> 1 GHz DIY differential probes - Page 1
Forum section: electronics / testgear

author: JohnG
author_rank: Frequent Contributor
timestamp: August 13, 2016, 03:37:19 pm
content: I'm wondering if anyone has a working design or knows of one for a DIY wideband (DC to > 1 GHz) DIY differential probe. I've search for this on and off, and found some attempts, but have not found a successful design. I have come across references to a 2 GHz design published in Elektor July/Aug 2015, but I don't have the article so I don't know if this is a working design or not. I'd buy the issue if I had some indication that this was indeed a working design.I've used these types of probes before, so I understand the voltage and impedance limitations they present.There are a few reasons I'd like to have a DIY design. First, to learn from. Second, to have a design that is not tied to a particular brand scope. Third, to extend or modify for different applications. In theory, one could have a few different probes for different voltage ranges, or similar.Any help appreciated!John

author: joeqsmith
author_rank: Super Contributor
timestamp: August 14, 2016, 02:09:15 am
content: I attempted to design a DC coupled wide band differential probe last year. Got as far as coming up with requirements. In the end I could not find parts suitable for the job.I had seen a few homemade probes people had came up with but nothing DC coupled and nothing as fast as I was looking to build.Beyond the DC-1GHz diff, what are your requirements?https://xellers.wordpress.com/electronics/1ghz-active-differential-probe/https://xellers.wordpress.com/2014/09/28/diy-active-differential-probe-characterization-round-2/

author: JohnG
author_rank: Frequent Contributor
timestamp: August 14, 2016, 02:53:41 pm
content: Since you asked about requirements:Bandwidth: > 1GHZ (risetime < ~300 ps) or so, faster is betterInput: >= 1k resistive, <= 2pF capacitive (higher resistance would be preferredAttentuation: 10:1INput voltage range: >10 V DM, >5V CMCMRR: >20 db at 1 GHz, if possibleIf I could do a 100:1 with a variation on the same circuit, that would be great. I could take some hit on speed at the higher voltage, but the input resistance would have to go up to at least 10k. It would not need to meet 100V continuous operation, but the ability to get capture short pulses (up to 1-2us wide) at 100V or close would be a big plus. Not many commercial probes can do this at this kind of BW.Here is what I really want, but it will only work with a Tek Scope, which I don't have. It will probably cost as much as the scope, or close:http://www.tek.com/dl/51W-60485-0%2520IsoVu%2520White%2520Paper%2520%2520TN%25203-25-16.pdfAll the CMRR you could ever want. I have seen it in action, and it works. But, I won't be getting one anytime soon.John

author: Marco
author_rank: Super Contributor
timestamp: August 14, 2016, 05:36:04 pm
content: Quote from: JohnG on August 14, 2016, 02:53:41 pmNot really. That's a high frequency 50 Ohm input isolation amplifier, not a high input impedance differential probe. You can simply add two resistors for a differential version of the passive attenuated probe, but just as with the passive attenuated probe that's not optimal as far as noise is concerned.The Elektor design uses a high frequency fully differential amplifier (ADA4927-1) as a differential to single ended converter, witch matched resistors to provide attenuation. BoringAlso they didn't actually measure CMRR, you can't rely on datasheet CMRR because that's for differential->differential and common mode errors for the two outputs could very well cancel out in differential mode.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 14, 2016, 05:47:04 pm
content: When I saw 1GHz to DC I was thinking digital, not something high voltage dual purpose. I was hoping to keep loading above a couple hundred ohms with maybe +/-2.5V differential and same for common mode. Maybe 0.2pf at the tip sort of thing, couple of GHz BW. I was thinking that with modern parts I could pull it off but I spent a few weeks looking and gave up. I was also having problems trying to figure out how I was going to tackle the DC part. My plan was to use 2 AC paths and two DC to something paths. Then somehow combine the whole mess and not ruin signal. The circuitry would have been fairly complex, at least for me.Keep us posted if you try it.On the bright side, at least for digital work the used probe market seems to be growing. Prices are all over the place. Thousands for broken probes with missing parts, to under a K for this baby!http://www.ebay.com/itm/Teledyne-LeCroy-D13000PS-13GHz-Differential-Probe-System-/131895698595?hash=item1eb598a4a3:g:4WEAAOSw~otWce6kI could see this for my hobby use.http://www.ebay.com/itm/LeCroy-D600-7-5-GHz-WaveLink-Probe/162154103823?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D20131003132420%26meid%3D1b91768d4ae64145a92f0a3a455d4605%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D311133558055That Tektronix probe looks slick. Having a 2KV CM standoff or galvanic isolation is not something I would need at home. I did find another article on it. Sounds like it may not yet be available.http://electronicdesign.com/blog/1-ghz-isovu-leads-tektronix-probe-performanceusability-charge-apecAnd a video on their facebook pagehttps://www.facebook.com/tektronix/videos/vb.50990632804/10153395327767805/?type=2&theater

author: Marco
author_rank: Super Contributor
timestamp: August 14, 2016, 06:26:23 pm
content: Quote from: joeqsmith on August 14, 2016, 05:47:04 pmHow do these actually work? From the pics and the impedance specs I would guess each signal is probed by a separate passive divider probe (with 500 Ohm tip resistance). Because it seems to have two separate transmission lines coming out.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 14, 2016, 07:15:58 pm
content: Quote from: Marco on August 14, 2016, 06:26:23 pmI have not idea but am guessing the changeable interconnect to a custom ASIC to the scope.Spent some time to see if they had a whitepaper or some other notes. May be of interest:http://cdn.teledynelecroy.com/files/whitepapers/wp_differential_measurements.pdfhttp://cdn.teledynelecroy.com/files/whitepapers/probing_technology.pdfhttp://cdn.teledynelecroy.com/files/whitepapers/technologies-for-very-high-bandwidth-real-time-oscilloscopes.pdfhttp://cdn.teledynelecroy.com/files/manuals/d13000ps-om-e.pdf

author: Marco
author_rank: Super Contributor
timestamp: August 14, 2016, 10:26:02 pm
content: That's the Lecroy, the IsoVu was at 80 dB at 1 GHz. As I said though, it's not a differential probe, it's a 50 Ohm input isolation amplifier.

author: tggzzz
author_rank: Super Contributor
timestamp: August 14, 2016, 10:36:26 pm
content: A novelverysimple and robust differential probe:http://emcesd.com/pdf/cd94scr.pdfNot DC, the article measures refers to 500MHz.

author: Marco
author_rank: Super Contributor
timestamp: August 14, 2016, 10:54:11 pm
content: Simple, but not cheap. It should be possible to make a combiner out of coax and ferrite beads.

author: nctnico
author_rank: Super Contributor
timestamp: August 14, 2016, 11:16:54 pm
content: Does it need to be DIY from scratch? Tektronix P6860 logic analyser probes can be found cheap (say $50) and they have a differential amplifier for the clock / qualifier inputs and regular FET probe inputs for the other signals. All in all one probe gives you 2 differential and 16 regular fet probes with >2GHz (probably 3Ghz) bandwidth.

author: lukier
author_rank: Unknown
timestamp: August 15, 2016, 12:18:37 am
content: Quote from: nctnico on August 14, 2016, 11:16:54 pmInteresting. Tektronix doesn't share much detail on the inner workings of their probes (but some companies seem to know more - e.g.http://www.movingpixel.com/SQUIRE.html). How do you power this probe (without TLA card/mainframe I assume in this thread) and terminate it for the scope (50 Ohm). Did you build an adapter from the probe high density connector (the side that goes to the TLA7AA4) to BNCs?

author: Cerebus
author_rank: Super Contributor
timestamp: August 15, 2016, 12:38:16 am
content: Please ignore this, just marking thus to come back to.

author: Someone
author_rank: Super Contributor
timestamp: August 15, 2016, 01:20:55 am
content: Quote from: JohnG on August 13, 2016, 03:37:19 pmYou'll have challenges simply driving the cable and scope input capacitance at 1GHz, you could start with a simple implementation of the THS3217 to get a feel for the project but it won't hit 1GHz. Cheap probes without manufacturer dependence are possible:https://www.eevblog.com/forum/projects/diy-100mhz-differential-probe/

author: joeqsmith
author_rank: Super Contributor
timestamp: August 15, 2016, 02:26:57 am
content: Quote from: tggzzz on August 14, 2016, 10:36:26 pmInteresting idea. Looks like the Mini-circuits 1-500MHz w/SMA cost about $65. Isolation is worse on their surface mount parts. I wonder why he did not have a resistive probe in his comparison.

author: nctnico
author_rank: Super Contributor
timestamp: August 15, 2016, 10:51:04 am
content: Quote from: lukier on August 15, 2016, 12:18:37 amI have not looked into the technical details. I'm just pointing towards an existing solution which could be hacked.

author: JohnG
author_rank: Frequent Contributor
timestamp: August 15, 2016, 12:46:13 pm
content: Thanks for the feedback, everyone.1 GHz is a hard requirement, and is probably barely enough for my application. I've seen the link to the design with the transformer, and it looks intriguing, but the bandwidth is not there and I would really like the ability to go to DC.I've looked at buying used probes and hacking at them, but this is for work, and my priority is not to design a probe, but get measurements. At some point it becomes cheaper to buy, just not there yet. I would like the flexibility of having a design - recently I have built transmission line probes into some designs and they work surprisingly well.The IsoVue may not look like a differential probe, but it sure acts like one. Yes, it looks like a 50 ohm isolation amplifier, but it is a high resolution analog one, and they are not exaggerating the CMRR. Because the CM impedance is so high, there is very little conversion of CM to DM. I'm not sure about the innards, except that I know they use an electro-optic modulator. These can be designed to be inherently balanced, so it may be that the only imbalanced part is the input coax/attenuator. So unless you are sure about the design, it may be premature to say it is not a differential probe. Here is a link with some interesting information:https://www.google.com/patents/US7310455John

author: tggzzz
author_rank: Super Contributor
timestamp: August 15, 2016, 12:50:01 pm
content: Quote from: JohnG on August 15, 2016, 12:46:13 pmHave you considered renting a probe? Last time I looked at renting, a long time ago, the monthly rental charge was 10% the purchase price.

