FieldFox Drift Compensation - Page 1
Forum section: electronics / rf-microwave

author: jjoonathan
author_rank: Frequent Contributor
timestamp: July 04, 2024, 01:31:50 pm
content: I recently switched from an 8510C to a FieldFox as my main VNA. As you can imagine, size and mobility requirements were the driving factor, but it did come at a cost to stability. A small cost in absolute terms compared to my needs, but large enough in relative terms that I can't help but want to chase it:https://jjoonathan.github.io/plot_drift.htmlthe time scale pretty strongly suggests that the effect is thermal in nature, and the piping hot ports on the FieldFox sort of point in that direction as well. There is a temperature reading in System > Service Diagnostics > Internal Temperatures, but I can't figure out for the life of me how to get it out of SCPI. It looks like Keysight's bigger VNAs have :SYSTem:TEMPerature? but that doesn't seem to be present on the FieldFox. So that's my first question, if anyone around here knows how to get at FieldFox service diagnostics through SCPI, or another way of getting at the temperatures. I might like to do some kind of automatic compensation, so taping a thermocouple to it isn't really what I'm going for, but I might get there eventually.My second question is more fundamental and has to do with the interpretation of the time domain results. Here's the setup:Let the FieldFox warm up for several hours and obtain a calibration using 85056D slotless standards applied directly to ports.Let the FieldFox sit and cool down for a day.Cold start FieldFox, attach 85056D short to port 1 (see picture), apply previously collected calibration.Start a script that grabs a trace every minute.Let it run for 16 hours.Time domain transform all traces using scikit-rf net.impulse_response()Plot ir-ir_final (the impulse_response function already stripped phase information)This results in the "TDR" plot in the link above. At first I was surprised to see the bump at 50ps (7.5mm) in front of the reference plane. Was I looking at a temperature sensitive standard? The more I thought about it the less likely that seemed. It's a short, so it should be all metal and I'd think that most of the expansion/contraction should show up near the reference plane, not 7.5mm into the thing. My current hypothesis is that the first order culprit for the drift is something that applies to the whole trace, like relative receiver sensitivity, and the fact that this "shows up" at 7mm is just because it changes the reflection and that's where the reflection is (see pictured time domain trace of standard).I don't really _need_ a good answer here, my primary purpose was just to become familiar with the first-order drift behavior of my new VNA so I can recognize it easily and not spend time chasing "DUT behavior" that is actually instrument behavior superimposed on the DUT. A little bit of tail-chasing up front saves a lot of it down the road, or that's the theory anyway. Still, I'd like to bounce the "receiver temperature sensitivity" hypothesis off the crowd. I was stuck on "but why is the short standard temperature sensitive?" for longer than I should have been and I'd like to be a little more sure that I've really got the correct mechanism in mind this time. Even just writing it down helps collect my thoughts, but I'd certainly love to hear from anyone who has spent time chasing the drift behavior of the FieldFox or any other kind of VNA.

author: KE5FX
author_rank: Super Contributor
timestamp: July 04, 2024, 09:56:33 pm
content: Interesting set of observations, and the time slider is very cool.I imagine the directional coupler (which I'd assume is a resistive bridge in the FieldFox) is what's doing the drifting. It is making the short standard's return loss appear to vary as the unit warms up. You could verify that easily enough by hitting it with some R134a and/or a heat gun while you measure it on either VNA after warmup.Something to remember about the 85xx VNAs is that they are based on technologies and components that predate digital error correction. They had to be ridiculously stable by modern standards, so it's no surprise that the FieldFox falls short in a head-to-head comparison.Also, calibrate them both now, then come back 6 months later and see how the calibrations are holding up. I'd expect the 8510C to win that contest for sure, for the same reason, but there's always room for surprise in this business.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: July 05, 2024, 12:07:13 am
content: Hmm, yes, answering this experimentally is probably quite practical. A short cable for thermal isolation would let me play with the standard and see if it generates similar results, and to test the source and receivers I could bring the FieldFox outside (it's summer and hot) or even put it in the refrigerator. Ha, that might have been tough with the 8510C!

