here is that same homemade resistive probe.That's s mighty big loop, lots of inductance. Not too surprised it peaks up. I would expect your probe would do a much better job with this 200ps edge.Scope is an old WM8500A 5GHz BW.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 09, 2023, 01:02:04 am
content: Using a different driver with my Tektronix P6202A probe. It makes a decent filter.

author: MathWizard
author_rank: Super Contributor
timestamp: August 09, 2023, 09:56:33 am
content: Cool project, I'd buy this probe down the road.How much worse would it be if you used all the same chips, but used all standard size through hole parts ? What if you didn't pay attention to spacing or layout ? I'm just trying to get a feel for how sensitive these circuits really are. If 2GHz is ~15cm waves, thats getting down to circuit size.1 of the next things I need to buy, is cables, terminators, and splitters, for using between my scopes and signal gen. And I want to try making 1 of those passive probes that's just coax and a 1k resistor, and has very low capacitance. Dave showed 1 in a video on probes. I can't remember their downside, besides low series resistance. But even mucking about with BJT amplifiers lately, I realize how big 15pF of 10x probes can be.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 09, 2023, 11:31:19 am
content: Quote from: MathWizard on August 09, 2023, 09:56:33 amMathWizard, for a 20:1 ratio, you want to use a 950 ohm resistor, not a 1k. The scope's input is 50, so the circuit you are probing sees 1000ohms (assuming a perfect world).The following was taken from this paper:https://people.ece.ubc.ca/robertor/Links_files/Files/TEK-Understanding-Scope-BW-tr-Fidelity.pdfAnalog bandwidth is a measurement specification that simply defines the frequency at which the measured amplitude of a sinewave is 3 dB lower than the actual sinewave amplitude (see IEEE-1057). Figure 1 shows an idealized amplitude roll off error as a sinewave signal approaches the specified bandwidth frequency of a measurement device having a first order or single pole Gaussian response. At the rated bandwidth, the measurement error approaches 30%! If you want to make a measurement on a sinewave that has only 3% error, you would only want to measure sinewaves much lower in frequency than the rated bandwidth of the oscilloscope, about 0.3 times the rated instrument bandwidth. Because most signals are more complex than sinewaves, it is a general rule of thumb is use a measurement device, like an oscilloscope, that has 5 times the bandwidth of the signal you intend to measure (explained later and shown in Figure 5).***In case it wasn't clear, the following was also taken from the same paper:In fact, every probe manufacturer assumes that at the maximum specified bandwidth, the probe’s frequency response is down 3 dB.

author: lasmux
author_rank: Regular Contributor
timestamp: August 09, 2023, 03:08:45 pm
content: Quote from: joeqsmith on August 09, 2023, 12:31:09 amYeah, that's a lot of ringing! Very neat how little it's loaded the original signal though.

author: lasmux
author_rank: Regular Contributor
timestamp: August 09, 2023, 03:16:57 pm
content: Quote from: MathWizard on August 09, 2023, 09:56:33 amThanks, I'll let you know when it's readyThe probe amplifier input and feedback network on the probe is all 0402 passives. Even with that I have to have various snubbing resistors etc as stray inductances/capacitances cause havoc with the frequency response. You might be able to get something working with through hole components, but it would take much more effort to get it working correctly. And you probably wouldn't be able to reach 2GHz bandwidth.Compared to the DIY 1k resistive probes, it is more general purpose due to the 1M input resistance, and in theory will have a more precisely tuned frequency response, but has lower bandwidth potential.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 09, 2023, 10:28:24 pm
content: Quote from: lasmux on August 09, 2023, 03:08:45 pmTo give you some idea of the effects of lowering the inductance, these are two poor man's 10X probes. The resistors are butted up against the coax center pin with a much shorter probe tip. All to reduce that big loop.As before, M2 is the unloaded waveguide. C2 is loaded waveguide with probe #2 attached. C3 is probe #2. M3 is probe #1. You mentioned the amount of variance seen with homemade probes but these two seem fairly well matched.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 09, 2023, 10:35:51 pm
content: Just for completeness, I reattached the homemade 20X probe to compare it with the 10X probe #1. Construction is everything and as you pointed out, I did try a few ways to improve it but they all failed to exceed what I could achieve with the simple techniques shown.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 10, 2023, 02:40:39 pm
content: I have thought about trying to add a blade for the ground rather than the wire to help reduce the inductance. One thing I have done in the past was when I wrap the wire around the coax to form the ground, I use both ends rather than one. I normally solder this coiled wire. The problem is getting them attached to the board. Picotech offers a probe like that today. Looks like a 20X 6GHz probe would cost about $1000 USD.https://www.picotech.com/accessories/gigabit-digital-passive-test-probes

author: tggzzz
author_rank: Super Contributor
timestamp: August 10, 2023, 03:48:57 pm
content: Quote from: joeqsmith on August 10, 2023, 02:40:39 pmLike this HPAK N2878A?

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 11, 2023, 12:37:29 pm
content: Quote from: lasmux on July 22, 2023, 07:59:21 amgood work!

author: joeqsmith
author_rank: Super Contributor
timestamp: August 11, 2023, 01:30:32 pm
content: Quote from: tggzzz on August 10, 2023, 03:48:57 pmYes, but with a smaller loop.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 11, 2023, 02:14:05 pm
content: I used some copper spring material to make an "L" shape for the blade. The tab was formed into a cylinder and soldered to the coax.Again, M2 is the unloaded waveguide (probe not attached). C2 is with the probe loading the waveguide. C3 is the measured signal from the blade probe. Of course there are some errors with gain and I can compensate for the phase. A bit too much gain in the plot but works fairly well compared with the others.***The blob used to mechanically stabilize the resistor is Devcon 5 minute epoxy. It doesn't appear to hurt the performance of the probe at least at these frequencies.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 12, 2023, 03:03:17 pm
content: Making similar measurement with an old LeCroy 7200. The only option for saving data is with a floppy drive which I can read with my USB floppy. For plotting, I use CERN's viewer.Shown looking at the 10X blade probe on channel B2. Most of the error is due to the how the scope works. It has a step generator built-in and can loopback that signal to the display. It's is not the same as probing at the end of the waveguide like I have been showing. I need one more channel for that.The scope doesn't have a lot of storage and I don't remember it having a way to adjust the trigger independently for each channel. Even at 1nS/div, I can't align the two waveforms. Consider we are using 30+ year old scope (the 7200 was released in 1989), results seem fair.

author: lasmux
author_rank: Regular Contributor
timestamp: August 12, 2023, 10:42:44 pm
content: Quote from: joeqsmith on August 11, 2023, 02:14:05 pmThis is really nice. I love the short L ground blade design. The probe response matches the source waveform really well! Even the ripple is very close.Quote from: joeqsmith on August 12, 2023, 03:03:17 pmCan you save waveforms on the screen? This is how I sometimes do it. I save and display a reference waveform, and then use the delay on the trigger to overlay the live view of another signal.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 13, 2023, 12:32:12 am
content: I believe it has the freedom to do what you suggest, if the scope was 100% working. It throws the BRAM (Battery RAM) fault on start as the internal battery is dead. I'm not sure if that is the problem but when I select the internal memory, the scope resets. The scope shows the memory is garbage and selecting clear will also cause a reset. I am using a version of firmware that is known to have problems but it supports the plug-ins that I am using. It's possible that the memory problem could be tied to firmware. 30 years old, maybe even a hardware failure. This thing is an old VME chassis.***This was a kickstarter resistive probe. While a try to reduce the inductance, if you read their comments, "the Vishay FC0402 resistors I use in the attenuator are rated for 30V each. I have a series string of 100-75-75-75-75-50 ohms so with 50V at the tip you shouldn't be over 30 on any single resistor." Hard to believe they would get any level of performance out of it.Note how they terminated the test board so we can't see the loading effects.https://www.kickstarter.com/projects/azonenberg/akl-pt1-2-ghz-passive-oscilloscope-probe

author: joeqsmith
author_rank: Super Contributor
timestamp: August 13, 2023, 05:35:17 pm
content: While we wait for your probes, attempted to low format the drive, which failed. The drive I have been using is an old Seagate ST125. Note the jumper wire is not factory but a mod I sorted out that makes the drive act like the old Mini-scribe drives that came from the factory. These scopes only supported a few different drives. The KALOK KL320 was another one it supported. I inserted my backup drive and again, fails out. That drive has been in storage for maybe 15 years or so.I had converted this scope to use rechargeable batteries for the backup. I had forgot that the low format would fail if they were dead. So after a full charge, low format, reinstall, the scope now works. I went through the same process with the second drive and it works as well. Putting it back together, it no longer resets when the internal memory is selected and does not show garbage when you try and display the contents.So I tried your suggestion of using the storage and delayed trigger using about a foot or so of coax for a delay. Works like a charm, well, for now.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 13, 2023, 07:20:55 pm
content: For fun, I took the ECL output into a DC block, into a Mini-Circuits 8,1GHz tripler and then into a wideand amplifier. The hope was to get a faster edge. Loaded waveguide is C2 and the bladed probe, C3. The WM8500A has a BW of 5GHz, or roughly 70ps. Signal level is a bit low but the homemade probe seems to do a fair job tracking it.Granted, you can't use it for high voltages and at low frequencies, even a basic 10X probe would present less loading but if you work with high speed digital, it may be difficult to match the price performance of a resistor and some coax. Well, and apparently a really good ground.

author: lasmux
author_rank: Regular Contributor
timestamp: August 13, 2023, 08:06:03 pm
content: I've seen that probe from Andrew Zonenberg before. He has some really nice other projects also. I think the main issue with that probe is the tip capacitance is actually quite high for a resistive probe, and the bandwidth stops at 2GHz. Not sure why it was necessary to use those Vishay FC0402 resistors. Under 2GHz I don't think a normal 0402 resistor would have been an issue, and they are miles cheaper.Glad you managed to fix your scopeI've never been good at fixing old oscilloscopes. Last time I took apart my HP 54111D, I managed to achieve nothing but waste time and stress out about killing myself on the CRT.Nice tuning of that delay haha. Very nice match between the source/measured signal. But yeah, damn dude, that resistive probe is working a charm. Really nice. As always, I think there is a place for active probes above resistive probes. Where you want reduced signal loading, probably sub 2GHz, and for frequency response stability.My original application was my actively quenched APD as a photon counting module, where I wanted to measure across a 1k ballast resistor restricting current through an APD. A passive probe had way too much capacitance and would have slowed down the response of the signal, a resistive probe would have made the ballast resistor ineffective and probably damaged the APD. An active probe was the only way that I could think of.On another topic, I was considering a differential version of the probe. I've been playing around in LTSPICE, and I could probably get over 1GHz of bandwidth, with the same 0.7pF tip capacitance, +/- 30V input dynamic range, but I suspect my CMRR wouldn't be great without a bit of work on matching the two inputs closely.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 13, 2023, 09:14:14 pm
content: I agree, active probes certainly have their place. I would like to see your diff probe if you pull it off. When I investigated it, I could not locate parts to do the job. That has been a long time ago and I suspect based on your goals, you will be able to pull it off. That would be a handy probe to have on hand. Much of the high speed digital has been differential for some time.Searching ebay, even junk 1GHz diff probes have an asking price of over $200. Not suggesting anyone would pay that. Looking for my D300, this one was about the cheapest I could find which at least appears complete and they offer returns.https://www.ebay.com/itm/175614100115Of course, that's about 4X the BW you are targeting but still, I think there is money to be made if you can keep the costs down.When my wife and I were first married, I was needing a higher speed scope than we had at work. I rented a scope similar to that and had it shipped to our house so I could sort out how to run it before taking it to work. My wife was worried that something would happen. I think the cost was more than our house at the time.