author: Marco
author_rank: Super Contributor
timestamp: August 15, 2016, 01:59:11 pm
content: Quote from: JohnG on August 15, 2016, 12:46:13 pmI thought that because they used MMCX connectors all the input impedances of the probes were 50 Ohm, but apparently they just ignore the mismatch (not big deal at 1 GHz) andthe attenuated probes have higher input impedance. They seem to be simply using simple passive dividers for attenuation, with no termination at the probe end.So yeah, it's a high impedance differential probe ... but a noisy one, because of the attenuation. 25x for 1.25k input impedance. I guess they want to sell an active probe separately in the near future, undoubtedly for a ton of money.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 15, 2016, 10:34:04 pm
content: Rental may not be a bad way to go but I too have not done this in many years. I wonder if the cost of a used one would be close to the rental fees. Something to consider if its a one off measurement.I looked around to see if I could find a better 180 degree combiner but did not find anything. May have been fun to try and replicate his tests.

author: David Hess
author_rank: Super Contributor
timestamp: August 17, 2016, 09:53:14 pm
content: Source impedance is a major problem with wide bandwidth differential probes and a major limitation of performance. If you are probing a differential transmission line, then it may be better to build a test point into the circuit with 50 ohm outputs to connect directly to the oscilloscope which can then do the differential measurement itself.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 17, 2016, 11:37:02 pm
content: Quote from: David Hess on August 17, 2016, 09:53:14 pmI guess that depends on what you consider a major problem. My old DC coupled diff probes are 300 ohms at 4GHz. Tip capacitance is a 1/10th of a pf.

author: David Hess
author_rank: Super Contributor
timestamp: August 18, 2016, 06:31:04 am
content: Quote from: joeqsmith on August 17, 2016, 11:37:02 pmHow much source impedance mismatch are you dealing with though? A 300 ohm differential probe is not intended for general purpose measurements; it requires closely matched source impedances to maintain common mode rejection ratio.This Tektronix article,Making Single-ended Measurements with a Differential Probe, refers to the problem on page 11:A 50 ohm source impedance mismatch for a single-ended signal measurement has only a minor effect on the DC CMRR of a probe with a 50 Kohm probe input impedance. As the signal frequency increases however, the probe input impedance begins to decrease and eventually this 50 ohm source impedance mismatch for a single-ended measurement can become quite significant. High frequency AC common mode voltage transients that may be largely rejected by the relatively high CMRR in a differential measurement may show a noticeable effect due to degraded CMRR in a single-ended measurement.

author: tronde
author_rank: Frequent Contributor
timestamp: August 19, 2016, 12:51:39 am
content: Quote from: JohnG on August 13, 2016, 03:37:19 pmI don't want to break Elektor's copyright, but this is the specs:Technical specification• Attenuation: 10:1 with a differential signal and 50 ? termination in the ‘scope• Differential input resistance: 5 k?• Single-ended input resistance: 2.5 k?• Output resistance: 50 ?• Bandwidth: 1.9 GHz (–3 dB)• Rise/fall time: 300 ps• Power supply: ±8 to 12 V DC.They also write:For readers who are interested the authoroffers ready-to-use and tested PCB modules, also a kit consisting of case, RF cablewith BNC connector and power supplylead with plug. Further information from:alfred_rosenkraenzer@gmx.de.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 19, 2016, 03:07:58 am
content: Quote from: David Hess on August 18, 2016, 06:31:04 amDifferential digital, LVDS etc.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 19, 2016, 03:19:41 am
content: Quote from: tronde on August 19, 2016, 12:51:39 amLooked to see if they had a website or something with more info. Do you have a link besides the email address?

author: Cerebus
author_rank: Super Contributor
timestamp: August 19, 2016, 12:48:57 pm
content: Quote from: joeqsmith on August 19, 2016, 03:19:41 amI finally managed to find a copy of this to look at. There's very little to this probe. An ADA4927-1 op amp, input and feedback resistors for it, two 7xL05 regulators and associated decoupling. That is pretty much all. The amp only has an input common mode range of +/- 3.5V. The most concerning thing from my perspective is that there is not even a nod to input protection - the author relies on 2297 ohms of input resistance and whatever the chip has on board as input protection. Given there is a differential output already available from the amplifier so some sort of bootstrapped protection network wouldn't have been too taxing to design.I'm pretty comfortable that posting just the schematic for study and discussion falls under fair use, so here it is:

author: JohnG
author_rank: Frequent Contributor
timestamp: August 19, 2016, 01:14:17 pm
content: Interesting. I would not have guess that you could do the differential to single-ended conversion that way.Also, a 300 ps rise and fall time does not correspond to a 1.9 GHz BW. That is more what I would expect from a ~1.2 GHz probe. Either the frequency response is not approximately single-pole near the BW, or 1.9 GHz is for small-signal only. I'll look up the specs on the chip.John

author: Marco
author_rank: Super Contributor
timestamp: August 19, 2016, 07:39:26 pm
content: Quote from: JohnG on August 19, 2016, 01:14:17 pmI'd like to see some CMRR measurements. As I said before the datasheet CMRR might be a result of errors in the differential output canceling out.

author: alfredr
author_rank: Newbie
timestamp: September 05, 2018, 06:53:58 am
content: Hi, my name is Alfred Rosenkraenzer.I am the designer of this active high speed probe described in Elektor 2016. This version is no longer available, but there is a new one described in ELEKTOR 3/2017(DE) using a USB power supply. The analog specs are similiar.I am selling loaded and tested pcb and finished probes (no blank boards). More info under alfred_rosenkraenzer@gmx.de

author: KE5FX
author_rank: Super Contributor
timestamp: September 05, 2018, 07:13:15 am
content: Quote from: alfredr on September 05, 2018, 06:53:58 amNice. Is that an LMH5401?

author: nctnico
author_rank: Super Contributor
timestamp: September 05, 2018, 07:39:09 am
content: Quote from: alfredr on September 05, 2018, 06:53:58 amVery interesting. Do you have some kind of datasheet? Also, why are you using RG316? In my experience this is already quite lossy at 'only' 1 GHz.

author: alfredr
author_rank: Newbie
timestamp: September 05, 2018, 07:48:06 am
content: No, it is an Analog Devices ADA4927-1. It gives you about 2 GHz bandwidth (-3dB). The differential input impedance is about 5.1 KOhmThe LMH5401 gives you even more bandwidth (maybe up to 4 GHz) but it lowers the input impedance even more.I use the LMH5401 in another application, but I did not test it for a probe.The higher bandwidth might not be usefull for most people since you need a expensive scope.There is a description with specs attached.I use the RG316 to stay at an affordable price.

author: nctnico
author_rank: Super Contributor
timestamp: September 05, 2018, 08:03:13 am
content: And what would the price be when using better coax? 1 meter of RG316 gives a loss of 1.2dB at 1GHz which is an amplitude error of around 15%. I could use this probe but I would like to have the best possible frequency response.

author: alfredr
author_rank: Newbie
timestamp: September 05, 2018, 09:51:02 am
content: I would propose you buy the loaded board and the housing (drilled holes and added connectors if you like) and install whatever cable you like.The USB cable should not be aproblem

author: nctnico
author_rank: Super Contributor
timestamp: October 14, 2018, 08:41:46 pm
content: I got a bit intrigued and rebuild the Elektor design but this time using 0402 capacitors & resistors to get better HF performance and an improved board layout which should minimise the HF peaking. This posting is an update of where I'm at now. Unfortunately my design also showed some hefty peaking so I didn't got very far. A 400MHz square wave for example:Things get even worse for a 200MHz square wave. Time to pull out the spectrum analyser and do some sweeps. Now a differential probe should show the same signal amplitude regardless the polarity of the input signal. Interestingly reversing the polarity of the input shows two entirely different traces. In one situation it peaks and in the other it drops. The effect seems to be the strongest between 700MHz and 800 MHz. A CMMR (common mode rejection ratio) measurement also shows poor performance in the same frequency range.This turned into a bit of a head scratcher. The simulation (using the ADA4927 model from Analog Devices) doesn't show this behaviour at all and I did not manage to find the cause in the circuit board layout. It has to be something in the amplifier which isn't in the pspice model!At one point this graph in the ADA4927 datasheet the caught my eye:This shows the common mode amplification with peaking in the 750MHz region! And since the ADA4927 is not used differentially but as a common-mode amplifier this graph just explains everything. Some further research showed me that all so called fully differential amplifiers will suffer from this behaviour because the amplifier for the common mode has a poor frequency response. IMHO the fully differential amplifier are simply not suitable for use in a differential probe if the output is used in a single-ended way.

author: Cerebus
author_rank: Super Contributor
timestamp: October 14, 2018, 09:17:55 pm
content: I find pretty much all of nctnico's findings unsurprising. Every commercial differential probe design I've seen has frequency response trimming between both halves of the differential input (one or both sides), CMMR trim, etc. The assumption that this design makes in omitting them and expecting to get acceptable results is, IMHO, a foolish one and seems to be bourne out by measurement.

author: Marco
author_rank: Super Contributor
timestamp: October 15, 2018, 09:18:07 am
content: Quote from: nctnico on October 14, 2018, 08:41:46 pmNeed a GHz version of the AD8129.

