Shunt capacitance of 1206 SMD resistors; Jeroen Belleman - December 2010 - Page 1
Forum section: electronics / rf-microwave

author: joeqsmith
author_rank: Super Contributor
timestamp: July 14, 2021, 12:16:38 am
content: Some time ago I had come across an paper written by Jeroen Belleman where he was studying the parasitic shunt capacitance of some 1206 resistors. The original paper may be found here:http://jeroen.web.cern.ch/jeroen/resistor/shuntC.htmlHe notes:QuoteI had been wanting to repeat this test to see if I could replicate that curve up. Shown are my 1206 test resistors plus an added 50 ohm part. Sadly my PNA can't run below 300kHz. What I have done is used my HP3589A to collect the data below 300kHz and then stitched the two files together. Their setup starts to curve up below 100kHz so I thought I would collect data down to 3kHz.The source was set to 0dB and the RBW to 1.2kHz (thought about lowering it and using the log sweeps). I have no additional attenuators inserted and see no signs of this upwards curve.****Change scale of my graph to allow easier viewing.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 14, 2021, 10:57:29 am
content: I never realized when the HP3589A starts a sweep, it causes a small disturbance in the data.Operator error..Shown with nothing inserted in the test jig (open) to look at the floor with a RBW of 150Hz. I would like to see about 10dB of headroom but it should be fine.It will be interesting to see if the METAS software can hand this much data and with it being stitched together.Note that I had started out with a cable that was loaded up with Ferrite where it it shown without. As expected, I saw no difference and plan to run it as shown (without the Ferrites).

author: joeqsmith
author_rank: Super Contributor
timestamp: July 15, 2021, 01:20:05 am
content: Shown is the combined data, sweeping from 900Hz to 6GHz with roughly 27,000 data points segmented quasi log sweep. A fairly decent test for both programs.All the Ferrite was removed when collecting this data. Once again, there are no signs that odd amplitude change at the < 100kHz.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 15, 2021, 11:41:16 pm
content: I converted the original data to Touchstone format and overlaid it with the data I collected.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 15, 2021, 11:53:35 pm
content: Beats me; does seem like an instrument error or something. Have you contacted Jeroen? He's... well at CERN still AFAIK, and also I think checks sci.electronics.design sometimes.Tim

author: joeqsmith
author_rank: Super Contributor
timestamp: July 16, 2021, 12:10:23 am
content: Quote from: T3sl4co1l on July 15, 2021, 11:53:35 pmI wrote Jeroen to see if he had any additional clues. They may attempt to repeat their test. I have provided them with a link to this thread as well.It seems strange that the problem would be below 100kHz. I'm actually surprised how well the two tests tracked.

author: David Hess
author_rank: Super Contributor
timestamp: July 16, 2021, 04:01:53 pm
content: It looks to me like hook in either the resistor substrate or test fixture substrate. It is in the correct frequency range.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 16, 2021, 05:50:09 pm
content: I thought of that, but that doesn't explain it being independent of value. It's got to be systematic.However, it probably explains the discrepancy between ideal RC and the slightly softer curve measured!Tim

author: David Hess
author_rank: Super Contributor
timestamp: July 16, 2021, 06:50:49 pm
content: Hook can be part of the resistive element itself. Some resistive materials display dielectric absorption.I can think of tests which might resolve if it is hook or not, but they depend on finding a bad part or test fixture first.My prime suspect is hook simply because whatever it is is in the same frequency range. Is that a coincidence?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 16, 2021, 07:35:48 pm
content: It's, kind of coincidence, kind of not, I guess? Probes are some megs and some pF, equivalent to the diagonal asymptote being further to the left (more C), and a bit lower down (more R). So, it's directly applicable when the components are crossing in that region -- but there's no reason to expect that time constant should be independent of value, indeed it should be proportional. Which means, that would indeed explain the soft corner; but not the tick up on the left. (And you can make probes with whatever resistances, and the hook scales proportionally, just as the soft corner does here.)Dielectric absorption, now that's interesting; I suppose it could even be something like substrate (does the fixture use FR-4?). But again it seems unlikely that it should be so strong (it's a constant number of dB from the flatband level, and independent of attenuation level?), or that it should attack so suddenly (it looks roughly like another simple pole; absorption I think I'd expect a diffusion mechanism instead? -- not that we have much of the curve to really infer much about its overall shape).Of the materials in the resistor itself, I would think that's very unlikely; the alumina substrate should be pretty clean, and the enamel is very thin. But Idunno; is alumina known to have significant absorption at lower frequencies? I suppose it could, if it's got like hydroxyls in the crystal, or adsorbed water or something. Neither of which should really be all that significant given how it's made (fair purity, high density (very low porosity), high firing temperature), but it would be interesting if true.It really just seems like AC coupling in the system... was an AC block accidentally left on, maybe?Tim