author: hpw
author_rank: Frequent Contributor
timestamp: August 14, 2023, 07:33:59 am
content: Quote from: lasmux on August 13, 2023, 08:06:03 pmYeah, as I currently use the AP034 diff probe (see picture) as nice using 0.1" pin headers for fixed or tips connections over the active HFP2500 or the crappy active Cal Test CT4121. Even the CT4121 as shielded front header still picks up RF waves.Hp

author: tszaboo
author_rank: Super Contributor
timestamp: August 14, 2023, 01:02:18 pm
content: Quote from: lasmux on August 13, 2023, 08:06:03 pmI just looked at those FC0402 resistors and they are 5 USD in single pieces.I really don't get it. 0402 SMD thin film inductor is going to set you back 2c, but if you want a resistor with the same technology, it's going to cost 300 times more.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 16, 2023, 05:05:00 am
content: S11 & S21 for the blade probe using the PNA. My homemade PCB waveguides are going to be useless. Starts out at 500 ohms as and drops off rather quickly reaching 100 ohms at 6 GHz. Mag isn't very stable (when holding all the parts in my hands, moving them around...). Then there is de-embedding the interconnect. Consider it all just a gross measurement to give us some idea how it behaves.

author: JohnG
author_rank: Frequent Contributor
timestamp: August 16, 2023, 02:02:07 pm
content: Quote from: lasmux on August 13, 2023, 08:06:03 pmThere is another way that might work. You make the 1k ballast part of your probe by putting a 953 ohm resistor in series with 50 ohm microstrip or similar transmission line right on the same PCB. Just don't forget to terminate the other end of the line if it is not connected to your scope. You get your intended loading and measurement at the same time. Note that this does not work as well if the ballast is connected to the positive bus, but sometimes it is possible to do something clever and make it work anyway, especially if you can put in a series dc blocking cap (if you don't need a dc response).John

author: tszaboo
author_rank: Super Contributor
timestamp: August 17, 2023, 12:26:39 pm
content: Quote from: lasmux on August 13, 2023, 08:06:03 pmYou made me do this.But yeah, you piqued my interest, I designed this small board to be able to measure passives with the VNA. I was always disappointed by the inductors in RF matching, and wanted a way to measure them, this gave me the extra push to make this.My issue now, how do I calibrate the VNA to 50 Ohm on a resistor, and then measure the DUT which is a resistor.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 17, 2023, 01:17:04 pm
content: Why have components mounted on the thru, open and short? For the load, why wouldn't you use a standard? Are you concerned the length of the load will made a big difference?

author: tszaboo
author_rank: Super Contributor
timestamp: August 17, 2023, 04:46:52 pm
content: Quote from: joeqsmith on August 17, 2023, 01:17:04 pmI don't intend to mount those components, but I placed the ~3mm long transmission line and the component footprint anyway.I guess I could use an FC0402 50Ohm as the load. Normally the length shouldn't make a difference, but from what I've seen, the closer you can make the calibration look like the real world condition, the better.Plus, I only have a female cal kit for for SMA (at work).But you know what, it's an interesting question as well. I'll calibrate it without the components on it and mount some 0402s and see if it makes a difference.

author: nctnico
author_rank: Super Contributor
timestamp: August 18, 2023, 12:30:18 am
content: Quote from: tszaboo on August 17, 2023, 04:46:52 pmWith an OSL you will be calibrating at the connector. So whatever is different (like the trace not having a perfect 50 Ohm impedance and FR4 loss factor) for the real measurement will show up. Then again, I'm missing the DUT position for a reflection based measurement so how useful are the OSL parts on your board anyway?But it does make sense to calibrate the board from an OSL calibration in order to determine whether the error is as expected c.q. up to what frequency your board is useable. But the through measurement will be the most useful anyway because the error is low over a much wider frequency range.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 18, 2023, 12:48:12 am
content: Board would be taken out and reference plane would be at the end of the transmission line. I assumed they were making multiple boards and populated the test parts on the second one.

author: JohnG
author_rank: Frequent Contributor
timestamp: August 18, 2023, 02:40:35 am
content: FC resistors may be overkill, but they are nice and accurate. If you get them, you should look at the datasheet and consider the difference between the "wrap-around" termination and the "flip-chip" termination, shown in the figure labeled "Internal Impedance." (Good god, I hate this graphic designer trend of eliminating section and figure numbers. I curse them...). It shows a few % increase in impedance at 2 GHz, and a lot more at 6 GHz, for the "wrap-around" versus the "flip-chip".https://www.vishay.com/docs/60162/fchpseries.pdfIf you use a more cost-effective thin film resistor, you can get the same benefit by mounting the resistor upside-down, i.e. with the resistive element adjacent to the PCB. It can save a few hundred pH. If you are measuring your boards and feel so inclined, you can measure this difference pretty easily at a few GHz.Hope you find this useful.John

author: tszaboo
author_rank: Super Contributor
timestamp: August 18, 2023, 10:41:20 am
content: Quote from: nctnico on August 18, 2023, 12:30:18 amI'm planning OSLT, the measurement is not a reflection based it's through.https://coppermountaintech.com/measurement-of-electronic-component-impedance-using-a-vector-network-analyzer/According to this article, measuring 100 Ohm and above, I should use the through method, and around 50 Ohm, I can use the reflectionbased on a S11 measurement.Quote from: joeqsmith on August 18, 2023, 12:48:12 amExactly.I'm going to open another thread for this, so we don't pollute lasmux's excellent design with off topic stuff.https://www.eevblog.com/forum/rf-microwave/measuring-passive-parts-with-vna/new/#new

author: RFKev
author_rank: Newbie
timestamp: September 01, 2023, 08:22:54 am
content: Quote from: joeqsmith on August 16, 2023, 05:05:00 amWhat was the measurement setup and METAS VNA Data Explorer configuration to get the probe impedance with frequency?

author: joeqsmith
author_rank: Super Contributor
timestamp: September 01, 2023, 02:10:36 pm
content: Quote from: RFKev on September 01, 2023, 08:22:54 amI showed the screen shot of the METAS setup and used S11 to measure the impedance. Guessing that is not what you are asking. Maybe consider posting a more detailed question.

author: RFKev
author_rank: Newbie
timestamp: September 05, 2023, 02:56:51 pm
content: Quote from: joeqsmith on September 01, 2023, 02:10:36 pmI can see the settings along the top of the METAS screenshot. Was there anything configured in the "Setup" dropdown? My main question is how was the probe interfacing with the VNA. Did you touch the probe on the port 1 connector, or probe a transmission line? If you probed a line connected to port 1, was it terminated with a 50ohm load?

author: joeqsmith
author_rank: Super Contributor
timestamp: September 05, 2023, 05:13:05 pm
content: Quote from: RFKev on September 05, 2023, 02:56:51 pmNo, just S11. Everything is defaults except the top. My comment about de-embedding the interconnect was the main point. Ideally, I would have a custom jig for this. You can't... well, er... you could probe an RF connector directly but nah.... I have a connector that matches with my home made standards and touched off on it. This gets up roughly the correct reference plane. Still, I expect some error. There is nothing attached to the test connector besides the probe (which is what I am wanting to measure). Think of it this way. If the test connector were terminated to 50, then I place the probe in parallel with that, we are not going to measure 500 ohms but rather 500 in parallel with 50, product over sum....

author: schratterulrich
author_rank: Regular Contributor
timestamp: September 10, 2023, 06:59:01 am
content: Your probe project is very impressive. It certainly took a lot of effort to achieve this high bandwidth.I came across this post because I probably took a similar approach to building an active probe.My intention was to get a probe with very high input impedance and low capacitance to test sensitive nets like oscillator signals and to get an high impedance amplifier for 50 Ohm instruments.I chose the ADA4817 operational amplifier and got the following characteristics for the active probe. (With direct SMA connection)500 MHz bandwith660 ps Rise Time1,3 pF input capacitance95 MOhm Input impedance

author: lasmux
author_rank: Regular Contributor
timestamp: September 14, 2023, 11:11:43 pm
content: Quote from: schratterulrich on September 10, 2023, 06:59:01 amHey, cool probe thereNice linear response out to 300MHz or so!Thanks for the feedback. It definitely did take a lot of effort, and even more learning. I am currently waiting for the 9th revision PCB to arrive (!!!). Fortunately active probe PCBs are small and relatively cheap.

author: hpw
author_rank: Frequent Contributor
timestamp: September 15, 2023, 05:01:49 pm
content: Quote from: lasmux on September 14, 2023, 11:11:43 pmAnd please do not forget any 2GHz differential mode(s)

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 16, 2023, 11:29:13 pm
content: What is the estimated price for the probe?

author: lasmux
author_rank: Regular Contributor
timestamp: September 19, 2023, 03:13:04 pm
content: Quote from: LooseJunkHater on September 16, 2023, 11:29:13 pmIt'll be around £160/$200, excl shipping. I'm thinking of just sticking with a 2GHz version, and not bothering with an artifically worse/cheaper 1GHz model. I'm building a (hopefully) final iteration at the moment which will likely improve most of the specifications, then go into production.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 19, 2023, 06:37:39 pm
content: Quote from: lasmux on September 19, 2023, 03:13:04 pmI'm not personally interested in the single-ended probe, but I'd absolutely love if you came out with a variable attenuation dual-ended probe. $200USD is a pretty great price! I'd imagine it would be difficult however for it to reach 1GHz. I wonder if you'd be able to make minor changes to your circuit to allow for it.

author: lasmux
author_rank: Regular Contributor
timestamp: October 15, 2023, 11:14:25 pm
content: After a bit of radio silence, I have a large update with big improvements to the probe:I have worked through another two iterations of the probe as there were various specifications that I thought could be improved upon. We're at V10 now.I also was in contact with Picotech, who generously offered to lend me a fast oscilloscope (Picoscope 9404-16, 16GHz sampling scope) for proper time-domain measurements of the probe. This has been a huge pleasure to use, and I am very grateful to them for lending me such a high specification oscilloscope. I unfortunately have to ship the scope back this week. So yes, big thanks to them! Buy their oscilloscopes!I now no longer use the optional resistive ground lead to reduce the DUT loading, instead the signal pin is now always resistive. As such there's no longer a trade-off between probe bandwidth and reduced DUT loading. You get both high bandwidth and reduced loadingSo the main new specifications are:Bandwidth: 2.7GHzRise time: 130psDUT loading: 120Ohms worst case (at 1.6GHz).Tip capacitance: 0.7pFReturn loss on a 50 ohm transmission line -15.4db maxNoise: 350uV RMS output, or 7mV RMS referred to input (20x probe)Now for the performance plots.Showing -3db at just over 2.8GHz. Linearity is still very good.Tip loading is now minimum 120Ohms, and stable out to 4GHz at 200 ohms. My VNA is unstable beyond this.Step response test from signal generator with 350ps rise time. Probe response is very close to the signal generator, and probe loading has not modified the signal at all. Note red and green lines are almost perfectly overlapping. Note that red and green traces are saved waveforms so they can be overlaid with the probe signal. The probe has a 5.4ns latency.Rise time measurement using fast edge (70ps) from fast signal generator. This edge has a bit of ringing, so I'm uncertain how much of the probe ringing is because of the source signal ringing vs ringing because the source rise time is so much faster than the probe rise time. Probe rise time around 130ps which corresponds to a 2.7GHz bandwidth. Note probe has only a minor loading on this very fast signal. The rise time is slowed by around 6ps and there is some reduction to the peaking.Nice flat-topped low-frequency square waves (1khz).Test setup for oscilloscope measurements. Signal generator routed into oscilloscope (channel 4) via intermediate coplanar waveguide board. Active probe measures signal on coplanar waveguide and is read by oscilloscope channel 2.I'm pretty sure I've said this before, but hopefully for real this time. I am looking at starting production of these. If anyone wants one, send me a PM to get yourself to the front of the queueThe cost will be between £180 and £200, depending on how much of my time I end up spending on each probe. Hopefully more on the lower end of that. The probe tips are hand-made, and there may need to be some tuning on each probe if the frequency response isn't as good as it should be when I test them.