author: dietert1
author_rank: Super Contributor
timestamp: October 15, 2018, 09:49:54 am
content: Happened to find this thread after completing various measurements with a probe i bought recently from A. Rosenkränzer. In my opinion the probe is useful to record a LVDS data stream, but i would not call it a 1.9 GHz differential probe. My measurements with our HP 8560A show that ADA4927 common mode is limited to about 600 MHz if you require 3 dB accuracy. The measurement shows gain on direct (faint curve) and inverting input (bright curve).I would consider the design incomplete. For example, he writes the probe is 1:10 while it really is 1:20. 400 mV difference on LVDS results in 20 mV in the scope (with 50 Ohm terminated input).Anyway the concept should be valid, when elaborated carefully, e.g. with metal enclosure. Maybe a LMH3401 serves better. It is specified with a common mode bandwidth of 3.3 GHz.Regards,Dieter

author: nctnico
author_rank: Super Contributor
timestamp: October 15, 2018, 10:17:24 am
content: The LMH3401 won't work because the minimum stable gain is >6.3 which together with the GBW product throws you back to little over 1GHz of bandwidth in an actual design. Most (if not all) of these fully differential amplifiers are designed as a front-end for fast AD converters which need a 1Vpp or 2Vpp (-ish) input signal. Also I don't think a metal enclosure will help much. All nodes in the circuit are low impedance anyway.@Cerebus: the problem isn't in the mismatch between the inputs. I've used a symmetric PCB design and 0.1% resistors. There is no amplification so any mismatch between the inputs doesn't get multiplied. I'm convinced the problem is in the behaviour of the common mode amplifier of the ADA4927.

author: dietert1
author_rank: Super Contributor
timestamp: October 15, 2018, 10:37:09 am
content: Where did you get that number "6.3"? Can't find that in the LMH3401 datasheet.The evaluation board manual describes a 50/200 Ohm configuration (gain = 4), so i unterstood that was supposed to be stable.

author: Marco
author_rank: Super Contributor
timestamp: October 15, 2018, 11:40:12 am
content: Why not build an AC differential amplifier from a pair of RF transistors?

author: tggzzz
author_rank: Super Contributor
timestamp: October 15, 2018, 01:39:31 pm
content: Quote from: Marco on October 15, 2018, 11:40:12 amOr from a few bits of coax and a combiner, e.g.http://emcesd.com/pdf/cd94scr.pdfNote: I haven't tried such a probe!

author: nctnico
author_rank: Super Contributor
timestamp: October 15, 2018, 01:46:30 pm
content: Quote from: dietert1 on October 15, 2018, 10:37:09 amThe number 6.3 is from the specs on the website. That corresponds with a gain of 16dB. I just checked the evaluation kit manual but I can't find any mention of configuring the chip for a gain of 4x. Creating an opamp with a high bandwidth which is stable at a gain of 1 is very hard because it needs compensation to be stable but at the same time the compensation eats into the bandwidth.

author: dietert1
author_rank: Super Contributor
timestamp: October 15, 2018, 02:35:35 pm
content: Yes, they use 16 dB as a basic configuration of the LMH3401 in the specs. But that is not a stability limit, see for example Figure 2 in the datasheet, which is for 12 dB and extends well up to 5 GHz. Schematic is in Figure 51. The 12 dB configuration is also mentioned in the evaluation board user manual, paragraph 4.4.As far as i understand one advantage of the LMH3401 over the LMH5401 mentioned before is the internal feedback path which helps for stability.

author: snoopy
author_rank: Frequent Contributor
timestamp: October 15, 2018, 11:11:24 pm
content: The probe heads on the commercial probes such as Keysight and Tektronix are usually built using strip-line techniques on special pcb materials with consistent dielectrics used for rf design so there is quite a bit of R&D in them to get them right. I just picked up a differential probe off ebay for a good price but they are getting hard to find at good prices. Considering the new ones cost megabucks your best bet is to look for used ones that are in working order.

author: Marco
author_rank: Super Contributor
timestamp: October 16, 2018, 06:22:41 am
content: Those aren't high impedance though, not unless you count a couple hundred Ohms as high impedance.GHz is in that sweet-spot where you can still have something relatively high impedance and general purpose.

author: dietert1
author_rank: Super Contributor
timestamp: October 16, 2018, 08:10:23 am
content: The offers of Keysight may be well competitive, but they forgot who are their customers: engineers and scientists. Sorry, they cannot address me with a TV spot talking about a spiral inductor and ESD protection. They don't offer a solution compatible with various makes of scopes.The calling price of a generic 1.5 GHz differential probe seems to be $ 1500, seehttp://www.significantdevices.com.au. Any hands on experience?

author: Marco
author_rank: Super Contributor
timestamp: October 16, 2018, 08:24:17 am
content: Quote from: tggzzz on October 15, 2018, 01:39:31 pmMore expensive than a bunch of BFU660F's though. How about something simple like this? Is the simulator really giving me that bad an impression of how it would work with a tight layout and some =<0204 components? R1=R2=0 is only useful for say 10s of mV of input, probably want to bias the bases to ~4V with opamps instead of resistor dividers, might want to clamp R5-R8 with Schottkys.PS. that Deltasense probe has nice specs, large voltage range and input impedance.

author: Cerebus
author_rank: Super Contributor
timestamp: October 16, 2018, 09:55:13 am
content: Quote from: Marco on October 16, 2018, 06:22:41 amWhat are you calling "high impedance" in this context, bearing in mind that 1pF across an input @ 1GHz would equate to 159 ohms, and that the stray capacitance across an 0603 part is around 0.04pF and an 0201 part 0.02pF (the latter being ~8k @ 1 GHz)?

author: Marco
author_rank: Super Contributor
timestamp: October 16, 2018, 10:14:08 am
content: High resistance at low frequency, high enough impedance to still be useful at high frequency. So still useful for general purpose probing in a pinch.The really high frequency differential probes all try to be constant impedance, just high enough to not fuck a 50 Ohm line too much ... and that's all they are useful for.

author: BravoV
author_rank: Super Contributor
timestamp: October 16, 2018, 11:32:57 am
content: Quote from: Mechatrommer on October 16, 2018, 11:09:31 amFemto farad ? I'm guessing its not that easy, example of a 3 decades old, HP 54701A 2.5GHz active probe (not a differential probe) has 0.6pF (typical) with input resistance of 100K Ohm.

author: Rerouter
author_rank: Super Contributor
timestamp: October 16, 2018, 11:44:20 am
content: If you want crazy low input capacitance, things get complex, but I would guess we can reduce what is currently there and iterate towards the goal, anything under 0.5pF already appears to be crazy level money,Most of the schematics I have glimpsed in the past from reverse engineered crazy high bandwidth probes seemed to use an input network to cancel out the apparent capacitance. and pretty much no protections at all.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: October 16, 2018, 11:51:31 am
content: I absolutely LOVE these HP probes, also the 1152A successor!The 0.6pF make the series resonance of the ground lead much less importantand you can probe around without fear of burning sth. @ less than 50V.regards,Gerhard

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 16, 2018, 12:12:28 pm
content: Quote from: BravoV on October 16, 2018, 11:32:57 amsorry i screwed up (msg deleted) i didnt see the low-f respond (1st attachment). so 1pF seems feasible? i got 0806 smd 1pF here(2nd attachment) they both load the 500 ohm node by a bit anyway @ 1GHzQuote from: Rerouter on October 16, 2018, 11:44:20 amhow about pcb printed capacitor? or diy like 2 plates separated by a piece of paper? is it wrong to be unacademical?

author: nctnico
author_rank: Super Contributor
timestamp: October 16, 2018, 05:22:05 pm
content: Quote from: Marco on October 16, 2018, 08:24:17 amI don't think this configuration will work very well (probably due to the miller effect). I did find a fast opamp and I'm going to try a classic difference amplifier approach. After all the reason to use a differential (instrumentation) amplifier is to have extremely high input impedances on both inputs but in this case a few kilo Ohms is already OK. Perhaps a discrete solution specifically designed to act as a differential probe can be pushed to a higher bandwidth especially if it doesn't need to amplify. DC offset and temperature stability will be challenging nuts to crack though.

author: Marco
author_rank: Super Contributor
timestamp: October 16, 2018, 05:37:55 pm
content: There's not a whole lot of amplification in that circuit, 1.5 or so since the emitter resistance of RF transistors is rather high, so Miller capacitance isn't a big deal. The simulator says the circuit has >400 Ohm of input impedance at 1 GHz. When you increase R1&R2 the Miller capacitance is reduced further still.

author: dietert1
author_rank: Super Contributor
timestamp: October 18, 2018, 07:14:15 am
content: Your difference stage (and whatever amplifier you make) will have an input capacitance of about 0.5 to 1 pF, which is much better than the usual passive probe and may be good enough for a 1 GHz probe.Yet if you want the 0.04 pF or 0.02 pF that cerebus proposed above and that Mr. Rosenkränzer implements in his probe, there will be a capacitive divider in front of the amplifier. This is also mentioned in the Keysight video. The difference stage always looses a factor 4, because you drop one of two output ports and you want to drive the line to the scope with 50 Ohm output impedance. In total you will have a factor 1:50 or 1:100.I think you really want some gain. If you put emitter followers in front of your difference stage this will get you roughly a factor 5 (BFU66F Cbe/ Ccb). Those who want DC to GHz might use THS4302 gain blocks.By the way, i received a LMH3401 evaluation board. That is an excellent difference stage, like +/- 1 dB up to 2.9 GHz. Image is boring.

