Hi Z probe for 50 ohm spectrum  analyzer - Page 1
Forum section: electronics / testgear

author: wd5jfr
author_rank: Regular Contributor
timestamp: September 05, 2020, 05:22:45 pm
content: Has anyone found a good reasonably priced Asian probe.Here's how to build one but I'd rather find a solution versus a DIY project.https://www.edn.com/high-impedance-fet-probe-extends-rf-spectrum-analyzers-usable-range/ThanksHank

author: joeqsmith
author_rank: Super Contributor
timestamp: September 05, 2020, 06:50:39 pm
content: I made something like this. There were a few places selling them at the time for $500 or so USD. A few links that you may find of interest:https://www.ebay.com/itm/High-Impedance-Buffer-Amplifier-1MOhm-FETAMP1-Similar-to-Tektronix-TCA-1MEG-/260750545356https://www.eevblog.com/forum/projects/1meg-to-50ohm-trans-impedance-amplifier-for-general-instrumentation-use/25/https://www.eevblog.com/forum/projects/high-z-wideband-buffer-amp-for-50-ohm-cro-inputs/

author: Kean
author_rank: Unknown
timestamp: September 05, 2020, 07:35:05 pm
content: Maybe this is suitable for your application. It is somewhere between DIY and a packaged solution."RF Active Probe 0.1-1500 MHz-1.5 GHz analyzer oscilloscope" from ebay seller 60dbmco in Ukrainehttps://www.ebay.com/itm/332393676082

author: joeqsmith
author_rank: Super Contributor
timestamp: September 05, 2020, 07:45:05 pm
content: Quote from: Kean on September 05, 2020, 07:35:05 pmNice find. Have you tried one to see how well it performs?

author: Kean
author_rank: Unknown
timestamp: September 05, 2020, 07:48:20 pm
content: Quote from: joeqsmith on September 05, 2020, 07:45:05 pmNope. I've only had it sitting on the bench for 12 months... so maybe soon...It is cheap enough to get one and try it out. The delivery time is the biggest issue right now.

author: Kean
author_rank: Unknown
timestamp: September 05, 2020, 07:55:37 pm
content: Apparently not that great - but still useful if you stay aware of its limitationshttps://www.eevblog.com/forum/rf-microwave/response-of-cheap-rf-active-probe-from-ebay/

author: Kean
author_rank: Unknown
timestamp: September 05, 2020, 08:49:26 pm
content: So I should be sleeping, but did a little more searching around this forum. The topic has come up several times before, with Joe frequently appearing.A couple of options from Mathews Engineeringhttps://www.mathews-engineering.com/store/p20/Active_FET_Probe%2C_1000_MHz.htmlhttps://www.mathews-engineering.com/store/p1/High_Impedance_Buffer_Amplifier.htmlAnother DIY implementation of the earlier active probehttps://www.instructables.com/id/DIY-1GHz-Active-Probe-for-Under-20/If you want something that will work reasonably well out of the box and well specified that is "affordable", check Auburn Tech P-20A or P-20B. Note that these are passive not active, so they may not be considered Hi-Z depending on your application.http://www.auburntec.com/This is another DIY passive RF probe designhttps://emcesd.com/1ghzprob.htmOr go crazy herehttps://ggb.com/

author: tautech
author_rank: Super Contributor
timestamp: September 05, 2020, 08:50:31 pm
content: Quote from: wd5jfr on September 05, 2020, 05:22:45 pmIt might help if we knew what SA you were planning to use it with.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 05, 2020, 08:53:14 pm
content: 12 months isn't too long to procrastinate. I keep meaning to run that trash picked Fluke 77 but the lab is in need of some major cleaning. I've been saying maybe next weekend for a few months.OK, I checked the link and you're right, it's not flat. Some of that may be the probing and at $12ish, it may be worth a look.I was looking to get something DC coupled with close to 100MHz. Once I get beyond this, I start using resistive probes. Shown was my final attempt which with a 1dB of gain, still isn't all that flat.

author: Kean
author_rank: Unknown
timestamp: September 05, 2020, 08:54:59 pm
content: Quote from: tautech on September 05, 2020, 08:50:31 pmI wouldn't think that would be as important as knowing more about the frequency range and type of circuits to be probed.