author: tggzzz
author_rank: Super Contributor
timestamp: October 16, 2023, 07:19:38 am
content: Looks goodKudos to you.Kudos also to PicoTech. Their attitude is like Bill and Dave, and that was instrumental (ho ho) in building HP into the great company it was.If I was younger, it sounds like I would want to work for PicoTech

author: lasmux
author_rank: Regular Contributor
timestamp: October 16, 2023, 08:52:29 am
content: Quote from: tggzzz on October 16, 2023, 07:19:38 amYes exactly. They do seem like an excellent company in all respects! Really cool vibes from them.I do also have a picoscope 2206B (50MHz bandwidth) which I use for all my embedded and lower speed development. It and its software are seriously good.

author: 2N3055
author_rank: Super Contributor
timestamp: October 16, 2023, 01:13:59 pm
content: I agree with Tggzzz, nice work on the probe...I also have great respect for Picotech...And 3 Picoscopes (actually 4, with retired one I don't use anymore)

author: lasmux
author_rank: Regular Contributor
timestamp: October 16, 2023, 10:40:13 pm
content: The noise calculation was a bit tricky as the scope noise was much worse than the probe noise level. For subtracting noise, I found thefollowing equation:𝑃𝑟𝑜𝑏𝑒𝑅𝑀𝑆= √(𝑆𝑐𝑜𝑝𝑒𝐴𝑛𝑑𝑃𝑟𝑜𝑏𝑒𝑅𝑀𝑆2− 𝑆𝑐𝑜𝑝𝑒𝑅𝑀𝑆2)So I'd measure the AC RMS noise of the scope without the probe plugged in, and then measure the AC RMS noise with the probe plugged in and tips grounded, and then perform the above calculation.The noise level on the oscilloscope with nothing plugged in was 1.986mV RMS, with the probe plugged in and the tips grounded, this rose to 2.0258mV RMS, meaning the probe had a noise output of about 0.35mV RMS.I think I had the scope set up right... I used the oscilloscope channel with the lowest baseline noise, which was channel 4. The scope was configured to operate with random equivalent time sampling, free-running mode, 5us per division, and 250000 points - for an effective sample rate of 5GS/s. When I selected shorter or longer time bases, it didn't make much difference to the final calculated noise level.Probe noise without probe attached:Probe noise with probe attached:

author: jmw
author_rank: Frequent Contributor
timestamp: October 17, 2023, 02:06:28 am
content: Nice work!I've been slowly working on a probe based on the BUF802, with some similar specs: 10x attenuation, input impedance of 1 MΩ || 1 pF, Zmin ≅ 100 Ω. I've been simulating and testing one piece at at a time, and put together the first complete prototype this weekend.It's hitting about 2.3 GHz, and there's no big resonant peak in the frequency response, so I'm probably even leaving some bandwidth on the table. I used a test jig PCB that fits against the front of the probe, but had to "cheat" a little by making another ground connection to lower the ground lead inductance. When held by hand as pictured, the bandwidth is closer to 1.6 GHz.I'm trying to think of practical ways of keeping the right ergonomics and pushing inductance lower. Unlike the input capacitance that is more or less fixed by the probe design, ground inductance is mostly determined by the user's probing setup. A handheld probe needs to have a pointy shape to get into tight spaces, so there's always going to be a little bit of a loop. Most commercial probes have similar input impedance, so they must have some way to get super low inductance to hit their bandwidth number. And even Keysight mentioned their probe bandwidth numbers are measured using jigs that are not representative of most real world use.I suppose more attenuation makes it easier to get lower input capacitance; is this the reason you went with 26 dB attenuation?

author: lasmux
author_rank: Regular Contributor
timestamp: October 17, 2023, 06:04:24 am
content: Hey, nice probe! Nice flat responseI've found that dealing with stray inductances and capacitances is quite difficult, and quite design/layout specific. Some designs were more sensitive to it than others. It's madeningly easy to set up some LC resonance somewhere which causes a big dip or peak in the response at some frequency. Maybe you have something similar at 1.6GHz and 1.9GHz? Those dips are just single data points however so it could just be noise?Dealing with ground lead inductance is difficult as a lot of things you can try either make the probe more difficult to use, or damage the response linearity/bandwidth. I stuck with using a spring pin probe for the ground lead because it makes probing real circuits much much easier if one of the connections is sprung, even though this ground lead has quite a bit of inductance. You don't have to get the probe angle perfect to get a good connection.I chose a 20x attenuation for a number of reasons. As you say, it makes it easier to reduce the input capacitance, also, the slew rate on the op amp I'm using is a bit slower so by increasing the attenuation it increases the bandwidth at which the probe becomes slew-rate limited for very large AC-amplitude input signals.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 11, 2023, 04:56:28 pm
content: Any progress with the latest revision?

author: lasmux
author_rank: Regular Contributor
timestamp: November 14, 2023, 10:36:27 pm
content: I'm organising an initial manufacturing run of a batch of 25 PCB assemblies now. Going through some normal back-and-forth with the assembly house over some of the details.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 15, 2023, 12:47:07 am
content: Nice. Just let me know when you are ready.

author: points2
author_rank: Regular Contributor
timestamp: January 15, 2024, 05:35:43 am
content: Quote from: lasmux on November 14, 2023, 10:36:27 pmHi lasmux,any info about when the probes will be available ?

author: lasmux
author_rank: Regular Contributor
timestamp: February 27, 2024, 09:51:37 pm
content: Quote from: points2 on January 15, 2024, 05:35:43 amSoon. Did I say that last time though

author: lasmux
author_rank: Regular Contributor
timestamp: February 27, 2024, 10:02:44 pm
content: In other news, I have the first batch of 25 PCBs here (at long last) and the first couple have tested fine!!I've also spent what felt like an eternity soldering together a mountain of probe tips. Each signal tip is in three parts. There's a pin to go into the probe, a series resistor, and then a sharp tip, all cut down and then soldered together. This is then carefully glued for rigidity, and finally tidied up with some semi-rigid heatshrink. 10 minutes assembly per tip. 50 tips. I've managed to burn through every decent podcast I know of. Fortunately the ground tip sockets are a bit simpler to assemble.

author: lasmux
author_rank: Regular Contributor
timestamp: April 17, 2024, 03:25:51 pm
content: I'm super proud to say that at long last, these are available at the site:http://www.lasmux.com/product/single-ended-active-probes/The datasheet has also been extensively updated, so worth checking that out!It's surprising how much time and effort it is to get something like this up and running. Production issues, delays, customs and shipping, fighting insurance companies... the list just goes on and on. My hourly wage for this first batch, even if I exclude the 1.5 year long design work, is very, very low. Hopefully the next batch will be smoother.

author: mawyatt
author_rank: Super Contributor
timestamp: April 17, 2024, 03:40:05 pm
content: Nice effort & probe, well doneIf/When we need an Active Probe we'll order oneBest,

author: lasmux
author_rank: Regular Contributor
timestamp: April 24, 2024, 09:23:58 am
content: I think one of the biggest things I've learnt from this project is that manufacturing (more than one-offs) things which aren't just straight PCBs is really hard, despite having some R&D experience in many of these aspects. It's also been surprising how long seemingly trivial tasks take. I had a bunch of estimates on how long various tasks would take for each probe. Pin assembly, testing, customs paperwork, etc. For almost everything I've underestimated the time requirements, despite having some experience in all of these things in the past.I have been thinking on ways to improve my workflow for subsequent batches, including test jigs, and assembly jigs for the pins. It's definitely tricky as these kinds of tooling jobs take time to set up, and the time-saving payoff will take a while to reach.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 14, 2024, 01:01:25 am
content: Lasmux has provided me with one of his new probes to review. I gave it a quick check and everything appears to work out of the box. The bug rug was not supplied with the probe. I added it for a bit of insurance.As always, if there is something specific you would like to see, feel free to ask.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 21, 2024, 07:30:21 pm
content: Making some progress on the review today. The probe came with baby papers (data from the mfg) which included S-parameters. I had asked Lasmux to provide more details about their setup and have asked that they make this public. Shown is the setup I am using.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 21, 2024, 07:35:45 pm
content: I'm not a fan of the placement of the probe tips having the offset as it makes it a challenge to attach to the board. Also, the shape of the probe body makes it difficult to clamp to. Note my use of a paper clip around the cables to hold the probe in place.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 24, 2024, 01:33:40 pm
content: My review.

author: dmderev
author_rank: Contributor
timestamp: May 25, 2024, 08:32:00 am
content: Could you share schematics, please?

author: 2N3055
author_rank: Super Contributor
timestamp: May 25, 2024, 08:57:17 am
content: Quote from: dmderev on May 25, 2024, 08:32:00 amNo.It is not open source..And schematic is minor part of the story. Most of the work is in physical layout...

author: joeqsmith
author_rank: Super Contributor
timestamp: May 25, 2024, 03:17:26 pm
content: Quote from: dmderev on May 25, 2024, 08:32:00 amYou would need to contact the manufacture for that level of documentation. They did provide some details about the design in the start of the thread:https://www.eevblog.com/forum/testgear/12-ghz-active-probe-project/msg4974892/#msg4974892For what they are charging, I can't see a reason why anyone would want to try and make their own anyway.I thought about showing the prototype on camera but they had posted photos already so it seemed redundant. And with the prototype being hand built, I didn't want to give viewers the impression that this was the same quality of the final product.IMO, if you are needing a higher BW, low capacitance probe with high DCR and you are working with low voltages, this thing is a steal at the price.

author: lasmux
author_rank: Regular Contributor
timestamp: May 25, 2024, 10:34:21 pm
content: Thanks for the review joeqsmith!! Excellent overview of it!I'm glad your experience was overall positive, albeit with a couple of niggles. I love the array of probes you have to compare it against! And of course, good to see the new resistive probes in action also.One thing that struck me was how well the Lecroy PP002A passive probe performed when you were loading the 1GHz clock signal later in the video. Despite it having a 14pF (!!!!) tip capacitance. It actually performed about the same as the reviewed 0.7pF active probe. I think this is because the clock is such a narrow band signal, and when I've done testing on passive probes, I've seen that their applied load is very frequency dependent. Maybe at 1GHz, the Lecroy load is particularly gentle on the probed signal? For example, the below graph was the S11 reflection coefficient (blue line) of a load I measured last year, with an HP 6pF 500MHz passive probe attached across the load. See at 1.5GHz, there's a pretty good reflection coefficient. At 600MHz, not good.As you noted, I forgot to post a more detailed method for measuring the probe frequency response. Here it is:There are three graphs provided with the datasheet (two of these measured for each individual probe in the test report). In the datasheet these are figures 1 top and bottom, and figure 2.Datasheet figure 1 is captured on a libreVNA, S21 and S11, with a custom coplanar waveguide connected to port 1, see photograph. The VNA has been calibrated with the coplanar waveguide in place, so the reference plane is at the end of the coplanar waveguide (where the load is fitted in the photograph). For the through calibration, the load and SMA-BNC adapter are removed to allow the blue SMA cable (port 2) to connect to the coplanar waveguide SMA. I've found that SMA-BNC connectors are often not very good at higher frequencies. It took me a few attempts to find one which didn't lead to nasty reflections.For the effective impedance measurement (datasheet figure 2), a custom SMA 50Ohm load was built so I could place the probe directly at the termination point, so the probe acts as a perfect(ish) parallel load to the termination/VNA reference plane. This was constructed from four 200Ohm 0402 resistors soldered to a trimmed SMA PCB edge connector. The custom load has a very similar return loss compared to a commercial load, measured up to 4GHz. The globs of solder on the load are there to allow the probe tips a larger targets to contact when taking impedance measurements.The VNA is calibrated with the custom load, and then the probe is placed across the load.The VNA is then set to output the impedance magnitude. The effective impedance of the probe is then seen as a load directly in parallel with the 50 ohm VNA load, so the probe effective impedance is calculated as 1 / ( (1 / measured impedance) - (1 / 50)).