author: dzseki
author_rank: Frequent Contributor
timestamp: October 18, 2018, 08:43:38 am
content: Quote from: BravoV on October 16, 2018, 11:32:57 amI have the HP 1120A which is only 500MHz active probe (not differential), it has 100kOhm||3pF input impedance in 1:1 mode and 1MOhm|| <1pF when using a divider (10:1 or 100:1).The point is this is a quite old stuff, appeared in one HP Journal from 1969 I think, the service manual do actually contains all the schematics, even the probe's hybrid, it might worth a look.

author: dietert1
author_rank: Super Contributor
timestamp: October 18, 2018, 09:29:22 am
content: In my opinion that old technology is completely obsolete and its analysis a waste of time. With the THS4302 gain block i mentioned you can make a high impedance 1:1 probe with 0.4 pF input capacitance and a bandwidth of 2.4 GHz.

author: nctnico
author_rank: Super Contributor
timestamp: October 18, 2018, 10:04:18 am
content: Quote from: dietert1 on October 18, 2018, 07:14:15 amThat sounds promising. Did you test single-ended in and single-ended out? I did some testing with an LMH6703 (dead-bug style) and that looks promising as well albeit with less bandwidth. At least there is no difference between reversing the inputs.

author: dietert1
author_rank: Super Contributor
timestamp: October 18, 2018, 11:25:27 am
content: Yes, the LMH3401 was tested with the HP 8560A again, so it is single ended in (TG) and single ended out. The curves from both inputs to the same output appear flat +/- 1 dB up to at least 2.9 GHz. I would not trust my setup to better than a dB.In the meantime i experimented a little with the ADA4927 probe from Mr. Rosenkränzer. I modified the difference stage to the standard setup with 4 301R resistors (inputs = Gnd). The 2K5 0603 input resistors were replaced each by a 2x 620R 0402 resistor pair. I also removed two pads and brought the coax cable closer to the amp.Now it works better, but the asymmetry of the ADA4927 still exists. With our WR64Xi and a Lattice FPGA measured risetimes are 345 psec on indirect input, 306 psec on direct input. The direct trace still shows more overshoot/ringing at 1.4 GHz .

author: nctnico
author_rank: Super Contributor
timestamp: October 18, 2018, 12:04:35 pm
content: OK. I think I'm going to re-design my board with an LMH3401 then instead of the LMH6703.

author: David Hess
author_rank: Super Contributor
timestamp: October 20, 2018, 02:35:21 am
content: Bob Pease's probe with 0.29 picofarads of input capacitance is shown below. The printed circuit board stiffener adds another 0.08 picofarads but drilling holes in it reduces this to 0.06 picofarads of added capacitance.Old designs like the Tektronix P6202 (which is what I would start with if I wanted to design a general purpose probe) use an input divider to reduce the capacitance of the input device, 2.0 picofarads in this case.

author: Cerebus
author_rank: Super Contributor
timestamp: October 20, 2018, 08:07:01 am
content: Both designs are well and good in themselves, but don't forget that the title contains>1GHzanddifferentialin its title.Both are single ended, the JFETs in Bob's design run out of steam at ~200MHz and the P6202 is a 500MHz design.

author: dietert1
author_rank: Super Contributor
timestamp: October 20, 2018, 08:44:31 am
content: Thanks for posting the probe schematics. We cannot emphasize enough the advantage of using a low capacitance active probe, even if its bandwidth is "only" 500 MHz instead of 1 GHz and even if it's not differential. When you try to make a probe for time domain measurements, you want the gain curve to level off smoothly at the upper bandwidth limit. Otherwise you will see ringing, like with audio brickwall filters. That may be a reason to prefer a discrete design since in general it will have less components. For example the proposed transistor difference stage levels off smoothly.Anyway i would recommend to have a look at the "contemporary" TI proposals for a 2 GHz scope frontend with LMH5401 and LMH6401. Using that kind of building block is roughly how Mr. Rosenkränzer arrived at a working DIY type GHz differential probe.Really, in 2018 we should have DSOs with builtin probe deembedding including some handy calibration scheme.

author: nctnico
author_rank: Super Contributor
timestamp: October 20, 2018, 12:03:03 pm
content: At some point you just can't get away with a discrete solution especially on readily available PCB materials. The parasitics will kill the circuit. Meanwhile I've updated my design with the LMH3401 to see how that works. The datasheet for the LMH3401 says that TI choose to have the feedback resistors on the chip itself to avoid the parasitic capacitance of the circuit board. Unfortunately these are 200 Ohm and for a 1:20 differential attenuation this means a differential input impedance of only 2k Ohm. It is still OK-ish because on a typical differential pair terminated with 100 Ohm it will result in an amplitude error of 5% due to loading the signal. Still seeing is believing. The simulation shows some peaking at higher frequencies (several GHz) so hopefully the loss of FR4 takes care of it.

author: dietert1
author_rank: Super Contributor
timestamp: October 21, 2018, 09:17:32 am
content: I bought some ATF-35143 FETs to make buffer input stages. That works +/- 1 dB at least up to 2.9 GHz with a 51K/10K input divider made from 0805 SMD resistors. Then the parasitic capacitance of the 0805 resistor roughly compensates the FET input capacitance. Low frequency noise is less then a mV. Attached schematic describes a test setup. With smaller parts and a careful layout a 10:1 divider should compensate. I will try to drive the LMH3401EVM inputs with two of these input stages.

author: nctnico
author_rank: Super Contributor
timestamp: October 21, 2018, 10:37:40 am
content: That is interesting! Are these FETs available as a matched pair? I think that will greatly improve the CMMR and DC offset matching.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: October 21, 2018, 10:57:20 am
content: Discontinued.No matched pairs, if that word is allowed at all in the context of FETs.Maybe one could enforce equal currents with a mirror.possible replacements:<https://www.digikey.de/product-detail/de/CE3521M4-C2/CE3521M4-C2CT-ND/6165474/?itemSeq=265991607><https://www.digikey.de/product-detail/de/CE3520K3-C1/CE3520K3-C1CT-ND/6165473/?itemSeq=265991735><https://www.digikey.de/product-detail/de/cel/CE3514M4-C2/CE3514M4-C2CT-ND/6165472>Also interesting:<https://www.digikey.de/product-detail/de/analog-devices-inc/ADL5565ACPZ-R7/ADL5565ACPZ-R7TR-ND/2773701>regards,Gerhard

author: Carrington
author_rank: Super Contributor
timestamp: October 21, 2018, 11:07:01 am
content: I just wanted to say that this is getting more and more interesting, and I think that something pretty good is going to come out of here.

author: Marco
author_rank: Super Contributor
timestamp: October 21, 2018, 07:38:56 pm
content: Quote from: nctnico on October 21, 2018, 10:37:40 amHow close can you match the capacitance of the resistors though? Seems to me the solder blobs would throw it off.

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: October 21, 2018, 10:56:24 pm
content: I'm aware it's neither differential nor >1GHz, and well, it's only available in German, but the most elaborate take on a DIY active probe I stumbled over in the last years was this one:http://welecw2000a.sourceforge.net/docs/Hardware/Aktiver_Tastkopf_mit_OPA659.pdfThe probe uses an OPA659 with a trimmable capacitive divider. The main developer and author of this PDF "branadic" is also active in this forum btw.Building some of these is one of multiple projects on my "stack".As a side note: I also bought some of the probes by Alfred Rosenkraenzer (PCB version). When I bought the first one, I was well aware that the bandwidth sounded a bit optimistic. Still, my main intention was to measure LVDS signals in the 20-40MHz range and the probes worked excellently for that. I.e. where the passive 500MHz probes just showed a sine shaped signal, I got a clear rectangular signal with steep edges. I already used them for 5V signals as well though (e.g. 100ns CS pulses) where they are also far superior to 500MHz passive probes. Admittedly, the scoped used was a 600MHz Lecroy so I don't really care if the probes really have a much higher bandwidth or not. And yes, looking at the differential amplitude, the probes show a 20:1 attenuation, I guess the 10:1 value was meant for each input. In the meantime, the description in the eBay offers was also changed as far as I can tell.

author: dietert1
author_rank: Super Contributor
timestamp: October 22, 2018, 09:20:56 am
content: Today i received a THS4302 evaluation module. Compared to the OPA659 the THS4302 has more bandwidth and some gain.The EVM gain curve measured by our HP 8560A shows 7.8 dB at low frequencies (G=5 gives 14 dB - 6 dB output line attenuation). 3 dB bandwidth is roughly 1.8 GHz. Up to 2.9 GHz it levels off smoothly, so this will result in low ringing.The evaluation module has a terminated strip line as input. A probe needs a different design. THS4302 input capacitance is specified as < 1 pF, so if you put a 10:1 divider in front, you may get a 0.2 pF high impedance probe.

author: Hydron
author_rank: Super Contributor
timestamp: October 22, 2018, 09:39:30 am
content: Looks like there's also a gain-of-10 version of the THS4302 too (the THS4303) if it's of interest.Thanks for all the work being done in this thread - sounds like the final result could be pretty handy.

author: dietert1
author_rank: Super Contributor
timestamp: October 22, 2018, 06:57:17 pm
content: Yes, definitely 10x gain is better than 5x. Also very interesting: The THS4303 datasheet contains a schematic diagram of the amplifier.When we compare the large signal gain curves (Figure 3), both amplifiers show the -3 dB point at about 1.4 or 1.5 GHz. The THS3402 has a minor bandwidth advantage only with small signals.My own 1.8 GHz bandwidth estimate above was "almost large signal": -10 dBm into 50 Ohm = 200 mVpp, 5x => 1 Vpp.