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: July 05, 2024, 10:36:55 pm
content: The fact that the bump in time domain appears pretty suddenly seems and occurs at a slightly positive time, and the phase of the S11 moves in a positive direction, implies the short circuit distance is getting shorter (if you used a calkit short, it is actually an offset short). If you looked at the raw response you might see a big discontinuity there, and it is being corrected for; but with time and temperature, something shifted (10 degrees phase at 50 GHz implies a physical distance in reflection of about 88 microns). So a slight shift in a discontinuity going through the test port interface to the port can account for the change in TDR response. You can think of it as subtracting a big error and the position of the big error shifts slightly.In Fieldfox, the bridge is on the PCB which is connected to the front panel through a small cable and connector. In the 8510 coupler, the test port connector is integrated into the bridge and so much less susceptible to any thermal effects. And as mentioned earlier, the raw performance is much, much better. So when the math is subtracting the raw performance to make it appear perfect, it is subtracting a much smaller number, so a change in that number has a much smaller effect.And, for reference, an HP 8510 system was about $200k in 1985; that's $600K in todays dollars. Fieldfox is a bargain compared to that.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: July 10, 2024, 12:19:30 am
content: Yep, it's an offset short at 6.75mm. I'd expect thermal expansion in the metal on the order of 10ppm/K * 30K * 10mm, but that's just 3 micron. If the reflectometers are on the PCB (similar to Shahriar's FieldFox video), perhaps that's where most of the expansion is coming from. The intuition that polymers have a CTE around 100ppm/K with a few cm of distance along the PCB would place us right at 90 micron of expansion, which agrees quite nicely with the observation. This is good news, it means that letting the FieldFox warm up really is a pretty good answer here, at least for my purposes. The measurements already showed this, but it's nice to have a mental model with numbers that check out.It's amazing how long the HP 8510 has held on. I am very glad to live in an era when instrumentation of this quality can drift into my budget range. 40 years and people are still doing real work with them. I had no trouble at all selling it on once Iconfirmedthat the FieldFox was workable. I saw the raw performance -- that's actually why I went to such pains with the confirmation -- but it pulled through. Besides, I need to leave myself some motivation to turn the pile of 1.85mm microwave bits I have accumulated over the years into an actual VNA. Most of the "marine snow" falling off the microwave industry whales seems to be Anritsu kit these days, but there is an HP doubler and some Centellax amplifiers in the pile.As you might have seen in the notebook I linked, I am working on getting the 2.4mm and 1.85mm HPAK calibration kits into scikit-rf (still working on banded cal). At one point I started writing a parser for the files, but then Keysight pulled the cal kit download page and pointed at a PDF, so now I'm thinking I should just commit the latest version of the hardcoded values and leave it at that. I suppose they probably aren't updated very often, but I don't have good visibility into how not-often that is. If the cal kit constants change less often than the file formats, that's probably the right approachbut in any case is there a mailing list for "ping me if the constants change"?

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: December 07, 2024, 05:50:54 am
content: I just stumbled across this and I'm curious: what doubler do you have?

author: jjoonathan
author_rank: Frequent Contributor
timestamp: December 07, 2024, 08:01:00 pm
content: https://photos.app.goo.gl/KwvnMGm5GyvnmgE58I was referring to a 40GHz doubler from an HP 8350 signal generator plugin with discrete amplifiers and "Kilroy Was Here" on the Wilkinson segment (5086-7468). The 8350 plugin came with a Navy Metrology Lab sticker and enough oxidation that I suspect it might have been stored in a seaside warehouse for a decade or three. It didn't work, but its doubler did, and its gold plating even protected it from the oxidation. Once I reverse engineered the package pins it became my project doubler for a few years. Right now it's integrated into a signal generator project and handling the 20-40GHz band. The low band is a demo board and there's now a 40-80GHz band using a Marki doubler and Norden amplifier that I obtained from a test jig someone put on ebay (along with two 67GHz Keysight attenuators, it was a good find). I'm eventually looking to integrate my signal generator project (which for me means modules->PCB, not PCB->chip), what with so many new synthesizers and SOI chips hitting the market, but it has been on the back burner for a while.That's partly because I have a good 40GHz and 50GHz 8360 (with 5086-7507 and 5086-7513 doublers) to run experiments with. The 40GHz doubler came in working condition but the 50GHz doubler had fused bond wires on the power stage. The three TC702 amplifiers were fine but the TC724 amplifiers both had fused drain wires. I (carefully) scraped off the fused wires and re-bonded them and they have been working ever since. The bias looked correct and even though I "brought it up slowly" after replacing the bond wires I wound up adjusting it to almost exactly the same gate voltage that it was set at before. The input signals and bias adjustment should have all been protected inside the signal generator so I'm not sure what happened to fry those bond wires, but it evidently didn't damage the TC724s too much and it hasn't happened again so... shrug.I don't actually own the Anritsu doubler on the bottom. Kalle Kempe posted it to the Anritsu list and I grabbed the pictures because the overall design was, err,very familiar. Given the HP Journal article that lays out the design, I wonder if this wasn't one of the reasons we can't have nice articles like this anymore. The slotline doubler, PIN-shortable stubs, and optimized switched filter on the output were inspired. Were you involved with the HP design, by chance?Speaking of multipliers, I was talking to a pipe mapping guy at a water utility conference the other day and I overheard him mention "harmonic generation." I wasn't expecting to hear that term in that place, but it turns out they were doing sonar, and it turns out that when you do sonar in utility pipes evidently a good way to do harmonic generation is to bang a check valve. Like a step recovery diode, but with water! The hydraulic analogy works for SRD multipliers!