author: joeqsmith
author_rank: Super Contributor
timestamp: May 26, 2024, 12:36:03 am
content: Thanks for letting me have a look at it.Quote from: lasmux on May 25, 2024, 10:34:21 pmFor those who did not watch just some background:The oscillator was in series with the waveguide and then attenuator before connecting to the scope. What was shown was what difference the scope sees when the test probes were attached to the waveguide. For the PP002A, I used the provided ground spring. During the video, I did not terminate the probe to an actual scope.Here I show it attached to the scope simulator used during the review as well as the actual scope. It's basically a wash but best to prove it. When looking at the return loss of the PP002A using the same technique used to measure your probe, you can see how it peaks at 500 then starts to drop. I suspect some of this is due to the long spring.Had I used a 500MHz clock, I would imagine your probe would have far out performed it. Similar, had I used a higher source impedance driver, I could have made your probe look much better than the home made resistive probe. This is why I started out by using that book to cover some of the basics about loading.Also lets be very clear and state the obvious. Had I shown what the scope measures when the using your probe vs the LeCroy probe on that 1GHz clock, there would be a MAJOR difference!! The only way I could have done this test is to use a homemade 1Meg/50ohm buffer. That would have been meaningless. The only other option would have been to use that LeCroy buffer with the WaveMaster along with the PP005 as that is a combination someone could possibly still purchase.Again, a bit difficult to know what to show.

author: lasmux
author_rank: Regular Contributor
timestamp: May 26, 2024, 10:43:31 am
content: Yeah I totally understand that it's difficult to show all the context details. Not critisizing, I was just originally puzzled.It was good to see a lot of the performance specifications get some external validation. I really appreciate all the work you put into the review!Would the following be a fair summary of your conclusions?ProsPretty good probe frequency response, with similar bandwidth to what's specified.Low tip capacitance leads to low impact on DUT signal, with performance in line with specification.Really good price/performance ratio. Multiple times cheaper than comparable used probes from ebay.9 hour battery life.Not a proprietry connection to test equipment.ConsNoise level is higher than other probes, so less useful for analogue work.This is unfortunately unavoidable due to the input design.Needs to be handheld to achieve measured probe response linearity.This is interesting as all of my testing has been handheld so far, which is maybe why I didn't observe this. I will look into this further for the next production run, although no promises of any improvement. If I can't fix this I'll stick a note in the datasheet about this.A bit tricky to clamp when not using handheld.Is this a good thing given the above issueOffset ground pin can be a bit fiddly.I agree it can be a bit tricky. It works best when you pop the ground pin in a ground via nearby the signal to be measured.Conclusion.General mark of approval, especially at price point. Best for high frequency digital measurements. Less useful for analogue measurements due to noise.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 26, 2024, 03:21:32 pm
content: Quote from: lasmux on May 26, 2024, 10:43:31 amNo problem. I just wanted to be clear about what was shown and why. IMO, the two primary criteria are 1) don't mess with the circuit I am probing. 2) let me see what is actually happening. Then comes all the other things like tip selection, body styles... Of course, no probe meets those first to constraints, so it's a compromise depending on what I am trying to measure.Just to drive that point, shown with the PP002A 350MHz 10X probe connected to the AP-1M buffer, then to the WaveMaster. During the review I showed how the loading for this probe was on par with your probe when attached to the 1GHz NEL oscillator. But when use the scope and probe to measure the signal, we get a flat line with a bit of offset (M3). Not at all useful.Compare that to your probe (M2). The signal is high enough that the noise isn't a factor and with that higher BW, we get some details about the clock.Now compare that with the HFP3500 (M4). What's the right answer? I would say I need a better scope and probes to answer that!Quote from: lasmux on May 26, 2024, 10:43:31 amBeyond these, I also felt the overall quality of the probe is good. The size of the body fits the hand well. It was provided with a good variety of tips. The hard carry case is a bonus.One thing you may want to consider is a way to guard the input from ESD when the probe is being handled. Hence my use of the bug rug.***Also, with there being no auto power off and having left the probe on a few times already, if it wasn't clear, I have been using the probe with a lithium ion rechargeable battery. These only output 7.4V when charged and it seems to be fine.***Quote from: lasmux on May 26, 2024, 10:43:31 amOne thing about SJL's scope, they advertise it as being 6GHz. The unit they provided not only is easy to verify, they surpass it with a wide margin. IMO, you appear to want to advertise the product right at the limits of what you could measure using a $1000 VNA and what ever standards it came with. Personally, I wouldn't do this. I would be much happier buying a product rated to 2GHz but seeing it perform at 2.5GHz.The whole hand placement on the body of the probe IMO effecting the measurements needs to be addressed. Then again, maybe not. It's similar to the noise level. I can tell you that my first dive into active probes, I would have been working on analog designs with higher voltage levels. This probe would have been a very welcome addition. It far out performs my first commercial active probe. I would say it has a place, even as it is today.At the selling price, I see it as a win. Hobbyist are not typically doing cutting edge designs at home and may just need something better than their 10X probe. IMO this probe could very well fit that need. Buyers just need to be aware of the limitations.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 27, 2024, 03:21:12 pm
content: A similar setup using a PECL driver but with a 10MHz fundamental. C1/M1 is directly looking at the output of the PECL driver through the waveguide and about 6" of coax. Consider this is still not what the actual signal looks like as we just added a fair bit of capacitance which is dampening that fast edge.C2/M2 is the Lasmux probeC3/M3 is the PP002A/AP-1M probe/bufferC4/M4 is the HFP3500I took the data zoomed out a ways to better show the noise and settling times. Then zoomed in, aligning the edges.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 27, 2024, 03:35:03 pm
content: Tektronix P6202 vintage active probe attached to C4.

author: lasmux
author_rank: Regular Contributor
timestamp: May 27, 2024, 03:41:36 pm
content: QuoteThat's good to hear!QuoteThe tips (and power inputs) are actually already ESD protected. The signal socket has a series 1MOhm resistor going into the op amp, which itslef has inbuilt ESD protection.Interesting thought about an auto power off function. I'd need to see what is available. Maybe a danger with that is that it powers off whilst someone is taking a measurement, without them realising. Of course the alternative is using a 9V battery every time you forget to switch it off for a few hours. I like that you've used a rechargable 2S lithium batteryQuoteI do agree with the sentiment. Almost all of the probes in the first production batch were over 2.7GHz by my measurements. I guess the issue is that different peoples test kit will measure different bandwidths, which may leave them dissapointed. I'll consider that.QuoteI've done a bit of testing on this today, and unfortunately there's not an obvious fix. I tend to get somewhat consistent frequency response results when holding the probe in various different positions by hand, with different levels of contact, but letting go of it completely does seem to affect the response negatively for some reason. I think it'll have to stay as it is for the time being at least, and I'll need to update the documentation/website to make it clear that it's performance is characterised during handheld use, and using a clamp/stand will have an effect on the frequency response. I do agree with you however, that it is still a very useful tool irrespective of this. I'll keep looking and maybe figure out a fix at some point.QuoteYes for sure.

author: lasmux
author_rank: Regular Contributor
timestamp: May 27, 2024, 03:48:18 pm
content: Quote from: joeqsmith on May 27, 2024, 03:21:12 pmNice plots.A couple of observations:Noise is definitely apparent there on the lasmux probe. The probe noise combined with the relatively low PECL voltage transition amplitude (400mV) makes for a noisy signal when zoomed out.On the zoomed in signal, the rise time of the original signal is around 1ns and the HFP3500 and the lasmux probe seem to track it fairly well, albeit with a small DC offset on the lasmux probe. Not surprising as it's well within the bandwidth of both probes. The P6202 has a bit of overshoot, and the PP002A probe doesn't seem to have the bandwidth to keep up with the edge.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 27, 2024, 07:15:24 pm
content: Quote from: lasmux on May 27, 2024, 03:48:18 pmI have the OEM tools for the P6202 to compensate it but you have to remove the plastic cover from the tip and expose the delicate circuitry (mechanically not very sturdy).I dropped the fundamental down low enough so at least we could see something beyond a flat line with that 10X probe. It does make for a nice filter.Attached showing the updated spreadsheet for some of my probes. Note the dynamic range of my active probes are all less than half of yours. Maybe between you and SJL, you may want to give up dynamic range to improve the noise.

author: lasmux
author_rank: Regular Contributor
timestamp: May 27, 2024, 08:18:41 pm
content: From the very simplified schematic I posted much earlier in the thread (see below), that would involve reducing the ratio R2/R3. R3 is 20k, so there's an attenuation of 50x on the input. The signal is then amplified by the (not-unity-gain stable) op amp back up to a 10x attenuation factor. This becomes 20x a net attenuation by the 50 ohm output impedance/oscilloscope impedance. The issue with reducing the attenuation on the input is that the capacitive divider of the parasitic capacitance across R2, and the C1 + parasitic capacitance across R3 has to maintain the same ratio for the AC gain. The parasitic capacitance across R3 is great enough that C1 would have to be a negative capacitance to maintain the correct ratio, which obviously isn't an option. Reducing the ground around that node to reduce the parasitic capacitance just makes external noise pickup a lot worse.Another option is to reduce the impedance of the op amp input node so external noise pickup is less of an issue. So say reduce R2 to say 100k, and reduce R3 to 2k. It makes it a 100k DC resistance probe though, not a 1M, although that's probably a reasonable tradeoff if it actually helps. I'd have to try it to see what the change is.

author: hpw
author_rank: Frequent Contributor
timestamp: May 28, 2024, 09:47:41 am
content: Quote from: joeqsmith on May 27, 2024, 03:21:12 pmIMHO, you have a noise & a sine signal on your C2/M2 is the Lasmux probe ... as an interval of about 17..18ns as about 60MHz.Just switch on your WM FFT Spectrum analyzer. In addition I use an 4..8x AVG to get ride of the WM 8bit ADC noise.One about overshoots: I used none differnetal and differential probes AB034 and did get ride of the ground connections as asymmetric added additional overshoots. In addition I love the AP034 pin header connection, as getting 2 free hands to tune at the gear. Also 2 hands free even two or more AP034 used O;)IMHO, may an option to use the Bodnar fast pulse

author: lasmux
author_rank: Regular Contributor
timestamp: May 28, 2024, 10:50:45 am
content: Quote from: hpw on May 28, 2024, 09:47:41 amYes I thought that also. Maybe some noise pickup from some other radiating piece of equipment?

author: joeqsmith
author_rank: Super Contributor
timestamp: May 28, 2024, 12:06:01 pm
content: QuoteI can't answer what your target application was or why you chose the parameters you did. For digital, the 1M DCR isn't really important.Quote from: hpw on May 28, 2024, 09:47:41 amI have not shown how my differential probes compare but could if you would like to see them.During review, I used a antique Tektronix pulse head with SJL's GigaWave 6400 oscilloscope.https://www.eevblog.com/forum/testgear/show-us-your-square-wave/msg5284957/#msg5284957As for the noise it is certainly possible it is coming from an external source. We have various radio/TV and other stations in the area. Keep in mind that all probes are exposed to the same environment and I do not normally make measurements at home in a screen room.Attached images show spectrum from 1 to 10MHz with the Lasmux probe attached. The ground and tip were shorted together with some wirewrap wire. Probe was allowed to float in the air so no other electrical connections were made except the cable to the spectrum analyzer.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 28, 2024, 12:16:14 pm
content: Probe wrapped in foil appears to knock out the low frequency.

author: lasmux
author_rank: Regular Contributor
timestamp: May 28, 2024, 02:23:47 pm
content: The 60MHz low frequency? It's not shown on your plots that only go up to 10MHz.