author: David Hess
author_rank: Super Contributor
timestamp: October 24, 2018, 09:08:57 pm
content: Quote from: Cerebus on October 20, 2018, 08:07:01 amI linked them as examples of what is involved. Both serve as benchmarks for what is possible without resorting to hybrid construction.I think a >1GHz differential probe is unrealistic without hybrid construction but performance better than the famous and very long livedTektronix 100MHz P6046 differential probeshould be feasible. I would take a very close look at maximum performance design using the AD8130 difference amplifier as the differential to single ended conversion stage to get better than P6046 performance.If there is a better part than the AD8130 for differential probes, I have not found it yet.

author: dietert1
author_rank: Super Contributor
timestamp: October 25, 2018, 07:25:45 am
content: The amplifier testkits i wrote about are no hybrids but standard PC boards. The LMH3401 board has four layers with the amplifier side made from RF material. Its common mode is near perfect up to 3 GHz, its differential mode extends up to 7 GHz (datasheet spec). The HEMT fet buffer i tested should be good for 5 or 10 GHz as well. The wiggles in my measurements are reflections and mean nothing. Some days ago i found better patch cables and some SMA attenuators which make the measurements smooth to about +/- 0.2 dB.If you prefer AD over TI, the differential amplifier ADL5565 mentioned above seems to be very similar to the LMH3401. Or the ADL5569 with 2x 20 dB gain on a single chip and good common mode suppression up to 3 GHz.If you want to make hybrids, please search for "agilent ghz die" at ebay. Wellcome to 2018!

author: Marco
author_rank: Super Contributor
timestamp: October 25, 2018, 02:42:42 pm
content: AFAICS it's impossible to guess what the CMRR is for the ADs for single ended output. Only the LMH3401 has anything in the datasheet about it (Fig. 42, Common-mode input, common-mode output transfer function).

author: nctnico
author_rank: Super Contributor
timestamp: October 25, 2018, 04:42:56 pm
content: Quote from: David Hess on October 24, 2018, 09:08:57 pmOne way to get in the ball park of 1GHz is to use 2 closed loop buffers (from TI for example) and drive a regular opamp or differential amplifier with these to get the differential signal. It should be possible to get over 20k Ohm of differential input impedance and more if you make it a 1:10 probe but I think the capacitive divider will be tricky to get right.

author: dietert1
author_rank: Super Contributor
timestamp: October 26, 2018, 09:38:20 am
content: If we are still considering DIY, a 3 mm "antenna" of thin wire soldered vertical to one end of a 0603 resistor helps. Then you can adjust compensation by bending the loose end in one direction or the other. Also "solder blobs" were mentioned above.

author: nctnico
author_rank: Super Contributor
timestamp: October 26, 2018, 11:55:11 am
content: The question is how low do you want to get the input capacitance. On one of my prototypes I measured less than 0.5pf between the tips (and that is with some uncertainties). The actual capacitance between the tips may even be lower. This also implies that aiming for a very high differential input impedance isn't going to do much good because at several GHz the input capacitance will dominate the input impedance anyway.

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: October 26, 2018, 06:26:03 pm
content: Quote from: nctnico on October 26, 2018, 11:55:11 amWhile it's true of course that a capacitance becomes low ohmic at high frequencies (e.g. 1pF at 1GHz equals ~159Ohm), your signal typically consists of a wide range of frequencies where the (real part) resistance will matter the more, the more low frequency components you have.

author: dietert1
author_rank: Super Contributor
timestamp: October 26, 2018, 10:28:18 pm
content: This will be similar to any other scope frontend. Once you know how to DIY you can solve the DC impedance problem using AC coupling or using an adjustable DC offset source or by adapting the division ratio. Depends on the application.For myself i want a high impedance 10:1 probe, so i designed a small 4-layer board with 2x HEMT buffers plus DC servos plus a LMH3401 difference stage plus low noise +/- 2.5 V regulators. A HEMT easily oscillates above 10 GHz and the probe will have two of them next to each other, so i already know i will suffer. Probably i should first try a THS4303 single ended 10:1 probe, will be much easier.

author: Marco
author_rank: Super Contributor
timestamp: October 27, 2018, 12:13:26 am
content: The quality to which Baluns can be made is quite impressive by the way. Marki hasonewhich does 40 dB CMRR out to 6 GHz ... they of course go higher, but for those the CMRR is reduced across the board.What's inside these things? I suspect ferrite on coax.

author: Marco
author_rank: Super Contributor
timestamp: October 29, 2018, 12:58:06 pm
content: Quote from: Gerhard_dk4xp on October 21, 2018, 10:57:20 amSuch a shame too. Some of those Broadcomm pHEMT's go up to a transconductance of 2000 mS, there's nothing else like them in the discrete world.

author: nctnico
author_rank: Super Contributor
timestamp: October 29, 2018, 02:14:55 pm
content: Quote from: Marco on October 29, 2018, 12:58:06 pmOff topic: IMHO it is a major PITA that Broadcom is killing all the nice RF parts from HP/Agilent/Avago. Also a lot of opto stuff is being discontinued.

author: LapTop006
author_rank: Unknown
timestamp: October 29, 2018, 02:53:08 pm
content: Quote from: nctnico on October 29, 2018, 02:14:55 pmGiven how hard it is to be able to purchase or even just get real datasheets for their parts that might actually be the better option, at least now a competitor knows they have a good chance at picking up customers.

author: Marco
author_rank: Super Contributor
timestamp: October 29, 2018, 04:31:02 pm
content: High GHz discrete RF is such a niche that probably no one will pick them up. CEL and Skyworks will probably keep producing their parts for a while, but those just don't compare to Broadcomm's as far as transconductance is concerned. I don't see them developing ones to rival Broadcomm's ... gone with the wind now.

author: nctnico
author_rank: Super Contributor
timestamp: October 31, 2018, 09:32:13 pm
content: Today I had some time to put a new board with the LMH3401 together. The differential impedance is 2k Ohm with a 1:20 attenuation. I'm contemplating having a 1:50 version with 5k Ohm differential input impedance but that is for later. Some initial testing shows promising results:This is with the input normal and reversed so it should give an idea on how good the common mode amplification is. I'll need to do a whole lot more testing but for that I need to finish the rest of the board first. This is just the amplifier (and 0402 decoupling capacitors) powered from a bench PSU.

author: David Hess
author_rank: Super Contributor
timestamp: November 01, 2018, 12:08:25 am
content: Quote from: Marco on October 29, 2018, 04:31:02 pmNXP has discontinued their 4GHz PNP RF transistors as well and unlike some of the Avago/Broadcom parts, there are no substitutes for them or at least I have not found any.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: November 01, 2018, 12:57:59 am
content: <https://www.digikey.de/products/de?keywords=hfa3096bz-nd>but yes, it's a dying species.regards, Gerhard

author: Marco
author_rank: Super Contributor
timestamp: November 01, 2018, 01:12:50 am
content: There are so many legacy BFT92 designs, they could charge 2$ a piece and they'd still sell them.

author: nctnico
author_rank: Super Contributor
timestamp: November 15, 2018, 11:58:49 pm
content: Time for an update on my design using an LMH3401. The differential impedance is 2k Ohm and the capacitance is less than 0.5pf (I can't measure any lower). The maximum differential input voltage is +/-20V RMS (based on dissipation). The maximum common-mode voltage is +/-13V. These ranges are wide enough to measure low-side current sense resistors and the gates of MOSFETs in the low side of a switching power supply as well.First some graphs from the spectrum analyser up to 3GHz.Normal and reverse connected bandwidth to show where the common mode amplification starts to deteriorate:Common mode rejection:It looks reasonably flat up to 2.2GHz and the CMMR is -40dB at the worst point. All measurements where made using a levelled RF generator set to 0dBm and 50cm of Huber+Suhner coax (and a DC blocker) connected between the probe and the spectrum analyser.On to the practical part. On the differential probes I've seen there are moveably probe tips so points with a variety of distances can be probeb. I wanted to achieve something similar. After some thinking I came up with the idea of using thin stainless spring steel rods inserted into sockets. The end result is this:I didn't expect the offset voltage to be as bad as it is (in the prototype it is 30mV) so I had to bodge a potmeter onto it. The ground lead is to equalise the ground of the probe with the ground of the DUT. The power is supplied using a micro-USB connector. A switching capacitor chip with linear post-regulators provides a positive and negative suply voltage.Let's measure some signals. First a 1.25Gbit ethernet stream:Secondly a 250MHz square wave:The stainless steel spring rods seem to work fine. Probing a 0402 component is doable as well:All in all I'm quite impressed. It took 3 board spins to come up with something decent. The results from the initial version with the ADA4927 made me sceptical about using a fully differential amplifier but I'm happy dietert1 held a strong argument in favour of using the LM3401.I still need to add the potmeter to the final PCB design. Perhaps I'll add a slit between the input divider resistors to reduce the input capacitance as far as possible. If there is some interest I could have a small batch made. For now I have used heat-shrink tubing as a casing but I have created some indents in the board so it can be held in place in a 3D printed housing.

author: Hydron
author_rank: Super Contributor
timestamp: November 16, 2018, 11:41:36 am
content: Looks great, definitely interested in more details/design info.RE the offset issue - I assume that you're terminating the inverting output in 100R, could you potentially use this output to sense the offset when inputs are shorted and use a micro to calibrate it out using a digital pot/dac when a button is pressed? If the offset doesn't drift then this is probably overkill and a pot is the best.

author: Carrington
author_rank: Super Contributor
timestamp: November 16, 2018, 12:21:14 pm
content: Cool!Quote from: nctnico on November 15, 2018, 11:58:49 pmI'm interested, and I think I'm not going to be the only one.