author: wd5jfr
author_rank: Regular Contributor
timestamp: September 05, 2020, 08:57:00 pm
content: I have a Siglent SSA 3021X hacked and and HP 8935 both 50 ohms.Filter sweeps at HF (IF filters) and maybe later at 6 meters

author: Kean
author_rank: Unknown
timestamp: September 05, 2020, 09:10:13 pm
content: What is your definition of "reasonably priced"?I've not come really across anything between the ebay $10 device and the $200+ ones I listed above.

author: wd5jfr
author_rank: Regular Contributor
timestamp: September 05, 2020, 09:19:01 pm
content: Thanks for the Ebay link. I got an Ukrainian RLB about a year ago and started to compare it to my Eagle RLB but got distracted. I will wait a couple of days and if there's no serious negative comment on the Ukrainian one I'll get it. The price is right, $19.00 as is the DIY part. By the time it gets here I might get distracted again (easy at 79) and it can join the it's country cousin on the shelf. Looks like I can run it with a couple of lithiums in series.Hank

author: tautech
author_rank: Super Contributor
timestamp: September 05, 2020, 09:21:44 pm
content: Quote from: wd5jfr on September 05, 2020, 08:57:00 pmOK, the SSA has a 50V max input with an inbuilt DC block.For additional sensitivity you also have a preamp you can engage and if getting near the limits of what it can handle audible and visual warnings are given so to not damage the input circuitry.Still, be careful with it.

author: Kean
author_rank: Unknown
timestamp: September 05, 2020, 09:32:30 pm
content: Quote from: wd5jfr on September 05, 2020, 09:19:01 pmRead that other topic I linked in reply #5 which shows some measurements with it.The active probe is AC coupled at input and output, but as tautech points out you always want to be careful with what you feed into your SA.QuoteEasy at any age. I'm in my early 50's now, but I remember having the same issue since my 20's.

author: wd5jfr
author_rank: Regular Contributor
timestamp: September 05, 2020, 09:35:29 pm
content: I'll use the builtin tracking generator.I have a quite a few lithium button cells but that would be a awkward fit for a probe case. I wonder if there cylindrical lithiums about the diameter of a pencil. Perhaps some knows.

author: wd5jfr
author_rank: Regular Contributor
timestamp: September 05, 2020, 09:45:03 pm
content: I just looked at Insatman's post linked by Kean and I'm ready to buy but maybe something more appropriate will show up in the next couple of days. Or who knows I may lose the urge!Hank

author: Kean
author_rank: Unknown
timestamp: September 05, 2020, 09:51:03 pm
content: Quote from: wd5jfr on September 05, 2020, 09:35:29 pmIf you mean 3.7V lithium ion rechargeable, then the AAA size aka 10440 is reasonably common.If you want a 3V lithium primary, then there are very small ones (3-5mm diameter) used for fishing lures like CR425 or CR435.Quote from: wd5jfr on September 05, 2020, 09:45:03 pmI think for your use case it should be quite suitable. In that topic they were testing it to compare against the sellers claimed specifications and it certainly doesn't meet those. Nor would I ever expect it to based on the PCB layout!

author: wd5jfr
author_rank: Regular Contributor
timestamp: September 05, 2020, 10:14:26 pm
content: Man am I ever behind times, never heard of lit lures or floats. I grew up on the Red Deer river and spent much of my time fishing and hunting till girls became the game. Wow it would have been interesting to see what kind of luck a lighted lure would bring.But getting back to reality, the task at hand, I wonder if that probe might work with one CR435 with the VR bypassed. I guess I'll find out if I buy one.

author: tautech
author_rank: Super Contributor
timestamp: September 05, 2020, 10:18:50 pm
content: Quote from: wd5jfr on September 05, 2020, 10:14:26 pmYou never used lumo lures/flys ?

author: wd5jfr
author_rank: Regular Contributor
timestamp: September 05, 2020, 10:28:28 pm
content: In the late 1940s and early 50s, we had Devil Bait red and white was best and I can't remember the name of the plug. There was a Ruby Eye. That was it, too poor to have anything else.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 05, 2020, 10:34:13 pm
content: Just for completeness, shown sweeping the homemade buffer to 1GHz. Not a real good test setup but gets us in the ballpark. Peaks up to 1.5 dB and starts to drop like a rock above 200MHz. -3dB is around 300MHz.Really, for the 50MHz high end mentioned and sweeping filters I would have just went with that little NanoVNA. I've played a bit with them and even for narrow band work, they seem pretty decent depending on how low a noise floor you need. Worse case, a costly repair is limited to the time it takes to get a new one shipped and $50.