author: hpw
author_rank: Frequent Contributor
timestamp: May 28, 2024, 03:28:21 pm
content: Quote from: lasmux on May 28, 2024, 02:23:47 pm60MHz, may some IM or simple GND connection pickups.... as I had on an 1GHz active Cal TestThe simple solution went as for xGHz differential probes

author: joeqsmith
author_rank: Super Contributor
timestamp: May 28, 2024, 11:47:42 pm
content: With probe leads shorted, wrapped in foil, placed in shielded box with ferrite at feed, sweeping from 9k to 9GHz with probe turned on.Output appears very clean but of course, it would be difficult to use it to take a measurement like this.***Also, I normally keep a 10dB attenuator on the spectrum analyzer. This data was collected with attenuator removed and in peak mode.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 29, 2024, 12:10:20 am
content: Showing the range you are interested in (15 to 100MHz) with same setup.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 29, 2024, 12:11:07 pm
content: Looking at the probe attached to the SA, without any shielding and leaving the two tips isolated. The probe was not connected to anything but hanging in the air. For all images, baseline is with the probe attached but turned off.I know it is sensitive to my hand placement on the body. I wonder if this is part of the same problem.

author: lasmux
author_rank: Regular Contributor
timestamp: May 29, 2024, 07:01:06 pm
content: Did you remove a 10db attenuator for the 100MHz-1GHz test? And was the signal tip floating or grounded?So I guess it's fairly clear from all the testing that the probe amplifier input is picking up RF noise from your lab surroundings, given the earlier shielded test you did. This is kinda expected to a degree due to the input impedance of the amplifier, however I'd say the amplitude of this noise is higher than ideal. It looks like your lab has a fair amount more RF noise than mine. I think the amplitude of the actual noise in the oscilloscope traces you've captured is still acceptable, given the probes low price. If it was a $4k probe from Tektronix it definitely wouldn't be acceptable. I'll have a look at reducing the input impedance of the probe this weekend to 100kOhm or 200kOhm, and potentially upgrading the remainder of this batch if it makes an improvement. I now just need to find/make some suitable radiators to test the changes. Also I'd need another batch of stickers for the probe

author: nctnico
author_rank: Super Contributor
timestamp: May 29, 2024, 07:42:07 pm
content: First of all I think it is fantastic to get to such a nice probe at this price level. With a lower input resistor, the attenuation also gets lower if I'm not mistaken. In my view having a 100k Ohm input impedance is still great for many purposes. Maybe it is an option to have 2 versions. One high impedance version with a slightly higher pickup of external noise (which can be eliminated by testing inside a shielded chamber, room or tent) and one lower impedance version more suitable for looking at high speed signals in typical 50 Ohm systems.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 29, 2024, 07:52:46 pm
content: For the three sweeps shown in my previous post, both the signal tip and ground tips are floating. The only electrical attachment was to the spectrum analyzer. The 10dB attenuator was inserted for all three sweeps. Note RBW was changed (set to auto).I think the noise level is fine, depending on the application. Same for the higher DCR. If you do decide to change it, you may want to consider offering two separate products. Personally, I would rather the lower noise, lower dynamic range, lower DCR but can certainly see wanting what you have as well.I think the biggest contributors of noise in my lab are the 2 & 5GHz and my PC. Of course, then there is the device I am trying to measure. If you do come up with a new design and you would like me to provide feedback, I'm certainly willing to try and help out.

author: lasmux
author_rank: Regular Contributor
timestamp: May 30, 2024, 10:29:33 pm
content: Quote from: nctnico on May 29, 2024, 07:42:07 pmThanks for the kind words!Having two versions with different input impedances is an interesting suggestion. It makes the manufacture process a bit more difficult/expensive, but ultimately, different people have different needs, so maybe it's worth it.Quote from: joeqsmith on May 29, 2024, 07:52:46 pmOkay, I'll have a look and see where I end up with a new alternate version. In general, thank you for your support in this process so far. I really appreciate the interest you've taken in this project.

author: lasmux
author_rank: Regular Contributor
timestamp: June 15, 2024, 07:48:36 pm
content: Announcement time! We've teamed up withSJL Instruments, creator of the GigaWave 6GHz sampling oscilloscope, to offer a US$300 voucher for an SJL oscilloscope when you buy a probe. Given the price of the probe is only $225, and the oscilloscope (which is excellent) was already really well priced at $2295, this is a great deal! In general, the two devices really complement each other given their pricing and bandwidth/performance range, so this collaboration was a no-brainer for us.As the Gigawave scope has SMA inputs, there’s now a new version of the probe available with an SMA-SMA cable. This is selectable on the site. If you’re wanting to use the probe for connection to a spectrum analyser of VNA, this is also more convenient.

author: lasmux
author_rank: Regular Contributor
timestamp: June 15, 2024, 08:58:29 pm
content: To add, if you've already purchased a probe and want a voucher code, let me know and I'll send you one.

author: lasmux
author_rank: Regular Contributor
timestamp: June 18, 2024, 10:53:21 pm
content: I've done a bit of work on several points. Firstly, the effect of using the SMA-SMA cable. Secondly, looking at the noise pickup in the probe. Finally, looking at the issue where the probe responds differently depending on whether it's clamped or held.Maybe unsurprisingly, using the SMA-SMA cable shows a fairly significant improvement in impedance matching to equipment with SMA connectors. It removes the need for an BNC-SMA adapter, which I've found in the past could be a fairly large source of error in the match. See for example this S21 measurement using the new cable:Much cleaner with reduced ripple/reflections.With regards to the noise pickup. After a bit of toying about, I have confirmed that the issue is relating to picking up noise on the signal input of the op amp. As the op amp isn't unity gain stable, its design gain unfortunately amplifies this noise. The obvious answer is shielding which can be achieved with copper tape around the front of the probe, near where the amplifier input is. There is much less benefit from wrapping any other part of the probe in copper tape, and it is not really recommended as it can damage the sticker on the probe. The copper tape can be grounded to the SMA connectior. Care must be taken to not have the tape any further forward than in the image, otherwise the ground starts to have an affect on the linearity of the probe, and even the probe loading on the measured signal if the tape is pushed much too far forward. Unfortunately, the noise pickup is dramatically improved the further forward the tape is pushed, so a balance must be achieved. With the tape as it is in the photograph, I see an improvement of the noise pickup of around -5dB, with little affect on the frequency response, although this is a little difficult to measure consistently given the nature of the radiators that I was subjecting the probe to. This is still going to be a fair bit of noise, but it is an improvement, with a fairly easy implementation if you do need to reduce the noise level.As an aside, this is one of the reasons why adding a metal enclosure is not really viable, as the closer the metal is to the signal pin and associated circuitry, the higher the tip capacitance. I think it would be quite challenging to achieve the 0.7pF tip capacitance with a non-insulating enclosure.Finally with regards to the issue where clamping the probe results in a poor looking frequency response. I've tested this and there is a difference in frequency response depending on whether the probe is clamped or held.Clamped (not handheld)Handheld (not clamped)Basically, I think this is caused by a specific grounding configuration, and the addition of a hand modifying this somehow. The addition of the copper tape shielding doesn't help here as it's not actually modifying the ground connection of the probe. I think I see some improvement by using a more substantial ground connection off the ground pin of the probe, by basically soldering together some of the pins in the pin kit to make a DIY ground blade pin. I need to do some more testing to confirm this. Something I've not tried is to use a soldered ground connection with a copper braid, I think that might make quite a difference. I'll continue working on investigating this issue, however I'm not sure it'll ever be completely 'solved' as different grounding configurations will always have an effect on the probe response. Also on my equipment the effect is much more subtle, which makes looking for improvements more difficult. For now I'd say keep using it handheld, but hopefully if you don't the modification of the probe response would be relatively minor so long as your probe grounding is reasonable.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 19, 2024, 01:15:11 pm
content: You make no mention of lowering input impedance to support low voltage signals where drive strength is less of a concern. Is this no longer being considered?For the effects I show using the holder vs placing my hand on the probe being much more perturbed than what you show, hard to say. I could set it back up and try to see if I could add anything further to the discussion. The probe holder is all metal. It was laying on top of a ground plane. The DUT and cabling were all laid across the ground plane as well. There was no good way to mount your probe to my holder, so I used a paper clip which is also metal. The metal bar that the probes would normally attach to, was a fair distance away from the probe tip. Lots going on there. When placing my hand on the probe, the probe was still attached to the fixture in the same manor.The point I was trying to make in the review is that how the probe was held made a difference above a GHz.

author: lasmux
author_rank: Regular Contributor
timestamp: June 19, 2024, 03:32:51 pm
content: Quote from: joeqsmith on June 19, 2024, 01:15:11 pmYeah, I tried this, it makes no difference at all to the noise pickup. Not surprising in retrospect as the existing op amp input node capacitive coupling to GND is going to provide a much smaller impedance to reduce noise pickup, than reducing the relatively large voltage divider resistor. ~5pF coupling to GND at the op amp input, at 500MHz is 63Ohms impedance. Reducing the resistor to ground on the input voltage divider from 20k to 1k (and the probe input impedance from 1MOhm to 50KOhm), isn't going to make much of a difference given the capacitance and the frequencies we're dealing with.Yeah, it does sound like there's a lot going on there! In my case it was a plastic clip holding the probe. Maybe it will have some effect. I'll need to do some testing.Quote from: joeqsmith on June 19, 2024, 01:15:11 pmYeah for sure. Currently it looks like I will have to keep on marketing it as a probe which performs best/closest to the specification whilst handheld, but in general, I may need to make more clear that the response will also vary depending on the grounding setup. It is kinda implied in the test sheet etc, but I will make it more clear in the "Performance Insights" section of the datasheet, at least until I decide whether it is resolvable or not.

author: lasmux
author_rank: Regular Contributor
timestamp: June 19, 2024, 10:18:34 pm
content: The issue with the probe response being different is definitely a grounding setup issue, where the ground between the local signal and the probe is insufficient. A hand touching the probe improves the probe grounding with some capacitive coupling between your body and the nearby DUT. This issue can be resolved with a bit of copper tape, connecting the SMA connector (which is grounded) to a DUT ground nearby the measured signal. I had the tape stuck to the 50 ohm terminator shell, but I think this could be any sensible ground connection on a PCB. This tape has a very similar effect as holding the probe and really smoothes out the response.A couple of things to be careful of.The added tape shouldn't be directly connected to the ground directly next to the waveguide being measured, as it will affect the actual signal.Don't let the tape get too close to the front end of the probe, particularly the signal pin, as this will affect the response linearity of the probe.So yeah, if you want to use the probe handheld, there should be no problems. If you want to use it in a clamp, you'll need to improve the probe grounding with some copper tape. Also, it does seem like all other active probes in general seem to suffer from variable probe response depending on the exact local conditions, so I think this is reasonable.Clamped with copper tape disconnected vs connectedWithout copper tape:With copper tape:

author: joeqsmith
author_rank: Super Contributor
timestamp: June 20, 2024, 11:43:25 am
content: Quote from: lasmux on June 19, 2024, 03:32:51 pmAs we have discussed, for digital you could potentially sacrifice dynamic range as well to improve the SNR. I have included the spreadsheet from my review which shows how the probes I have on-hand compare. Even my old Tektronix probe was only rated to +/-6V. Again, I'm not suggesting this would be a replacement for what you are offering but an extension.QuoteOther snips from the review showing my setup as well as my with and without touching the probe. There's a fair bit of difference between what you show. I could try to compare how sensitive my other probes are to the proximity of my hand and holder to the body if you like. I don't recall the handing ever being a concern but that may be my ignorance showing. Most of the LeCroy probes were provided with their own holders. I got the idea for my Harbor Freight probe holder from the Tektonix PPM100 which I use for work.https://www.ebay.com/itm/175205288879

author: lasmux
author_rank: Regular Contributor
timestamp: June 20, 2024, 12:47:30 pm
content: Quote from: joeqsmith on June 20, 2024, 11:43:25 amThat would require reducing the input resistive and capacitive voltage divider ratio, which isn't as simple as it sounds. There are two issues. As the divider ratio reduces, the ground half of the capacitor divider would need to become significantly smaller than the existing parasitic capacitance of that circuit node, which is problematic. I could compensate for this with a larger capacitance across the input resistor, but then that would increase the tip capacitance/loading of the probe, which I don't want to do. Secondly, the output would in general have larger voltage swings, so the slew rate limitation of the probe would become more pronounced.QuoteYeah I see. My assumption here is that in my setup when the probe is clamped, I have more coupling between the probe and signal grounds than you do, due to my specific setup, so the worst case when clamped isn't as bad. The probe being handheld further improves this response and improves the grounds coupling. In your case, with the copper tape added to better couple the probe and signal grounds as per the photo above, the specific local setup shouldn't matter as much and you should just get a good response without it being handheld. If you have time (and some copper tape handy), it would be interesting to see if it makes a difference in your setup also.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 20, 2024, 02:06:59 pm
content: In real world use, adding a strap would be difficult, or at least it is not something I have ever considered when making a measurement. Still, I'll set it up again and see what effect the strap has.Agree that it's not so simple to develop such a probe with good performance. There's a reason we don't find them for $20 from China. Not yet anyway.