author: nctnico
author_rank: Super Contributor
timestamp: November 16, 2018, 03:11:17 pm
content: Quote from: Hydron on November 16, 2018, 11:41:36 amThat could be an option. The datasheet from TI actually suggests to use an extra opamp to correct the offset error of the LMH3401. I'm wary to do that because it complicates the PCB design and some extra load needs to be connected to the output of the LMH3401. I suppose the offset doesn't drift that much but it is something I could test to see if it depends a lot on the operating temperature.

author: JohnG
author_rank: Frequent Contributor
timestamp: November 19, 2018, 02:34:27 pm
content: Quote from: Carrington on November 16, 2018, 12:21:14 pmI would also be interested.Thanks,John

author: dpenev
author_rank: Regular Contributor
timestamp: November 19, 2018, 04:18:34 pm
content: I am interested too.Sent from my MI NOTE Pro using Tapatalk

author: rx8pilot
author_rank: Super Contributor
timestamp: November 19, 2018, 05:16:42 pm
content: This is a very interesting and practical project. Very impressive results.Digging through the rest of the thread looking to see how you measured impedance and capacitance - delicate measurements.Short and misplld from my mobile......

author: 2N3055
author_rank: Super Contributor
timestamp: November 19, 2018, 09:57:48 pm
content: I would be interested too.Nice work.

author: mk_
author_rank: Regular Contributor
timestamp: November 25, 2018, 09:05:41 pm
content: still working on my 400Mhz-diffprobe... I`m interested too

author: nctnico
author_rank: Super Contributor
timestamp: November 25, 2018, 09:24:36 pm
content: I still have to update the PCB design to include the offset pot. I hope I can squeeze this in this week and then get a quotation from an assembler to have a couple of these probes produced professionally. It is not going to be really cheap (likely >100 euro) but still extremely competitive compared to 'the real deal'. I want to sell the probe as a kit complete with SMA to BNC cable, USB power cable and stainless steel wire to make probe tips.

author: mk_
author_rank: Regular Contributor
timestamp: November 26, 2018, 10:17:43 am
content: Quote from: nctnico on November 25, 2018, 09:24:36 pmAt least here there are enough SMA>BNC-Cables aviable.I use for my 400MHz-Probe something like this:https://www.schukat.com/schukat/pdf.nsf/index/D796B48DFC770906C1257933002CFFBC/$file/PTR_Serie_1012_E.pdfAnyway, 100€ are fine and if you need an assembler mail me PM, I`m working "inside" an assember....

author: nctnico
author_rank: Super Contributor
timestamp: December 05, 2018, 09:33:22 pm
content: Quote from: nctnico on November 25, 2018, 09:24:36 pmI just ordered a first batch of 10 pieces. The price for the complete kit is going to be 140 Euro including VAT. I'll create a user manual / specification sheet as well.

author: bitbanger
author_rank: Regular Contributor
timestamp: December 28, 2018, 05:50:35 am
content: Expressing interest for one of these.

author: BFX
author_rank: Frequent Contributor
timestamp: December 28, 2018, 09:28:02 pm
content: This is really greatBut isn't possible to do that in little bit better formfactor?Slim to be able to put inside some nice aluminum cover?

author: nctnico
author_rank: Super Contributor
timestamp: December 28, 2018, 10:22:42 pm
content: Quote from: BFX on December 28, 2018, 09:28:02 pmI tried to make the board as narrow as possible however the width is dictated by the width of the micro-USB and SMA connectors. IMHO the design isn't lumpy compared to differential probes you can buy from R&S, Keysight or Lecroy but yes, it definitely is more lumpy than a passive probe. BTW I'm having a plastic case designed & manufactured because the heat-shrink sleeving just didn't sit right with me.

author: mk_
author_rank: Regular Contributor
timestamp: December 28, 2018, 10:39:06 pm
content: Quote from: BFX on December 28, 2018, 09:28:02 pmI buildt a 400MHz-Diffprobe some time ago and decided against a nice aluminiumcover because if it sits somewere on the circuit the chance for a destructive short is high.

author: BFX
author_rank: Frequent Contributor
timestamp: December 30, 2018, 04:13:53 pm
content: Quote from: mk_ on December 28, 2018, 10:39:06 pmIt's all depends on design. If it's done properly there is no chance for some shorts. But yes it's necessary to have access to some CNC machine.Here is for example my noise source.

author: joeqsmith
author_rank: Super Contributor
timestamp: December 30, 2018, 04:17:39 pm
content: Quote from: BFX on December 30, 2018, 04:13:53 pmThat's some nice work.

author: mk_
author_rank: Regular Contributor
timestamp: December 30, 2018, 04:49:54 pm
content: QuoteI didn`t mean short in the diffprobe itself but that the diffprobe, housed in an aluminiumcover and lying around can easily create shorts in the circuit it is probing below.

author: David Hess
author_rank: Super Contributor
timestamp: December 31, 2018, 06:19:46 pm
content: A conductive case is actually desired to prevent degradation of the AC common mode rejection from variation in the probe's environment. Plastic enclosures should have a conductive film on the inside.

author: BravoV
author_rank: Super Contributor
timestamp: December 31, 2018, 06:40:16 pm
content: Quote from: David Hess on December 31, 2018, 06:19:46 pmWill something like the old conductive carbon spray for coating the CRT TV case internal or the CRT tube work ?Example :

author: luma
author_rank: Regular Contributor
timestamp: December 31, 2018, 09:06:50 pm
content: I’m definitely interested in placing an order when you think you’re ready for testers. I have a fair bit of experience designing 3D printable enclosures for OSHW projects and would get to work on that immediately upon receipt of an assembled board.

author: bitbanger
author_rank: Regular Contributor
timestamp: December 31, 2018, 10:56:52 pm
content: ^ with him/her. I'd be perfectly happy with as-is stated performance and 3D printing an enclosure.

author: David Hess
author_rank: Super Contributor
timestamp: January 01, 2019, 02:57:36 am
content: Quote from: BravoV on December 31, 2018, 06:40:16 pmIf it will work for EMI shielding then it will probably work. The cases on the Apple 2 computer used something like that on the inside.

author: _Wim_
author_rank: Super Contributor
timestamp: January 01, 2019, 08:45:27 am
content: Quote from: David Hess on January 01, 2019, 02:57:36 amI think for EMI it is better to use this one on a plastic enclosure (kontakt chemie EMV 35 or EMI 35):https://uk.farnell.com/kontakt-chemie/emi-35-200ml/coating-conductive-emi-35-200ml/dp/2142398

author: the Willows
author_rank: Newbie
timestamp: January 04, 2019, 10:52:19 am
content: Very interesting, was on the way to order from the German guy, but this seems to be a much better design.Please write me up for 2 pcs.We own (me and Daniel from Xtreemtec) since last december a LeCroy WavePro 7300, but no probes yes. Hard to find item or huge prices.So this is a good alternative.

author: paul67
author_rank: Newbie
timestamp: February 19, 2019, 07:19:29 pm
content: Are there any probe kits remaining? I'm interested in one. Thanks.

author: paul67
author_rank: Newbie
timestamp: February 19, 2019, 07:23:29 pm
content: Quote from: nctnico on December 05, 2018, 09:33:22 pmMeant to 'quote' that. I'm interested in one of those probe kits...

author: nctnico
author_rank: Super Contributor
timestamp: February 19, 2019, 09:45:18 pm
content: I'm almost ready to put the first batch together. Give me a few more days to make the official announcement.

author: technogeeky
author_rank: Frequent Contributor
timestamp: February 20, 2019, 12:14:18 am
content: Quote from: nctnico on February 19, 2019, 09:45:18 pmI'm also interested.

author: the Willows
author_rank: Newbie
timestamp: February 20, 2019, 08:32:59 am
content: Tonnie Wittenaar wrote on the 4th of January:QuoteSo if Designer can give us an update how things are going, or project has died, please let us now.

author: nctnico
author_rank: Super Contributor
timestamp: February 20, 2019, 09:42:11 am
content: Nearly ready! Everything except the SMA to BNC cables has been delivered. The boards have been tested and adjusted. I'm going to assemble the first batch in the next couple of days. The datasheet is also nearly ready. I want to do a few more measurements regarding spurs and noise.

author: the Willows
author_rank: Newbie
timestamp: February 20, 2019, 09:44:24 am
content: Your are the best.Thanks!

author: nctnico
author_rank: Super Contributor
timestamp: March 02, 2019, 07:45:58 pm
content: The first batch of 9 probes is ready to go. The product name has become 'DIP1400' (DIfferentialProbe1400MHz). The bandwidth is +/-3dB up to 1.4GHz.Datasheet:http://www.nctdev.nl/NCT%20Instruments%20DIP1400%20V1.0.pdfThe final price is 125 euro ex. VAT ex. Paypal / wire transfer fees. Shipping in Europe is 16 euro, worldwide 27 euro including insurance.

author: paul67
author_rank: Newbie
timestamp: March 03, 2019, 09:51:27 pm
content: Looks great nctnico :-)PM Sent with purchase request...

author: rsjsouza
author_rank: Super Contributor
timestamp: March 04, 2019, 01:32:57 am
content: Great work, Nico! It will be on my wish list as my finances for hobbies are quite depleted at the time.I read the datasheet and spotted a couple of typos:Page 3: pads cannot be torn off very easilyPage 6: A more coarse measurement of the bandwidthOne question:Page 4: do you have a characterization for the worst case scenario on the table?Again, great work!

author: paul67
author_rank: Newbie
timestamp: March 14, 2019, 10:29:26 pm
content: Thanks for the delivery of the probe, Nico. It works really nicely! I'm very happy with it. I wondered, what do you do with the LMH3401 inverting output... connect directly to 0V, or...?I looked at the datasheet but couldn't see guidance for using in single-ended output mode.