author: tautech
author_rank: Super Contributor
timestamp: September 05, 2020, 10:35:01 pm
content: Quote from: wd5jfr on September 05, 2020, 10:28:28 pmBefore I was even a twinkle in my fathers eye !Since then there are some nice lumo products we can tie into lures which you can charge with a torch or better an old camera flash unit which works much better and the glow last much longer. A flash unit is a must for the serious night fisherman.

author: wd5jfr
author_rank: Regular Contributor
timestamp: September 05, 2020, 11:11:36 pm
content: I thought about a nano but then I have to set it up, learn, and waste a bunch of time.With my Siglent SSA 3021X good to 50VDC and a probe running at 5vdc it should be plug and play with no pray and pay.That's a busy screen, pretty nice, which model is it?Hank

author: Kosmic
author_rank: Super Contributor
timestamp: September 05, 2020, 11:16:43 pm
content: Not really HighZ but the Tektronix P6056 goes to 3.5Ghz. It's a passive 10x 500\$\Omega\$ probe. Output is 50\$\Omega\$ so you can use it with a spectrum analyzer. Got one for 20$ on ebay the other day.http://w140.com/tekwiki/wiki/P6056

author: joeqsmith
author_rank: Super Contributor
timestamp: September 06, 2020, 01:12:01 am
content: Many years ago, I picked up an old (at that time) Tektronix P6202 active probe. It can drive both 1M and 50ohms and had an optional attenuator and DC block.The newer version's manual can be found here:https://www.tek.com/p6202a-manual/p6202a-instruction-manualThis is the one I have:https://www.ebay.com/itm/Tektronix-P6202-500-MHz-10X-FET-Pobe-Kit-Full-Accys-Manual-Case-Tested/324172342811?epid=1718864835&hash=item4b7a2d661b:g:zC8AAOSw01lexYjtLooks like you can find them for $70 on eBay.Quote from: wd5jfr on September 05, 2020, 11:11:36 pmNow days you have access to what I understand it some very good open source software for the NanoVNA. For the filters, you would be using the Nano's port 1, so there would be very little risk of damage. Of course, if you wanted to play with something with gain....That particular one is an old Agilent PNA that I bought it to experiment above 2GHz. The hardware is really nice but the software looks to me like Agilent fired every senior software developer they had, hired a bunch of freshout Windows CS majors and then managed the group with people who had no idea what a VNA was.

author: Kosmic
author_rank: Super Contributor
timestamp: September 06, 2020, 02:11:13 am
content: Quote from: joeqsmith on September 05, 2020, 07:45:05 pmI have one of those. Look like the seller tried to implement the design published in Elektor magazine a while ago. On his ebay listing he even use some schematic from the article.Most of the problem come from C1. For some reason (limit attenuation between 0-500Mhz), he decided to use 2 x 2nF in series. Was supposed to be a 0.75pF PCB cap created with 2 planes on each side of the PCB.This is what the original probe look like.This is the frequency response from 50 to 1500Mhz.Now a bodge to try to fix the probe. I relocated the pogopin on the other side of the board. It was soldered on a piece of copper tape. I added some super glue to make sure the copper tape and pogopin will stay in place. The Kapton tape was just added to isolate the ground pin.Now the frequency response of the fixed probe.For reference, the Elektor article about the active probe is in attachment:https://www.eevblog.com/forum/testgear/hi-z-probe-for-50-ohm-spectrum-analyzer/?action=dlattach;attach=1060862

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 06, 2020, 03:00:06 am
content: Quote from: Kosmic on September 06, 2020, 02:11:13 amtry to change the tip setup again, say using 50mm of wire betwen probe tip and pogo pin and plot the respond again. i bet you'll get another entirely different respond. i experienced this while testing an active probe in just hundreds of MHz range not long a go, its all over the placei'm not sure whats going on, my best guess is... reflection...

author: Kosmic
author_rank: Super Contributor
timestamp: September 06, 2020, 03:05:49 am
content: Quote from: Mechatrommer on September 06, 2020, 03:00:06 amIt has 1nF input impedance, not 0.75pF like the probe described in the Elektor article. Sure my, probing technique is probably ideal, and my bodged input cap is probably also not ideal but, I'm getting a lot closer to the specified -20db and the response is a lot more linear than the original one.