author: guagg
author_rank: Newbie
timestamp: December 08, 2024, 06:56:08 am
content: I have a 8510 whitch consist of 8515 and 83621a. I want to follow your method to modify 83621A to work as a 83631. Not sucess.The method to zero serial number in eEPROM need more information.Thanks,jjoonathan.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: December 08, 2024, 02:09:49 pm
content: Did you clear the SRAM? If you don't clear the SRAM, it doesn't read configuration from EEPROM. Also, some things to be aware of for the 20-26.5GHz bandThis is where the YIG tuning starts to go nonlinear. In lower bands, the master slope trimpots (YTMG/YTFG) and slope/gain software adjustments are enough. There's an auxilliary slope pot on Band 3, but you can usually get away without touching it. In Band 4, the breakpoint/slope pots really matter. The procedure in the manual is not great, you should get tuning vs frequency and power vs frequency on your oscilloscope and play with the pots a bit until you understand how they control the tuning curve. Look at theI(f) plot here, you are targeting a similar shape but with nonlinear trouble starting around 20GHz rather than 50GHz.The 26.5GHz YTF is different from the 20GHz YTF.It has a TC702 amplifier inside.This matters for the power budget of the 83630A.I don't remember if the 83631 is supposed to have a YTF (the 20GHz headless units only had a YTM IIRC), but if you are adding a YTF rather than replacing a Filter with an Amplified Filter, you'll also need a "dual ball" driver board. The YTM-only units have a board that only drives a single YIG.The high level challenge is marginal power at the top of the 26.5GHz band.Here's the 8360 Project Manager talking about it.If the 8510 complains about phase lock at 25GHz, that's the problem. 40 years of amplifier progress have given you options, but you also might be able to get away with simply ignoring the message -- it seems to trigger long before the power issues start to degrade any but the most demanding measurements.

author: guagg
author_rank: Newbie
timestamp: December 11, 2024, 07:48:59 am
content: Thanks a lot. My question is still on how to clear serial number.I used a miniPro to read the X28256D.No more information about the serial number.😭

author: jjoonathan
author_rank: Frequent Contributor
timestamp: December 11, 2024, 07:08:42 pm
content: Look onhttps://groups.io/g/HP-Agilent-Keysight-equipment/messages. This isn't the thread.

author: guagg
author_rank: Newbie
timestamp: December 14, 2024, 02:09:41 am
content: Thanks for information.Good job.

author: guagg
author_rank: Newbie
timestamp: March 16, 2025, 05:45:59 am
content: Hi JjoonathanI have done some tries for last days.I had tried a copy of x28256 from a 83640a.I found that the 8360 SM introduce a method to copy cal datas.I succeeded.as shown in Pic 3.At the beginning, I followed you introduced method and succeeded.Then I got a doubler for 8516A by chance.Foutunately ,I fond the module has same design as 83640 in a article of HP journal whitch pulished in April ,1991,volume 42 ,number2.Then I fond the connector definition for the doubler bias board as in Pic 4.I thought maybe I can modified my 83621 to a 83640a. I tried to found detailed usaged for every pin.......Now, it seems not a easy work.I don't success.A hard journey to 40g.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: March 16, 2025, 07:20:11 pm
content: Good news and bad news.Bad news: the band plan of the old doubler, the one from the 83597A and 8516A and Apr 1991 HP article, is very different from the band plan of the new doubler. At one point I wanted to upgrade an 8360 with my 8350 doubler so I checked every combination of hardware flags in the 8360 settings and none supported the old bandplan. I'm not 100% sure I covered every possibility -- I have seen 83640A doublers using the old housing, maybe it switches off model number or some other hardware flag I didn't know to check -- but as far as I know this would be a difficult upgrade involving code changes.Good news: if you just want to drive an 8510, the 8516A interface is simpler than you think. In fact, the 8516A does not talk to its doubler at all: the 8340/8360 source is responsible for toggling the filters on the doubler inside the 8516A test set and the signals are routed through some undocumented pins on the auxiliary interface. I happened to take notes on this the other year:https://www.dropbox.com/scl/fi/3isgo74bsihds6iqtyuhg/8516A-Doubled-Test-Set.pdf?rlkey=7qimqjtsih72b3tlezdjm3v85&st=cw49g8tm&dl=0so if you want to make an 8510 source, you just have to get your test set to identify as 8516A and route power + "undocumented" aux to your doubler and you should have a fully functional 40GHz source.

author: guagg
author_rank: Newbie
timestamp: March 18, 2025, 12:45:08 pm
content: Thanks for your tips.

author: guagg
author_rank: Newbie
timestamp: March 22, 2025, 01:24:21 pm
content: hi,Jjoonathanmaybe 8360 to 40G journey is hard.I do other tries ：as i have a 8340B . a 8517B and 83554A ,so i make a switched 40g test system.it is similar to 83640A: through path and doubler.As I saved two setting,i used state 1 for 45M~26.5GHz and state 2 for multiplier source whitch work at 26.5~40GHz.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: March 26, 2025, 12:58:33 pm
content: Nice setup! Climbing the GHz ladder one step at a time

author: guagg
author_rank: Newbie
timestamp: August 31, 2025, 02:22:18 am
content: I just got a hp8516A .It worked when i connected it to 8510c .Meanwhile i tested my module in 8516a and found it worked.😁😂

author: jjoonathan
author_rank: Frequent Contributor
timestamp: September 01, 2025, 08:01:55 pm
content: Congrats!