author: nctnico
author_rank: Super Contributor
timestamp: March 14, 2019, 10:35:13 pm
content: Quote from: paul67 on March 14, 2019, 10:29:26 pmThe inverted output is terminated to ground with a (in total) 50 Ohm resistor so both output see (about) the same load.

author: Evodad
author_rank: Contributor
timestamp: November 01, 2019, 06:04:13 pm
content: Hi ntcnico,Any chance you have any probes left ?This was an interesting thread. Why did it not survive longer ? Any spin-offs ?// Per

author: nctnico
author_rank: Super Contributor
timestamp: November 01, 2019, 06:07:36 pm
content: Quote from: Evodad on November 01, 2019, 06:04:13 pmActually I have 20 new probes fresh from the assembler.

author: Evodad
author_rank: Contributor
timestamp: November 03, 2019, 07:04:20 pm
content: That was good newsAny modifications since the last batch ?May I buy 2 of you ?)) Per

author: teomondo
author_rank: Newbie
timestamp: September 03, 2020, 04:20:27 pm
content: Dear ntcnicoIs DIP1400 1.4GHz differential probe currently avaliable?I have an Operating manual dated 2019 any new version avaliable?What about the price of DIP1400 1.4GHz differential probe and how I can buy it.Thanks for your answerRoberto

author: KE5FX
author_rank: Super Contributor
timestamp: September 03, 2020, 11:01:01 pm
content: Unsolicited testimonial: having used it for a while now, Nico's probe works really well. It's comparable to the Tek P6248 at a small fraction of the size/cost. If you do anything with differential signals -- or just want an active probe in general -- it's definitely worthwhile.(Edit: by "size" I'm not referring to the probe head, but to the large box at the other end of the P6248. When I use the P6248 with my HPAK scope, I also have to find room for an even-larger Tek 1103 power supply. The DIP1400's probe head is somewhat bulkier than the P6248's, but not excessively so, and it can be powered by the scope's USB jack.)

author: tautech
author_rank: Super Contributor
timestamp: September 03, 2020, 11:37:54 pm
content: Quote from: KE5FX on September 03, 2020, 11:01:01 pmYeah but where is he ?Not logged in for nearly a month so hope this C thing hasn't caught up with him.Stay safe and be careful out there.

author: Cerebus
author_rank: Super Contributor
timestamp: September 04, 2020, 12:18:03 am
content: Quote from: tautech on September 03, 2020, 11:37:54 pmLast seen being a bit argumentative with Simon, so his temporary absence might have less ominous overtones.Or he might just have got sick of the sight and sound of some of us and wanted some peace and quiet, can't really blame him if he has.

author: rsjsouza
author_rank: Super Contributor
timestamp: September 04, 2020, 01:50:00 am
content: Quote from: Cerebus on September 04, 2020, 12:18:03 amI suspect I might have contributed a bit also to his (hopefully short) self appointed vacation... We were sparring a bit about 'scopes and perhaps it was time for a break. I hope he eventually comes back, as he has a lot to contribute and in general he seems to be a nice fellow.

author: Kean
author_rank: Unknown
timestamp: September 04, 2020, 06:24:43 am
content: I was thinking that once he was happy with the design he would list it on his Tindie store. Still only has his CP2100 dual channel current probe listed.And now I have a Micsig CP2100B it might be confusing to have an NCTi CP2100...

author: Noy
author_rank: Frequent Contributor
timestamp: September 04, 2020, 08:17:40 am
content: Nice thing. I'm also interested to get one.

author: 2N3055
author_rank: Super Contributor
timestamp: September 04, 2020, 11:03:02 am
content: I went and checked, he hasn't been on EEVBlog for almost a month. I hope he's OK, and only just busy...

author: Noy
author_rank: Frequent Contributor
timestamp: September 20, 2020, 11:32:21 am
content: Any news here?

author: sciguy14
author_rank: Newbie
timestamp: September 30, 2020, 08:41:58 pm
content: I would also like to express my interest in purchasing two of these!

author: nctnico
author_rank: Super Contributor
timestamp: September 30, 2020, 09:05:40 pm
content: Quote from: 2N3055 on September 04, 2020, 11:03:02 amI'm OK (and busy). I have been cutting back on posting on EEVblog though.About the probes: I have these in stock ready to ship out. PM me when interested in purchasing.

author: Noy
author_rank: Frequent Contributor
timestamp: October 01, 2020, 08:30:31 am
content: I'm interested in one but currently I'm unsure if it is the right probe for my needs.I only have 350MHz scopes (Rigol MSO5000 (external termination) / Hameg HMO3524(internal 50 Ohm temination)).I need a probe to measure / qualify µSD signals in SDR104 mode somtetimes eMMC in HS400 mode (200MHz clk)...Sometimes "slow" LVDS / MIPI without eye measurments (cant produce eyes on my scopes)..Currently i only have passive 350MHz probes (they have too high loading for the 200MHz clk) / Micsig 100MHz high voltage differential probe (divider too high for the small 1.8V signals)..I think two single ended active probes with >=350MHz BW with High resistance and low capacitance input for low loading would be the best fit for my needs. I can use 2 probes + math to measure LVDS/MIPI.And for the "fast" SDIO signals i use both probes to measure 1 Data + CLk at the time.So your differential probe has 2k Ohm input impedance? I think that is to low for me? And the 1.4GHz is over the top for my scopes..?So has anybody another good solution for my needs? A DIY active probe with qualified behaviour (i can build one myself but i don't have equipment to measure the bandwith / impedance) and not too expensive?Or is the DIP1400 useable for my needs?

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: October 01, 2020, 02:03:02 pm
content: I'm using the differential probe from Elektor 4/2017 by Alfred Rosenkränzer discussed earlier in this thread. It has slightly different specs (~5kOhm differential resistance), but should be somewhat comparable.At work, I'm using these probes mainly to measure a 40MHz LVDS signal (MSC aka "microsecond channel") on a 600MHz LeCroy. Compared to the default 500MHz passive LeCroy probes, the difference is like night and day. I.e. with the passive probes, the signal looks like a distorted sine and with the active probes, it's a nearly perfectly rectangular signal.Actually I also use the probe for single ended measurements (i.e. "-" connected to GND) when very steep edges are involved and when the capacitance of the passive probe would somewhat invalidate the measurement.IMHO, an active probe with low capacitance is always worth it even on a 200MHz scope when looking at sharp edges. Of course if you don't need a differential probe, there might becheaperoptions.

author: tggzzz
author_rank: Super Contributor
timestamp: October 01, 2020, 04:04:59 pm
content: Quote from: 0xdeadbeef on October 01, 2020, 02:03:02 pmI expect you mean a 40Mb/s LVDS signal. The maximum frequency will be much higher, dependentsolelyon the transition time.It is unsurprising you see a distorted waveform, given that a *10 "10Mohm" impedance probe has a (capacitive) input impedance of <100ohmsat the frequencies you are likely to see on an LVDS signal.A pair of resistive divider Z0 probes would be a better bet, since they have a much lower capacitance and a well defined input resistance of 500ohms (*10) or 1000ohms (*20). They are easy to make at home.

author: nctnico
author_rank: Super Contributor
timestamp: October 01, 2020, 04:52:27 pm
content: Quote from: Noy on October 01, 2020, 08:30:31 amI think the DIP1400 is useful for your purposes. One of the advantages of a differential probe is that you get the signal between two point (close together) and not via a ground point connected to a long lead. For example: I have used the DIP1400 to troubleshoot a power supply problem on a SOC design because I wanted to make sure I wasn't looking at spikes induced into the ground connection. 2k Ohm should be OK for almost every digital signal; one of the design goals was to make the input impedance high enough so the amplitude error on signals using a 50 Ohm (single ended) connection is below 10%.

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: October 01, 2020, 07:31:50 pm
content: Quote from: tggzzz on October 01, 2020, 04:04:59 pmWell, one of the LVDFS channels is a 40MHz clock. So since when is frequency only a valid unit for sine waves? Of course the sharp edges have much higher frequency components. No need to mention that in this board, do we?Quote from: tggzzz on October 01, 2020, 04:04:59 pmFor a 11pF probe, a 400MHz component will "see" a resistance of ~36Ohm, while the DC components will "see" 10MOhm. How is it surprising that this distorts the waveform?Anyway, have you ever actually tried to measure a 40MHz square wave with a passive probe?Quote from: tggzzz on October 01, 2020, 04:04:59 pmA typical active probe has like 0.5pF capacitance, so anything you attempt to attach it somewhere will actually add a capacitance in the same magnitude or higher. I don't see how a purely resistive probe would help there. Plus, its low resistance makes it problematic for digital outputs which might as well create a burst clock only now and then.

author: nctnico
author_rank: Super Contributor
timestamp: October 01, 2020, 08:58:09 pm
content: Quote from: 0xdeadbeef on October 01, 2020, 07:31:50 pmWell, you should try the low-Z passive probes. These have very low capacitances and high bandwidth. I have a bunch of Tektronix P6156 probes (http://w140.com/tekwiki/wiki/P6156) and these work very well (3.5GHz bandwidth with less than 1pf of capacitive loading!) but you'll need to use the 1:20 attenuator (or higher) to get an input resistance of at least 1k Ohm otherwise digital signals like LVDS will be loaded too much.

author: tggzzz
author_rank: Super Contributor
timestamp: October 01, 2020, 11:56:29 pm
content: Quote from: 0xdeadbeef on October 01, 2020, 07:31:50 pmYes, it isfrequently(ho ho) necessary to mention it on this board - including in this case.I mentioned the abbreviated version. For a longer version seehttps://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/QuoteEr, I noted distortion isunsurprising. It appears that you had not considered the way in which it will affect the waveform.I havefrequently(ho ho) measured such waveforms with passive probes over thedecades.QuoteMy resistive divider probes are "<0.7pF", so yes it does help considerably compared with the typical *10 passive probe you used.This is nothing new; I have an "operating note" dated1972for my hp10020a resistive divider probes. Here's the context you chose to snip...Quote from: 0xdeadbeef on October 01, 2020, 02:03:02 pm