author: JohnG
author_rank: Frequent Contributor
timestamp: June 20, 2024, 03:27:25 pm
content: That the copper strap helps suggests that there might be common-mode pickup that is contributing to the problem. Maybe it's worth trying to loop the cable through a toroid for a few turns and see if that helps.Just a thought.John

author: lasmux
author_rank: Regular Contributor
timestamp: June 20, 2024, 05:07:35 pm
content: Quote from: joeqsmith on June 20, 2024, 02:06:59 pmI think it's easier than soldering in a in a probe ground, which is not unusual when using active probes apparently. Hence all the solderable pins included with the probe. Personally I always use it handheld.Thanks very much. Looking forward to seeing if it makes a difference for youQuote from: JohnG on June 20, 2024, 03:27:25 pmI've just tried this now with a ferrite I had on hand, and I don't think it made any difference. I'll have another look later on.I think another option may be a better low-inductance ground pin, such as a ground blade pin, or perhaps a pin made with some braid soldered to it. This would be a lot more difficult to make cheaply however.

author: nctnico
author_rank: Super Contributor
timestamp: June 20, 2024, 05:23:50 pm
content: At some point you'll need to move to soldered-in probes or add signal pickup points in the design.

author: luudee
author_rank: Frequent Contributor
timestamp: June 25, 2024, 02:58:16 am
content: Dear Lasmux,have you considered printing the enclosure from conductive ABS ?Kind Regards,rudi

author: lasmux
author_rank: Regular Contributor
timestamp: June 25, 2024, 01:11:44 pm
content: I did consider a conductive enclosure. I didn't go for this as I was concerned about increased capacitive coupling to ground on the signal pin, which would negatively affect the return loss on probed lines. I really want to keep that 0.7pF input capacitance, which was very difficult to achieve.Another issue is that the enclosure is a very small print, and it has to interface with a real PCB. When I was prototyping I found that if the print wasn't dimensionally accurate enough, the PCB just wouldn't fit, especially around the probe tip socket area. Conductive ABS is normally printed via an FDM process which doesn't have the precision needed. Not to mention that FDM prints don't look as aesthetically pleasing as SLS prints.

author: tggzzz
author_rank: Super Contributor
timestamp: June 25, 2024, 01:30:28 pm
content: Quote from: lasmux on June 25, 2024, 01:11:44 pmIgnorant suggeston: could you use a conductive paint? I assume you could mask off the areas where you want to keep capacitance low.I've no idea about how well paint would or wouldn't adhere to the plastic, but it would be in the protected interior. The sintered nylon I've used is relatively rough (compared to SLA!), which might help.

author: lasmux
author_rank: Regular Contributor
timestamp: June 25, 2024, 01:45:44 pm
content: Quote from: tggzzz on June 25, 2024, 01:30:28 pmThat could work tbh. The coating would have to be on the outside though as the enclosure at the front of the probe is not split into two (it's in one part) so you couldn't mask off the area near the front of the probe which is most susceptible to noise. As it's on the outside it would need to be reasonably aesthetically pleasing when applied. I've never tried to spray paint 3D printed parts before though. It sounds... time consuming...

author: tggzzz
author_rank: Super Contributor
timestamp: June 25, 2024, 04:08:24 pm
content: Quote from: lasmux on June 25, 2024, 01:45:44 pmI'm presuming you don't want anything conductive near the tip, but you do want a conductive shield away from the tip. I'm also presuming that, say, a 90%/0% coverage in the right/wrong places is better than a 0%/0% coverage, so "missed" coverage isn't too critical.Hence, could you insert a plug, possibly 3D printed to the right shape, then (spray) paint the interior, then remove the plug? I have no advice about making contact between the paint and the circuit board.There is also conductive silver-plated copper heatshrinkhttps://ph.parker.com/us/en/series/electrically-conductive-heat-shrinkable-tubing.There are also microwave absorbent foams, designed to minimise cross-coupling and cavity resonancehttps://ph.parker.com/us/en/series/microwave-absorber-materialsFor the avoidance of doubt, I am not recommending Parker; they are simply the first products I came acrossThey also do conductive paints and epoxy.

author: lasmux
author_rank: Regular Contributor
timestamp: June 25, 2024, 09:03:19 pm
content: @joeqsmithThanks, that's very intereting!Quote from: tggzzz on June 25, 2024, 04:08:24 pmYes exactly. The important thing is to keep the copper away from the signal tip and the first ~1cm of the probe body. This also applied to the PCB layouy. There are parts of the PCB where there is no ground plane under the signal traces to reduce ground coupling.Quote from: tggzzz on June 25, 2024, 04:08:24 pmPossibly could use a plug. On reflection, I think a spray solution will be too difficult to implement reliably, without also ensuring there's no conductive paint that accidentally finds itself in the wrong place. I think I'd have to spray directly into the 'cavity' where the plug is, and the pressure of the spray may allow paint to get where it shouldn't be, despite the plug. I might also look into a paint 'dip', where I can dip the back end of the enclosure to a controlled dip. Interesting about the conductive heatshrink.

author: tggzzz
author_rank: Super Contributor
timestamp: June 25, 2024, 09:18:34 pm
content: Quote from: lasmux on June 25, 2024, 09:03:19 pmAll valid concerns, of course.The design of the plug would be critical, of course. Would something like cotton wool be sufficient?I'm sure there are conductive paints, so it may be possible to just coat part of the interior with a brush rather than a spray. A littlemechanicalexperimentation is in orderCan I suggest starting a separate topic on the subject? I'm sure many people here have experience with generic EMI/EMC mitigation techniques/technology - and they may not be following a long thread about a scope probe.

author: KungFuJosh
author_rank: Super Contributor
timestamp: June 26, 2024, 12:57:16 am
content: Conductive paints are a gamble. You can do them with a brush instead of a spray, which is better, but... still a crapshoot. Many conductive paints require multiple coats to get any real conductivity. They also require specific conditions to apply them, and usually require 24 hours between coats, also depending on climate conditions. It's a lot of work and likely some headaches that may or may not pay off at all. Even if you get it good at first, the paint can eventually flake off and lose it's conductivity as it ages. It might still be better than nothing, but the labor involved doesn't seem like it's worth it to me.If conductive heat shrink would work, I would try that first. If it works, cool.

author: tszaboo
author_rank: Super Contributor
timestamp: June 26, 2024, 07:42:14 am
content: Quote from: lasmux on June 25, 2024, 09:03:19 pmIf you look at the design of the N2795, there is a hard plastic part and a soft silicone rubberized part, where the MK1 bionic grabber is supposed to handle the probe. I'm going to guess that this part is shielded for sure.

author: Dennis Frie
author_rank: Contributor
timestamp: September 30, 2024, 06:54:07 pm
content: Thanks for a great product. A 2.7 Ghz active probe at that price-point is amazing.This is now my go-to probe to verify signal integrity of digital interfaces. Compared to the Siglent SAP2500 we have at work, this is an amazing price-performance ratio.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 01, 2024, 02:46:57 am
content: Quote from: KungFuJosh on June 26, 2024, 12:57:16 ami have MG CHEMICALS 841WB-150ML NICKEL CONDUCTIVE COATING bought from digi, its worth it sure better than nothing, but you need airbrush spray tool and compressor, tubing etc... using brush is a no no as conductive particles will just stick to the brush, not the enclosure to be coated. and EM protection is in term of dB, more layers, more dB attenuation, maybe 2 or 3 layer is ideal, more than that you wont get further noticable protection. if you tight arse, 1 layer can give you some protection already. ymmv.Quote from: KungFuJosh on June 26, 2024, 12:57:16 amyes, painter's skills are really needed to be applied here, covering/masking from spray here and there etc... and conductive particles is special type, we need to keep stirring spray tank on each spray pass otherwise they will settle to the bottom very quickly and clog the spray tool.Quote from: tszaboo on June 26, 2024, 07:42:14 ammost likely yes, my agilent 1141a has silver coat on inside of enclosure, its durable and scratch resistant type, unlike my water based coating type. ymmv.

author: lasmux
author_rank: Regular Contributor
timestamp: October 02, 2024, 10:11:09 pm
content: Quote from: Dennis Frie on September 30, 2024, 06:54:07 pmHi Dennis, thanks! I'm glad you're enjoying using the probes and are making good use of themIt really brings me joy that they are providing a useful service!@Mechatrommer, I've tried a couple of attempts at painting with a conductive paint, by hand. It's not worked very reliably, so I'm not planning on using that method going forwards. It's just not worth the time to be honest given the low price of the probe. I'll leave it to the customer to use copper tape if they are so inclined. I'm working on a new version of the datasheet with instructions on applying copper tape properly.I only have one probe remaining in stock. I am in the process of ordering/manufacturing more. I've unfortunately been hit by part shortages up till just a week ago, but they seem to be resolved now. My PCBA manufacturer is about to start kicking into gear in the next few days.

author: KungFuJosh
author_rank: Super Contributor
timestamp: October 02, 2024, 10:52:53 pm
content: @lasmux,It might be easier to go the other way- use a conductive material for the case (or part of it) and then insulate it.Thanks,Josh

author: lasmux
author_rank: Regular Contributor
timestamp: October 03, 2024, 01:52:46 pm
content: Quote from: KungFuJosh on October 02, 2024, 10:52:53 pmYeah, this is something I've been considering. I originally discounted the option, but I think it could be done. Perhaps a conductive body, but with just the tip around the inputs as a non-conductive part.Probes are now out of stock. I've put up an interest form on the product page so you can get notified when they go back in stock.

author: KungFuJosh
author_rank: Super Contributor
timestamp: October 03, 2024, 04:09:54 pm
content: Quote from: lasmux on October 03, 2024, 01:52:46 pmI think that option is 100x easier than applying shielding reliably otherwise. You might want an insulated coating on the whole outer body, but even a good powdercoating might be sufficient and would like nice. You can have a powder coater insulate both sides, and mask the contact points you need to keep the shielding solid.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 03, 2024, 11:08:16 pm
content: either way, the shielding need to be connected to the circuit ground.

author: nctnico
author_rank: Super Contributor
timestamp: October 04, 2024, 10:12:54 am
content: I don't know how the PCB design is currently, but it could help to put all signals on inner layers and have as much ground on the top & bottom layers as possible.

author: lasmux
author_rank: Regular Contributor
timestamp: October 04, 2024, 06:34:34 pm
content: Quote from: Mechatrommer on October 03, 2024, 11:08:16 pmYeah for sure. It would be connected at the SMA side-launch connector, as the copper tape shielding method is.Quote from: nctnico on October 04, 2024, 10:12:54 amThat's not really feasible with this design.I've had another look, and a conductive enclosure will really push the BOM price up, and that would push the probe away from the ethos of great performance at a very low price. I think the method of using a deliberate application of copper tape is cheap and easy to apply if the user thinks the noise pickup is too high. Again, this would only be the case where the noise level is high enough and measured signal small enough simultanously.

author: KungFuJosh
author_rank: Super Contributor
timestamp: October 04, 2024, 06:47:48 pm
content: Quote from: lasmux on October 04, 2024, 06:34:34 pmBy how much? If it's not too significant, and if it accomplishes the goal of improving the probe, I'm pretty sure people would happily pay it.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 04, 2024, 07:34:31 pm
content: and i suspect, switching to shielded design from open space no hand condition will throw away the probe's output profile requiring another series of R&D, profile tuning and/or pcb respin drama.. ymmv.