author: 2N3055
author_rank: Super Contributor
timestamp: September 06, 2020, 11:54:00 am
content: It has 1nF input capacitor connected to what? To mosfet gate and loaded with 10M resistorInput capacitance of that probe is 1 nF input capacitor in series with gate capacitance of that mosfet ( Which is typically around 2 pF), that is in parallel with parasitic capacitance of that 10M resistor .So how much of that capacitive load is presented to DUT?1nF or something about 2pF?

author: Kosmic
author_rank: Super Contributor
timestamp: September 06, 2020, 01:34:58 pm
content: Quote from: 2N3055 on September 06, 2020, 11:54:00 amFrom the article:QuoteSo no, I don't think substituting the 0.5pF cap by 2 x 2nF cap will really give the same input capacitance of 0.75pF. Unfortunately I'm not equipped to measure the input capacitance of the probe so I can't validate.

author: 2N3055
author_rank: Super Contributor
timestamp: September 06, 2020, 02:00:01 pm
content: No, input capacitance will be somewhere in 2 pf region... But it would have better low frequency response...

author: Kosmic
author_rank: Super Contributor
timestamp: September 06, 2020, 03:33:53 pm
content: Quote from: 2N3055 on September 06, 2020, 02:00:01 pmYes it should be sold as a 0-500Mhz probe. In that range it's OK-ish.

author: Kosmic
author_rank: Super Contributor
timestamp: September 06, 2020, 03:47:30 pm
content: There's also this one on ebay for 30$. A bit more expensive but specs might be authentic since they didn't simply copy the specs from the Elektor article like the other one.https://www.ebay.com/itm/RF-Active-Probe-DC-Out-100KHz-1GHz-HF-VHF-UHF-SHF-analyzer-oscilloscope-SDR/184372665711

author: joeqsmith
author_rank: Super Contributor
timestamp: September 06, 2020, 05:00:46 pm
content: Quote from: Kosmic on September 06, 2020, 02:11:13 amI remember seeing that article or at least the first picture from the article. -20dB loss and +/-2.5dB flatness. They talk about using a scalpel to trim the PCB which I think we all have all found to be common practice. The lack of a DC coupling is a deal breaker for me.As others have said, I too find it odd not to see the ground on these probes. That alone tells me the person was just trying to follow someone else's design without understanding it. Your addition of this is a big step in the right direction. For comparison, attached are a few pictures of one of my vintage LeCroy probes. The probe comes with a test PCB. Sadly this particular probe met it's match and required repair. The two drops of glue to help stablise the needles after reattachment is my work.Many years ago Pease showed a simple FET probe. I found his old book where he shows it (link below). He shows the BW is 90MHz and a slew rate of 300V/us. This probe is DC coupled. I still have my original copy of this book and highly recommend it. It was very practical.http://www.introni.it/pdf/Bob%20Pease%20Lab%20Notes%20Part%208.pdfWhile trying to find RAP article on-line, I came across this old video on scope probes....

author: mawyatt
author_rank: Super Contributor
timestamp: September 07, 2020, 03:00:15 pm
content: Quote from: joeqsmith on September 06, 2020, 05:00:46 pmThat book along with the one from Jim Williams are masterpieces for analog-types, I have both somewhereI think William's book discusses the Ross "T" network that Tektronix kept as a trade secret, and the Ft doubler amplifier that they patented, with these it took HP almost 20 years to duplicate the 1GHz real time scope Tektronix had in 1969!!The mentioned FET probe using the two JFETs actually came well before this book by Bob, we were using the concept back in ~1980. Think it originated from Tektronix and used in their scope front ends. With just 2 JFETs (2N4416, 2N4393) and a couple resistors you can have a really good high BW, high input Z buffer amp. The complementary emitter follows aren't necessary, you can take the output signal directly from the input JFET source for a low impedance (50 ohms with Rsource ~33 ohms & 2N4393), or from the bottom of the source resistor for a higher output impedance signal that is ~DC correct.Anyway, great stuffBest,

author: joeqsmith
author_rank: Super Contributor
timestamp: September 07, 2020, 04:34:09 pm
content: I am interested in seeing the original article if you are able to find it.

author: Bud
author_rank: Super Contributor
timestamp: September 07, 2020, 05:34:20 pm
content: Hmmm.... Where did I see that ?