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: October 02, 2020, 12:27:55 am
content: Yeah, I really don't know if this brought up anything worthwhile. I can't day it did for me. Hope you feel better at least.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 02, 2020, 06:25:06 am
content: Quote from: tggzzz on October 01, 2020, 11:56:29 pmwhile Z0 probe is the cheapest way, it doesnt help with high impedance nodes such as 1Kohm lines (regardless of frequencies low or high)... most logics will simply follow the distorted clock fine, but some other circuit will be screwed, maybe like comparator or serial comm/timing that needs strict synching... ymmv.since at it, here is a 50ohm 40MHz clock line loading effect i probed with my diy 1/10X passive probe vs Rigol bundled probe (DS1054Z grade BW) last night just out of curiosity. 1st and 2nd picture is for reference only (unloaded signal on 1GSa/s 1 channel activated only vs 500MSa/s both channel activated) granted most of high frequency content of my 6GHz function (clock) Gen already absorbed by DSO capacitance and phase distortion (delay or synch) is not so obvious from this snapshots. anyway, for 1Kohm nodes such as usually found in mcu output, Z0 probe will not be suitable however slow the clock is, this is where active probe is more of a general purpose probe ie we dont need to keep track of using different probes...

author: tggzzz
author_rank: Super Contributor
timestamp: October 02, 2020, 08:03:50 am
content: Quote from: 0xdeadbeef on October 02, 2020, 12:27:55 amThere's an English proverb: "you can lead a horse to water, but you can't make it drink".Perhaps it will avoid other readers being mislead.

author: Noy
author_rank: Frequent Contributor
timestamp: October 02, 2020, 01:03:44 pm
content: I think i will respin the opa probe.Even 4 active probes will be cheaper than one of the diffs here. I will try.Plan is to use the wson package of the opa (maybe a bit better choice than sot23) and to add a sma connector + micro usb for power supply.Is a LTC1044 useable for the OPA? With lots off caps + filtering? To get from 5V (maybe a powerbank / oszi output?) to +-5V for the opa..

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: October 02, 2020, 01:57:26 pm
content: I used the LT1054 (which is an improved/beefier version of the LTC1044/ICL7660) to to create +/-5V from ~7V and the MC79M05/MC78M05 for stable +/-5V. There is quite a bit of supply voltage filtering done on the probe itself, so this should be OK. However, creating -5V from +5V without any step up converted seems a bit optimistic. The negative voltage created with such an inverting charge pump will not have the exact same amplitude and the voltage drop increases with the load current.For the record: I converted the eagle schematics and PCB design to DipTrace some years ago if this would help.

author: JohnG
author_rank: Frequent Contributor
timestamp: October 02, 2020, 02:08:09 pm
content: Quote from: Noy on October 02, 2020, 01:03:44 pmThis OPA probe looks nice, and well documented, so thanks for pointing it out. The documentation looks especially good, but my German is preschool level, so I will need to run it through a translator.However, keep in mind that two single-ended probes is not the same as a differential probe. Aside from the obvious, like using more scope channels (I fall in the camp where 4 is almost enough), it is very difficult to get good common-mode rejection at high frequency. Scope channels rarely have good accuracy, and if the cables and probes are not the same at all frequencies, this eats into input CM rejection. For lots of small-signal stuff, this might not matter, but if you are troubleshooting in an electrically noisy environment, it can make a big difference.Just my $0.02,John

author: Noy
author_rank: Frequent Contributor
timestamp: October 02, 2020, 03:02:26 pm
content: I have a PCB from this project:https://xellers.wordpress.com/electronics/1ghz-active-differential-probe/Still in my "build in future" box. But currently i do not find the little bag with the already purchased parts.. Maybe i will order them again with the other parts for the Opa Probe... Then i have both.Its not a 1.4G but i think enough for my scope.Maybe i will rework this design (4 layer, jlcpcb, 0402 parts, micro USB) together with the opa design.Found same nice parts: LM27762 (+ pre boost converter?? Or only round about +-4.5V for the opa) / TPS65133

author: teomondo
author_rank: Newbie
timestamp: October 30, 2020, 06:02:14 am
content: Dear ntcnicoIs DIP1400 1.4GHz differential probe currently avaliable?I have an Operating manual dated 2019 any new version avaliable?What about the price of DIP1400 1.4GHz differential probe and how I can buy it.Thanks for your answerRoberto

author: nctnico
author_rank: Super Contributor
timestamp: October 30, 2020, 10:36:27 am
content: Quote from: teomondo on October 30, 2020, 06:02:14 amThe probes are a standard product so usually in stock; the manual hasn't changed. The price is 125 euro ex. VAT and ex. shipping. Please PM me for more details.

author: Noy
author_rank: Frequent Contributor
timestamp: November 14, 2020, 10:58:23 am
content: Your datasheet states 2k ohm impedance at DC between the inputs.Do you have a chart where the input impedance is shown over the full bandwith?

author: nctnico
author_rank: Super Contributor
timestamp: November 15, 2020, 04:33:53 pm
content: Quote from: Noy on November 14, 2020, 10:58:23 amNo. But 2k parallel with 1pf is likely a good worst case assumption.

author: JohnG
author_rank: Frequent Contributor
timestamp: April 13, 2021, 12:38:22 am
content: Nico's probe does what it says. Even nicer, I can power it right from one of my scope's USB ports. I am a happy customer! Already solved a problem for me.Nico, if you design one with higher input impedance, I would likely get one of those, too... Just sayin'Cheers,John

author: mawyatt
author_rank: Super Contributor
timestamp: April 13, 2021, 01:57:14 pm
content: Nico,Are these differential probes still available?Best,

author: nctnico
author_rank: Super Contributor
timestamp: April 13, 2021, 05:17:10 pm
content: Quote from: mawyatt on April 13, 2021, 01:57:14 pmYes but I'm currently out of stock; a new batch is being produced.

author: beardedhustler
author_rank: Newbie
timestamp: June 04, 2021, 11:52:09 am
content: Sorry for hijacking the thread, but since it's a bit related and I eventually will look into the topic (differential design) of this thread as well I hope it's ok.We have a Signatone probing station with (passive) "micropositioners" (probes) that we use to probe on bonding pads on various chips and read data with some reader attached. Coaxial cables from probes to reader are ~1m, so signal integrity isn't always great when the frequency increases, thus the need to add an active stage/probe. For now we've mostly dealt with various SPI flash chips, eMMC chips in 1-bit bus width mode, up to ~36MHz clk speed (I got enough probes that I can go for the 4-bit bus width mode which supports clk speeds up to 200MHz if I want, but it's not always beneficial to spend time and energy trying to place six probes within the small area where the bonding pads are located and at the same time not break any bonding wire between the pads and the controller, etc, so usually I stick to using a 1-bit bus width..).Due to the possibility of using the HS200 mode for eMMCs I think a 1GHz BW is an ok goal to aim for here, and since eMMCs use single ended signals I don't have the need for a differential probe (for now - that might change later on if we decide to try probing UFS chips, but that's another story - we'll cross that bridge when we get to it...). Another important thing to mention is that the goal here is to make the pcb that we can custom fit on the micropositioners in some way to utilize what we already have and thus buying a general purpose probe isn't an option for this now.I've started on a few single-stage designs trying out various op-amps from TI (LMH3401, LHM6702, OPA855/OPA858), and getting a BW > 1GHz seems quite straight forward with the op-amps mentioned, but I'm a bit unsure on the pitfalls when it comes to designing a active "probe". I guess trying to get as high input impedance / low input capacitance should be a goal as well. Anything else to consider other than trying to impedance match output from op-amp to 50Ohm (coax cable) to maximize power transfer?It's been a few years since I did design and back then it was mainly high power RF PAs with different things to consider during the design process. I've only used TI's spice simulator for now, and although it's a nice tool I guess it has its limitations. I tried importing some of the spice models they offer on their web page into Keysight ADS, and although I get almost identical results for similar simulations I'm not sure if e.g. the models are valid for other simulations, e.g. S-parameter simulations, which for some devices result in S11>0dB for a wide frequency range (and in my experience from the RF PA design days that's a big no-no).Suggestions to components, literature, etc, is greatly appreciated!

author: tggzzz
author_rank: Super Contributor
timestamp: June 04, 2021, 03:13:43 pm
content: Quote from: beardedhustler on June 04, 2021, 11:52:09 amYou would probably get better responses if you started a new thread with your questions. Plus, if the responses are useful, other people are more likely to find them and benefit from them.

author: beardedhustler
author_rank: Newbie
timestamp: June 07, 2021, 06:19:41 am
content: Quote from: tggzzz on June 04, 2021, 03:13:43 pmYou're probably right; I'll make a new thread about it.Edit: new thread up athttps://www.eevblog.com/forum/projects/input-to-single-ended-probe-designs/Any suggestions are greatly appreciated!

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: December 24, 2023, 08:01:37 pm
content: Quote from: nctnico on April 13, 2021, 05:17:10 pmDo you have any intention of making some high voltage (500v+) differential probes?

author: nctnico
author_rank: Super Contributor
timestamp: December 25, 2023, 03:59:33 pm
content: Quote from: LooseJunkHater on December 24, 2023, 08:01:37 pmNot at this moment. There are enough high voltage diff. probes on the market for reasonable prices. And such a product should also have a CAT rating which adds safety testing as an extra development cost.