author: lasmux
author_rank: Regular Contributor
timestamp: March 30, 2025, 09:27:32 pm
content: Hi again. Update time.I've gotten my next iteration up on the website for sale now.I've spent quite a bit of time investigating various iterations to manufacturing processes, enclosure designs, etc.The tip manufacturing process was improved to save time. I spent a very long time trying various batch manufacture ideas with machined aluminium jigs and solder paste, which just utterly failed. I couldn't effectively control the solder flow, and solder was going everywhere it wasn't supposed to. Ultimately these are still assembled one-by-one, but with some somewhat better processes/jigs to speed things up and make them more consistent.(the failed machined aluminium jig for batch soldering tip probes)What is updated on the probes themselves is the following:Enclosure updated to improve fit between top and bottom sides. Now the gap between these is covered by a lip so you can't see the PCB in the gap (you sometimes could).The enclosure supporting the power cable has been improved to reduce the likelyhood of a failure in the power cable (precautionary, this never happened).PCB has been updated to remove a design flaw which could have led to a failure where the power cable +ve lead shorted to ground. For the original batch the PCBs were manually modded to correct this.The datasheet has been updated to include information about grounding when not holding the probe, and tips to reducing the effects of ambient electrical noise on measurements.On a performance note, the second batch perform very consistently with the first. The only real change is there is a bit more gain/peaking at around 2.4GHz compared to the previous batch (maybe around 0.5dB), which also slightly increases the bandwidth. I think this might be because I've changed cable manufacturer and I believe the new cables/connectors have slightly less loss at 2.4GHz, and have a better 50 ohm impedance match.

author: lasmux
author_rank: Regular Contributor
timestamp: March 30, 2025, 09:40:22 pm
content: Here's an example test report from the batch. The response has basically the same shape as before.

author: Noizzar
author_rank: Newbie
timestamp: April 01, 2025, 02:55:39 pm
content: Hi everybody, thanks for the everybody’s input,I am considering buying one or two and I’ve read all the thread and also watched the video kindly recorded by member JoeQSmkth (thank you!) but I cannot help feeling that more live demonstrations of the probe with different signals and usage scenarios are needed to encourage more general potential users like myself, like (including but not limited to):full range large signals capture, with actual slew rate limitation shown, working with spectral analyzers as a preamp for high speed FFT spectrum analysis, extreme frequencies and avalanche pulses (~30 ps) response, feeding signal into higher impedance lines and associated solutions like via the 50 Ohms thru-terminators into 1M oscilloscope, etc.Questions to the design roadmap,Have you been considering adding multiple ranges (switchable), to address noise issues somewhat (as it relates to input and thus is multiplied by attenuator ratio). I understand this is impossible to have simultaneously low noise and high bandwidth but maybe adding several ranges would allow for addressing different usage cases so user trades bandwidth for noise when needed?I also used with success sprayed copper paint which quickly dries and can be mask sprayed and then optionally annealed with IR beam or hot air into plastic which holds the particles rock solid. Have you considered using this technique?Also I’d like to know if your idea to make 1GHz 30V p-p differential probe based on this active probe moved towards something one can touch?)Regards, N.

author: lasmux
author_rank: Regular Contributor
timestamp: April 01, 2025, 11:27:52 pm
content: Hi Noizzar,I appreciate that some more live demonstrations of the probe would be helpful. I am considering putting together a video at some point, however this is not something I can do soon. I am considering sending a probe to a couple of other test equipment reviewers for their perspective, however I don't want to do that until I have the manufacturing really nailed down as I can't deal with a higher volume of orders right now. I feel like I'll getting there with this next batch.In general, I've now sold around 30 probes, and I've not have any complaints so far. I've even had a couple of repeat orders from some companies. So I would hope you wouldn't be disappointed!Quote from: Noizzar on April 01, 2025, 02:55:39 pmThis would be impossible with the input architecture that I have. It is a cool idea though. Ultimately shielding is the way to improve the noise pickup. This could be added by the user by applying shielding to the probe (copper tape), or by probing in a quiet environment.Quote from: Noizzar on April 01, 2025, 02:55:39 pmI am trying to reduce manual assembly steps from here on out. This is a big task and would substantially increase the price of the probe.Quote from: Noizzar on April 01, 2025, 02:55:39 pmI have been playing in LTspice simulating this probe, and it is quite a challenge. Still tbd.

author: lasmux
author_rank: Regular Contributor
timestamp: July 28, 2025, 11:46:25 am
content: Update time. I have a few things to talk aboutI've done a bit of work on differential measurements using two single-ended active probes, with their ground pins tied together with a bit of copper braid (and not grounded to the DUT). The resultant waveforms from each probe is then subtracted via a math function on the scope. See attachments of setup and of an initial result from last night. The measurement is of a HDMI differential pair clock signal. It's also much much easier to get differential measurements of signals which are physically close together when you don't also have to worry about the ground pins making good contact. The one issue with this technique is that triggering is a bit more difficult as you are potentially triggering off a signal which is bouncing around, so you might not trigger off an actual edge. This makes say generating an eye diagram tricky as successive waveforms aren't necessarily aligned. A solution is to use a third probe elsewhere as a trigger signal.I've also been doing some work investigating the usage of stands and the effects on the frequency response of the probe. I've found that this is only really the case when using a VNA to measure the response of the probe. In usage with an oscilloscope, there is no affect on the output of the the probe, whether it's handheld or held using a stand. I'm still not sure the underlying reason why with VNAs, the probe benefits from grounding with a copper strip when using a stand.On the topic of stands... I have developed a stand to ship with these probes. I'm in the final stages of sorting out manufacturing these. This makes life much easier when taking longer measurements, or measurements with multiple probes, such as the aforementioned differential measurements. See attachment for picture of stand in use. The stand itself is weighted with a large block of steel to keep it stable and compress the sprung ground pin. It also has some rubber feet to stop it sliding about. There is a replaceable clip which attaches to the probe SMA plug.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 28, 2025, 12:33:44 pm
content: Thanks for the update.Quote from: lasmux on July 28, 2025, 11:46:25 amI doubt there is a difference, outside of what you are measuring. The last data I collected showed the 3dB loss at 1.5GHz. I could certainly repeat this test using a 1.5GHz source and scope.***https://www.eevblog.com/forum/testgear/12-ghz-active-probe-project/msg5547989/#msg5547989

author: lasmux
author_rank: Regular Contributor
timestamp: July 28, 2025, 01:12:08 pm
content: Quote from: joeqsmith on July 28, 2025, 12:33:44 pmOkay yes, that would be a more precise measurement. Specifically looking at 1.5GHz, where you saw the change. I have measured signals with harmonics around that frequency band (1.5Ghz-1.8GHz), with which I've not noticed a change, but I can have another look with the fundamental frequency of a measured signal being 1.5GHz-1.7Ghz. Now that I have a stand built it's very straightforwards to test for me

author: lasmux
author_rank: Regular Contributor
timestamp: July 28, 2025, 05:25:46 pm
content: Quote from: joeqsmith on July 28, 2025, 12:33:44 pmI just did the test quickly at 1.6GHz and 1.5GHz. A 1.6GHz signal went from a 506mV peak-to-peak, to a 525mV peak-to-peak readout, when I grab the probe whilst its in the stand. A 1.5GHz signal went from 645mV to 670mV. So about a -0.33dB difference for both.Like I said from earlier testing, signal traces didn't change visually when handling or not handling the probe.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 28, 2025, 06:20:28 pm
content: Quote from: lasmux on July 28, 2025, 05:25:46 pmYour setup even peaks up rather than attenuates the signal.***Reading through the old posts, are you testing with the same hardware revision of what was sent? You also talk about adding a ground strap. Can you post a photo of your setup when you ran this test?

author: lasmux
author_rank: Regular Contributor
timestamp: July 28, 2025, 07:07:09 pm
content: The signal is attenuated whennotholding the probe. I see that my wording before was potentially a bit misleading. This is the same behaviour (but greatly reduced) as what is observed in a VNA I think?The hardware revision is different, but nothing has changed in the sensor/amplifier part of the probe, only the power supply. With this hardware revision, I still see the same large attenuation effect when using VNAs with a stand (not handheld), and without the ground strap.Theres a photograph of an example ground strap setup in the datasheet on page 7:https://www.lasmux.com/wp-content/uploads/2025/04/LD-ASP-2.7_r1_4.pdfEdit:Very rough diagram of setup which doesn't show distortion in frequency response.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 28, 2025, 07:25:07 pm
content: I don't remember and will need to review that part of the video to know for sure what I was seeing at the time.For this test, are you grounding the probe like figure 9, or figure 7 of your document?***Also, when you are using the holder in place of your hand, is the holder metal? Grounded?***Reviewing the video I made, it does peak up as I place my fingers near the probe's body. Which is what you are seeing. Keep in mind that the probe holder I am using is metal. It is also grounded through the base. Think of it as being a metal rod placed along side of your probe, then I add my fingers to the mix. I'll set it back up and let you know if I can provide any additional insight. It should reproduce.I wonder what you are using for the USB signal generator? Are you trying to use one of the low cost VNAs in CW mode as an RF source? If so, which?

author: lasmux
author_rank: Regular Contributor
timestamp: July 28, 2025, 09:32:25 pm
content: The stand has a large piece of metal inside of it to act as a weight, but this is isolated from the probe. There is no electrical contact with the probe. The metal part is probably 10-15mm from the probe body. So I'm guessing it's not doing anything.I will try to be more specific on what test setups I've been working on. The signal generator I am using is the USB LTDZ 35-4400M spectrum analyser in CW mode. I use this because it generates a very fast 75ps edge, and is useful for evaluating the bandwidth of the probe. It outputs a fairly sensible waveform normally. I think I have also seen the same lack of effect when using the LibreVNA in CW mode, but I don't exactly remember.Test setup 1.No change in frequency response with or without stand/touching the probe.Test setup 2.Probably better ground connection between signal generator and scope.No change in frequency response with or without stand/touching the probe.Test setup 3.Using different scope. USB sampling scope from SJL instruments.No change in frequency response with or without stand/touching the probe.Test setup 4.Measuring frequency response on LibreVNA, with ground strap fitted. (figure 7 in datasheet).No change in frequency response with or without stand/touching the probe.Test setup 5.Measuring frequency response on LibreVNA, WITHOUT ground strap fitted.Notable change in frequency response when not holding the probe by hand.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 29, 2025, 04:10:26 am
content: I first did a sanity check using the LiteVNA. No attempt was made to calibrate the VNA as we just want a relative measurement. I used the same holder as before, mounting the probe using the same paper clamp. I laid everything on top of a section of PCB as before. I used a homemade coplanar waveguide that is setup as a load standard. I just want to know if I would see the same notch.LiteVNA_mount_hand: showing S21 without my hand touching the probe (RED) and with my hand (YELLOW). About a 2.5dB difference. So, at least this setup behaves reasonably close to what I expected.I used SJL Gigawave 6400 with a BNC to SMA adapter to attach your probe to channel 1 as shown. I used my Marconi 2024 RF generator for a source, set to 1.4GHz.SJL_mount_hand: Orange showing with my hand attached. Yellow/Red showing with my hand away from the probe. I would need to hunt around a bit to find the sweet spot but I can clearly see a difference with both the VNA and scope, as I would expect.

author: lasmux
author_rank: Regular Contributor
timestamp: July 29, 2025, 06:41:47 am
content: Thanks for the testing Joe. I'm going to have to do some more testing to see if I can reproduce your results. Do you have a bit of copper tape handy to test the ground strap?

author: joeqsmith
author_rank: Super Contributor
timestamp: July 29, 2025, 11:37:53 am
content: Quote from: lasmux on July 29, 2025, 06:41:47 amI'll solder up a dedicated test board with a strap and retest.