author: wd5jfr
author_rank: Regular Contributor
timestamp: September 16, 2020, 08:00:27 pm
content: Quote from: joeqsmith on September 06, 2020, 01:12:01 am

author: joeqsmith
author_rank: Super Contributor
timestamp: September 17, 2020, 12:55:29 am
content: Quote from: wd5jfr on September 16, 2020, 08:00:27 pmI assume you were not able to download the manual and read it, so I have attached the page with with the power supply you would need to go with this probe.As far as finding one, try eBay, salvage yards, other auctions, craigslist.

author: wd5jfr
author_rank: Regular Contributor
timestamp: September 17, 2020, 05:47:31 pm
content: Hey JoeI guess I should have said I downloaded the manual but couldn't tell much about the power connector and after looking at 1101 power supplies I couldn't recognize the plug or like the prices.. And after that somehow I recalled that I had picked up a Tektronix probe power supply because it was such a good deal and looked nearly new. So I dug around, found it and sure enough I had one and it's an 1103! So now I'm looking for a good deal on a Tektronix P6205A.

author: wd5jfr
author_rank: Regular Contributor
timestamp: September 20, 2020, 03:57:19 am
content: After scanning the pertinent info I could find online I'm considering a Tektronix P6202 500MHz that includes a line cord power supply (Model # 119-07744-00) using Lemco connectors, around $100.00. I've not seen any individual line cord power supplies for sale and the Tek 1101probe power supply is about $80.00.Since I have a tested working Tek 1103 probe power supply I'm considering a Tektronix P6205 750 MHz which plugs in directly. (I made an error in the previous post indicating a P6205A which doesn't exist.) I see that there is a chance of getting a good P6205 for about $!00.00.From what I could tell from examining eBay listing pictures many of the P6202 have probe pins that are bent, broken, missing or a stub. I'm guessing the pin is a straight wire (poorly selected metallurgy & form) that can't withstand the. stress of poking around and bends, finally breaking. Replacement pins in packs of 10 and a special tool are listed in a Tektronix catalog. I haven't seen any listed for sale. This point is a huge negative for the P6202 and unless I find an exceptional deal it not a wise choice. There's some pics on the old Yahoo Tektronix Groups site showing a tip replacement, not a task I want try. The P6202 also has a manual switch to select high or low impedance, another failure point.The P6205 has a redesigned pin which is a conical, pointed tip being much thicker at the probe entry transition so that it can better withstand the stress of being poked around. It doesn't have a manual switch to change impedance, the scopes that are designed for it detect it and automatically switch to 50 Ω. So I assume the P6205 is always 50 Ω. If I'm wrong someone please correct me. Also if there is something else I should consider please let me know.I'm not in a big rush as I'm still enjoying learning how to use the features of the Siglent SSA 3021X. I'm also wondering what if any influence a DC block will have on results.ThanksHank

author: joeqsmith
author_rank: Super Contributor
timestamp: September 20, 2020, 04:03:33 pm
content: Quote from: wd5jfr on September 20, 2020, 03:57:19 amThe manual for my 6202 dates from 1976. I would imagine that many scopes were still analog and most did not have 50 ohm terminations. The 6202 was designed for a 50 ohm system and they included a switch selectable terminator rather than requiring an external one be used for the scopes of this era.I've seen a lot of mangled probes over the years. I am not sure why people want to drive over the cables with their chairs and use the tips for pry bars. Still to this day I will see people who can't seem to reinstall the end caps.These were shipped with two replacement tips in case they were mishandled. I bought mine at Dayton maybe 25 years or so ago and still have all of the original accessories, including the two replacement tips. I'm not sure what the P6202 would have cost in today's dollars but I'm sure it wasn't cheap. I would expect the people that used them would have taken care them (mine obviously has been), but then again, there's that LeCroy probe.I've attached a few pictures of mine.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 20, 2020, 05:19:10 pm
content: Using my NanoVNA, with my home made PCB standards to evaluate the P6202.After performing a SOLT, an open test board was used to look at S11 using the probes IC cover and spring clip. I measure roughly 2pF up to 300MHz. At 500MHz, the Nano measures 3pF. I don't have a lot of confidence in the Nano to make this measurement at these higher frequencies. I suspect that the probe is performing better than what is shown.Next I installed the home made T-Check (50 ohm thru) along with 2X10dB attenuators to simulate the 10X probes gain. This seemed fine, so I installed the load standard and attached the probe with the external 50 ohm terminator selected. The manual shows -3dB at 500MHz. The Nano uses the harmonics for the source above 300MHz and we can see that. Still it appears well within spec.****The DC block was added as I wanted to see what effects if any, using the offset would have.I have also included a 20 minutes of data after power on. I was expecting the probe would have some drift to it but it looks really stable.

author: BravoV
author_rank: Super Contributor
timestamp: September 20, 2020, 05:35:46 pm
content: Took me almost 4 years of lurking to score this one at "decent" price.HP 54701A, basically at 10:1 probe drives 50 ohm, at 2.5GHz with input capacitance 0.6pF and input resistance at 100kOhm.

author: wd5jfr
author_rank: Regular Contributor
timestamp: September 20, 2020, 06:40:24 pm
content: Hey JoeIt didn't take long! Got offered this one for $101.18 because I looked a couple times and it looks like it might have a good tip, at least it's got spares and doesn't look to beat up, tested with 30 day return, maybe not as complete as yours. So now Ill have a dual purpose probe not sure how the higher impedance works and its uses.Hankhttps://www.ebay.com/itm/324172342811?boolp=5&ul_noapp=true

author: joeqsmith
author_rank: Super Contributor
timestamp: September 20, 2020, 06:59:21 pm
content: It looks like it's all there, wasn't abused and the 30 day return is more than fair. Hope it all works out.

author: wd5jfr
author_rank: Regular Contributor
timestamp: September 28, 2020, 11:23:57 pm
content: Well I got the P6202 on Saturday and is all there as described and not used much very clean maybe used a few times? Before using it on my Siglent SSA 3021X 50 ohms input I decided try it on my Tektronix 1012b using the calibrator and found that the impedance switch doesn't do anything to the trace except sometimes when moved from EXT to INT the trace flashes to about 1/2 size and returns to full scale. With my Fluke 189 I measured across the output BNC and it measures around 10 ohms regardless of the impedance switch position and I guess the Fluke is not quick enough to pick up the "flash".. The schematic shows that the INT switches in a 50 ohm resistor across the output and EXT disconnects it so a 10 ohm reading is quite puzzling. It says TESTED, who and how was it tested? I've contacted the seller. I think it was a hangar queen for years because of this problem. I wonder if someone with one can tell me what the resistance across the output BNC is in EXT & INT positions?ThanksHank

author: joeqsmith
author_rank: Super Contributor
timestamp: September 28, 2020, 11:43:09 pm
content: It could be a user problem. If the probe is powered up, I would expect you to get bad values. After all the probe is driving the output.Assuming you had the probe powered off, I would expect it to be 50 ohms in internal andin external. Certainly in the K ohm or higher. Again, it not really a resistance you are measuring.*************See attached

author: wd5jfr
author_rank: Regular Contributor
timestamp: September 29, 2020, 03:16:32 pm
content: I should have been clearer so I took some pics. The pics show power unplugged and IN and EXT switch exercised then firmly pressed into each position and it shows 8.83 ohms in both positions. So I assume there's a problem with the 50 ohm resistor or the switch or both, it's puzzling.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 30, 2020, 12:09:31 am
content: If it were the 50 ohm, you could still switch it out. Seems like an odd failure for the switch. Guessing it's not these but upstream. Maybe C33 or a bad cable. Maybe your meter or your connectors. Should be a few minutes to hunt it down.

author: Kosmic
author_rank: Super Contributor
timestamp: February 07, 2021, 03:50:51 am
content: Quote from: Kosmic on September 06, 2020, 01:34:58 pmI recently acquired a nanovna, so I did more test with the cheap active probe. I removed all my modifications and tried to measure the impedance of the probe in the state it was received.the input capacitance is around 3.9pF at low frequency but get quickly to 6.5pF at 543MHz and 17.8pF at 726MHz. We are really far from 0.5pF.It's a shame since the author of that probe could had made it better just by closely following the Elektor article.

author: mnementh
author_rank: Super Contributor
timestamp: August 06, 2021, 05:29:31 pm
content: Quote from: Kosmic on September 06, 2020, 02:11:13 am[https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/msg3621622/#msg3621622For those who care... 60dbm has updated his design with a proper gimmick cap on the input as in the original Elektor project. I've designed a shell for it which includes a holder for A23 car remote battery; the ~20mA draw sucks it down pretty quick, but still quite usable and mucho convenient for occasional use.https://a360.co/37CE8ElI'm waiting on pogo pins for the (GND) side then I'll try and get some data from my nanoVNA for y'all.mnem