author: lasmux
author_rank: Regular Contributor
timestamp: July 29, 2025, 11:50:57 am
content: I am wondering whether the metal stand you are using has something to do with this also. As you noted, there is a large grounded metal rod right next to the whole probe, but I see from the photograph that the bottom of that rod extends well beyond the bump grips of the probe. The circuitry beyond the bump grips is particularly sensitive, which is why it's recommended in the manual to only hold the probe from above the bump grips. I will run my own tests as well later.

author: hpw
author_rank: Frequent Contributor
timestamp: July 29, 2025, 02:25:50 pm
content: As you have a LeCroy WaveMaster 7300a.Just compare with an AP034 differential probe or any none differential how it perform.I meanCMRR & channel skewwise... while your dual probe use may looks as working but measurement wise

author: joeqsmith
author_rank: Super Contributor
timestamp: July 29, 2025, 03:12:21 pm
content: Quote from: lasmux on July 29, 2025, 11:50:57 amI made up two different fixtures, added a few inches of braid to one, and picked some O-rings that will allow me to ditch the paper clip and secure your probe to the holder without it extending beyond the grips.I'll run it later today.

author: lasmux
author_rank: Regular Contributor
timestamp: July 29, 2025, 10:34:30 pm
content: Having a piece of metal (grounded or otherwise) near the probe tip does indeed attenuate the output of the probe, so I think that's definitely something to try. I've still not been able to recreate the -3dB change going between handheld and stand held on a scope.Quote from: hpw on July 29, 2025, 02:25:50 pmUnder around 400MHz it's pretty well behaved, but yeah, I just ran some tests and the CMRR goes below 10dB at around 600MHz. It was around 30dB at 100MHz. And yes, you do have to tweak the skew on one of the channels to account for differences in cable length etc. But that's quite easy to do. Just measure the same signal with both probes and adjust the skew to minimise the subtracted signal.In general I've not put a lot of effort into this so far, so these are only early unrepeated results. And also, the performance will depend on how well the probes are matched. In general for measuring differential signals though, I think it is still very useful as most common mode noise on a measurement will be at relatively low frequency where the CMRR of this setup seems to be reasonable, and so a differential measurement will improve the noise on any measurement by a factor of 2x. Also, given there is no ground connection, potentially the signal loading is improved also. I need to test this also.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 29, 2025, 11:18:34 pm
content: Quote from: lasmux on July 29, 2025, 10:34:30 pmSetup1-3Showing new fixtures. Cut down wire wrap pins were soldered to the coplanar waveguides to allow a more repeatable connection to probe. Also shown is ground strap attached to the load board. The alligator clamp's teeth were cut off and braid was soldered in to get a better surface connection to probe.Setup4With new O-rings, I can attach the probe to the holder without the holder extending beyond the grip area of probe. Note how clamp attaches to the SMA body.The LiteVNA was again used. Yellow with my hand off the probe, Red with it attached. It certainly had an effect on the overall flatness. Fingers were placed higher up on the probes body as shown in Setup 5.The probe was then attached to the SJL Gigawave 6400 and the test fixture stimulated with my Marconi 2024. Shown with my hand off the probe Yellow/Orange and fingers just below the O-ring as before, Red. Note, that at higher frequencies we would see it flip (signal decrease with my finger touching probe).

author: lasmux
author_rank: Regular Contributor
timestamp: July 30, 2025, 10:52:06 pm
content: That's a very serious ground strap!!! A bit more significant than my simple copper tapeThanks for the extensive testing there. It looks to me like the effect has indeed been reduced with the movement of the metal stand away from the probe front end and the addition of the ground strap. Again, for me I only need to use the ground strap when taking VNA measurements whilst also using the stand (not handheld).In terms of what we have in common between our setups, we have the same probe, the same scope, a fairly similar signal generator, a fairly similar test fixture. Despite that we are getting some differences in results still, although closer now. Given the improvements by reducing the proximity of your metal stand, my feeling these anomalous readings are due to the large metal objects very near or directly pressed against the probe, especially if they are beyond the bump grips, which I don't have because of the plastic/isolated stand I'm using.In terms of the probe itself. The only difference between my probes and yours is that the revision I am using has a different grounding setup at the battery input. I don't think that would affect this though.I have done some more testing and confirmed that placing metal objects beyond the bump grips, within maybe 5mm of the area beyond the bump grips can modify the response of the probe.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 30, 2025, 11:53:20 pm
content: One thing I show in the photo, but may not have made clear, my wrist is laying on the PCB, which makes a difference.Also, keep in mind that someone could very well be trying to probe a board near metal.

author: lasmux
author_rank: Regular Contributor
timestamp: July 31, 2025, 08:43:57 am
content: I have also tried to recreate your results with my hand being 'grounded' to nearby metal etc, without success. I think there may be a couple of effects going on. Firstly, grounded metal increasing the capacitive coupling between the amplifier inputs and ground, which will affect the frequency response. Secondly an affect on the general grounding quality of the probe. I suspect the first one is the dominant effect due to the reduction in the effect when you moved the metal stand away from beyond the bump grips. The way to resolve this would be shielding which we've already discussed as not feasible at this price point, or increasing the capacitive coupling between the amplifier signal input and the signal being measured. This would have the effect of increasing the capacitive loading of the probe, so isn't something I want to be attempting as that is probably the parameter that the probe is most optimised for. It would also require retuning the frequency response of the probe. I will expand the current direction within the manual to 'not hold the probe beyond the bump grips' to include a comment about metallic objects beyond the bump grips somewhat affecting the frequency response.On the topic of capacitive loading on measured signals. I've done some more testing with the twin probe differential setup, and the S11 return loss on a probed line drops even further from a worst case -15dB to around -16.5dB worst case. I can't find much data about other probes that note this. Is anyone aware of what the comparable value would be for say a Teledyne/Keysight/Tektronix/etc? I will post some more details later on. I need to do a bit more tweaking of the ground connection between the two probes. Currently it is quite long which I don't think is ideal.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 31, 2025, 12:20:08 pm
content: Quote from: lasmux on July 31, 2025, 08:43:57 amList of my probes:https://www.eevblog.com/forum/testgear/12-ghz-active-probe-project/msg4988716/#msg4988716You are using the coplanar waveguide, terminated to 50 ohms, measuring S11, then attaching the probe? You would like to see that same measurement using my diff probe? They do provide the capacitance, noted in that list if that helps.

author: lasmux
author_rank: Regular Contributor
timestamp: July 31, 2025, 04:28:50 pm
content: Quote from: joeqsmith on July 31, 2025, 12:20:08 pmI am calibrating the VNA with a 50ohm terminated coplanar waveguide in place, and then measuring the return loss by attaching the probe to the waveguide. So there is -60dB S11 worst case return loss before the probe is attached, and then -15dB when it is attached.I would love to see the return loss measurements on those probes if you don't mind! Maybe the HFP3500 which is the same as the Lasmux probe capacitance. Also the WL300 would be a glorious measurement with a 0.1pF tip capacitance!!

author: joeqsmith
author_rank: Super Contributor
timestamp: July 31, 2025, 06:26:37 pm
content: Quote from: lasmux on July 31, 2025, 04:28:50 pmYou attach the WG to the test cable, then cal at the opposite end of the WG, then replace with terminator?OrShort / open at and of cable, then attach WG with load?OrSOL at end of cable, then attache WG with load?

author: lasmux
author_rank: Regular Contributor
timestamp: July 31, 2025, 09:46:24 pm
content: Quote from: joeqsmith on July 31, 2025, 06:26:37 pmThe first one. I attach the waveguide to the test cable, then SOL calibrate on the opposite side of the waveguide to the test cable.Port1 || ---- test cable --- || --- waveguide --- || SOL

author: joeqsmith
author_rank: Super Contributor
timestamp: August 01, 2025, 04:25:00 pm
content: Quote from: lasmux on July 31, 2025, 09:46:24 pmI used the LeCroy WR64x rather than the WaveMaster as we just need to power up the probes. I used the Agilent E8357A (hacked to 9GHz) with ecal. Used unknown thru to cal the VNA and left port 2 attached allowing us to collect all four S-parameters if you want them. I used some very short pins in the HFP3500. The pins on the 300 are fixed.While I am confident in the data, be aware that Keysight no longer serviced these units when I bought mine. I wrote some custom software to allow me to align it. The equipment I used to align it were checked against equipment from where I work that are under calibration. The ecals and adapters were characterized using Keysight standards that I loaned out.

author: lasmux
author_rank: Regular Contributor
timestamp: October 04, 2025, 08:13:55 am
content: Arg, sorry Joe I thought I had replied to this.Thanks for that test data. Very interesting results.That 0.1pF tip capacitance of the WL300/D300 probe is just marvellous! No idea how they manage that.Interesting how the return loss of the HFP3500 is a bit worse than the lasmux probe, despite being a similar tip capacitance.

author: lasmux
author_rank: Regular Contributor
timestamp: October 09, 2025, 07:58:37 pm
content: Alongside the new stand which I'm going to start shipping with the probe soon, I also have a new EMI shield to go over the probe. This is still being developed, but is essentially a semi-conductive (carbon filled plastic) shoud that covers the sensitive parts of the probe. I'm currently evaluating different materials that are better and worse at shielding. Not quite ready to share the shielding noise attenuation level just yet, but it's going in the right direction.The stand went through a last series of design edits to make the legs fold so it fit better in the case. I'm building these now. They have a large steel weight in them to weigh down the probe and keep it in place when measuring circuits. It's a bit of a game-changer for usability IMO.I've also got a much improved case it's now shipping with.See attached images

author: Andree Henkel
author_rank: Regular Contributor
timestamp: October 10, 2025, 11:58:24 am
content: For people like me, who already ordered have probes will there be a way to order only stand and EMI Shield?

author: skander36
author_rank: Frequent Contributor
timestamp: October 10, 2025, 01:23:45 pm
content: Quote from: Andree Henkel on October 10, 2025, 11:58:24 amIt should be possible, in my opinion ...

author: lasmux
author_rank: Regular Contributor
timestamp: October 10, 2025, 02:53:19 pm
content: Quote from: Andree Henkel on October 10, 2025, 11:58:24 amYes, I will be able to set up some sort of purchase link for just those. I'll do this once I've got the EMI shield design completed.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 10, 2025, 03:46:54 pm
content: We were playing around with some microwave horn antennas that were printed. We tried a few different conductive plastics but none of them worked well. We tried some copper the we printed oversized then attempted to sinter them. While this sort of did something, we could not get the process down.If you get something working, I am interested in knowing more details about it.

author: lasmux
author_rank: Regular Contributor
timestamp: October 11, 2025, 10:50:06 am
content: Quote from: joeqsmith on October 10, 2025, 03:46:54 pmYes I saw the thread. What conductive filaments did you use? I didn't see you mention it in the thread.As an aside, I was wondering if pulse plating with a low duty cycle would give better results if electroplating 3D printed parts that can't handle the heat generated from DC plating. Not that I'm planning on electroplating anything.

author: lasmux
author_rank: Regular Contributor
timestamp: October 14, 2025, 05:52:17 pm
content: So it turns out that the EMI shield does reduce the EMI... but also attenuates the probe response at higher frequencies. A lot of the improvements I thought I was seeing was actually just the probe response at that frequency being attenuatedAt 2.4GHz the EM inteference is reduced by around 8dB, but the output of the probe is also attenuated by around 4dB. So there's only around a net 4dB of EMI reduction, and the probe response is nowhere near as linear. The EMI attenuation reduces at 2GHz, and is basically 0dB attenuation by 1.5dB.On a plus note, the S11 return loss improved from worst case -15dB to worst case -16dB for some reason.As a result I don't think it'll be worth including the shield, and the recommendation for people working in noisy environments remains to try to isolate the probe from these sources of inteference.For reference I was using a semi-conductive 3D print filament from protopasta, which has graphite embedded in it.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 15, 2025, 02:32:41 am
content: I am not sure what material those were. One was an ESD type, but the other was listed as conductive. I was unsuccessful at plating them and they were far too lossy to make waveguide components from.