author: mnementh
author_rank: Super Contributor
timestamp: August 08, 2021, 11:21:24 pm
content: This is what I've got so far; it told me whatIneeded, which is that it would present reasonably low loading on a CPU crystal oscillator I wanted to probe for signs of life. I'm still working between bouts of packing and looking at properties to move; I'll be back with more data ASAP.One thing I like aboot 60dbm's new probe is the big pads for soldering pins to; you can easily reconfigure to match almost any needed pin spacing. Of course that is probably also why ~1.1 puff loading instead of 0.5, but hey... usability factor is important too; that's why I went faffing aboot with a little car remote battery.mnem

author: Kean
author_rank: Unknown
timestamp: August 09, 2021, 02:43:31 pm
content: Quote from: mnementh on August 06, 2021, 05:29:31 pmHey mnem,Did you intend for the F360 link to allow downloads of your model? Pretty pleaseSince I bought the ebay probe a few years ago, I have since acquired a pair of used & working HP 54701A 2.5GHz Active RF Probes (100k 0.6pF input).I may never use the ebay probe again, but it would be nice to have a case protecting it so if ever someone wants to borrow one of my RF probes, I can offer that one.Kean

author: Kean
author_rank: Unknown
timestamp: August 09, 2021, 05:38:04 pm
content: For anyone coming here later, mnemeth has just posted the STL and Fusion design files in another threadhttps://www.eevblog.com/forum/3d-printing/3d-printer-yet/msg3623898/#msg3623898

author: mnementh
author_rank: Super Contributor
timestamp: August 09, 2021, 05:49:00 pm
content: LOL.... you done stole muh thunder.Just be aware that this fits the new, blue version of this probe/PCB as shown above pretty snugly. PCB dimensions are 9mm W x 1.6mm thick exactly; approx 57mm long not including the SMA connector. I don't know if it will fit the old green version; the cutouts for that cap and diode in the battery compartment are specific to the blue one.mnemDirect Link to ZIP file:https://www.eevblog.com/forum/3d-printing/3d-printer-yet/?action=dlattach;attach=1243488

author: Kosmic
author_rank: Super Contributor
timestamp: September 06, 2021, 02:42:33 am
content: Since mnem got a new model with the gimmick capacitor on the input, I decided to order another one to compare.The 2 probes. I replaced the original pogopin and used shorter one I had.I changed my setup a little bit to be able to measure S21 on top of S11. The VNA has been previously calibrated with the small test PCB and the probe is connected near the load.Old probe (yellow) VS the New one (blue):New probe (yellow) VS the Old one (blue):Overall, the new one is an improvement. Perform relatively well up to 1G and input capacitance is a lot lower (1.2 to 1.5pF compared to 3.5 to 8.5pF for the old one). The gain is unfortunately not linear and starting at -25dB (and not -20dB as mentioned in the Elektor article).

author: harrimansat
author_rank: Regular Contributor
timestamp: October 02, 2021, 10:58:22 pm
content: Can you please take S21 curve without probe ground connection?ThanksRegardsMiguel

author: Kosmic
author_rank: Super Contributor
timestamp: October 03, 2021, 08:37:36 pm
content: Quote from: harrimansat on October 02, 2021, 10:58:22 pmIn this case, the source and the receiver are in the same VNA so they share a common ground. Also, even if the probe is battery powered, the ground is shared (connected by the SMA connector).This is a sweep with the ground pin floating. The reference in blue is a normal sweep with ground. In yellow there is no ground.You always need a good and short ground to avoid problem like that

author: harrimansat
author_rank: Regular Contributor
timestamp: October 05, 2021, 04:24:43 pm
content: My DIY fet probe

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: October 06, 2021, 02:59:01 am
content: Here's one I help out with as a youngster. Maybe not quite the price point but 30 years later it still has quite good performance, and we still actively sell it:https://www.keysight.com/us/en/product/85024A/high-frequency-probe-300-khz-3-ghz.htmland tech overview:https://www.keysight.com/us/en/assets/7018-06843/technical-overviews/5968-2101.pdfNo ground pin as the barrel held in your hand provided a low enough impedance to ground that no ground clip was needed. AC only of course.0.7 pF input impedance, flat to past 3 GHz.You could DYI it if you had your own custom GaAs fab.

author: Kosmic
author_rank: Super Contributor
timestamp: October 06, 2021, 03:07:43 pm
content: Brand new it might be a bit much but I can always try my luck with ebay. Added to my watch list