author: joeqsmith
author_rank: Super Contributor
timestamp: July 17, 2021, 12:40:34 am
content: It's too bad they didn't take the time to document the their experiment better. Just a few pictures of the setup may have helped. I doubt you would find 10 different valued resistors all having the same problem. I would also expect that when the system was calibrated that nothing odd turned up which would rule out the cables. Then again, maybe it wasn't checked. I would suspect something with the fixture. From their write up, I envisioned two SMAs soldered back to back with the part mounted between them, similar to this:https://www.eevblog.com/forum/projects/20db-rf-attenuator-seeking-feedback-to-improve/msg2965430/#msg2965430I doubt that would cause any problems assuming decent parts were used. I would think they checked it but I have no idea. Hopefully they will take the time to repeat the test and report back their findings.I'm sure a few may find it a bit strange that the part of their test I am interested in has nothing to do with the problem they were trying to solve.

author: G0HZU
author_rank: Super Contributor
timestamp: July 18, 2021, 06:03:38 pm
content: Stumbled across this on a google search. I think this is my first post for over two years on eevblog!I think the issue with the 8753D VNA could be that is has internal bias tees on each port and the cold (VNA) side of the internal bias tee will have a series capacitor to protect the deeper innards of the VNA from any externally applied DC at the bias tee port at the back of the instrument. This means the mismatch uncertainty (due to uncorrected VNA port mismatch) could be significant down towards 30kHz if the operator doesn't calibrate the VNA correctly for a two port measurement. The alternative to a proper calibration is to fit decent quality attenuators inline although these will eat away the dynamic range of the VNA.This mismatch uncertainty issue also applies over the full 6GHz range of the plots. Both the Jeroen and the joeqsmith plots look really poor considering a lab grade VNA has been used. If calibrated correctly I'd expect to see a smooth trace for a chip resistor right up to 6GHz. The wiggles in the response shouldn't be there. Ideally the VNA and the test fixture needs to be calibrated properly for a full two port measurement because the mismatch uncertainty (when measuring something like a series resistor of value 100R to 1Meg) will be very significant otherwise.

author: G0HZU
author_rank: Super Contributor
timestamp: July 18, 2021, 06:15:09 pm
content: As you have a decent (but old!) PNA VNA my advice would be to study up on how to calibrate the system when making measurements in a test fixture. I think the older PNAs can support two port fixture simulator models for example. To avoid the wiggly plots shown in this thread you really do need to cal/correct the system right up to the pads of the series resistor under test.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 18, 2021, 08:23:25 pm
content: I can't speak for Jeroen but in my setup, I have only normalized the data. I haven't thought about how to calibrate with that fixture. Sounds like you have some ideas how to pull it off.

author: G0HZU
author_rank: Super Contributor
timestamp: July 18, 2021, 08:36:51 pm
content: Yes, your plots do look like normalised measurements. I suspect Jeroen has done the same and this might explain the 'curve up' in his plots because the port match of the 8753D might be compromised slightly down at 30kHz and this could be partly due to the internal bias tee hardware. It would explain why he saw the problem go away when attenuators were added inline. However, all we can do is guess at the problem.I haven't used an 8753D in many years so I can't remember how good the port match is at 30kHz. At work all the 8753D and 8573ES VNAs were disposed of about 3 years ago so I can't do any tests on one. The 8753D datasheet suggests that the port match is still quite good at 30kHz but I think there are different versions of the 8753D VNA. I think there is a version that uses an external s-parameter test set for example.To get good plots up to 3GHz and hopefully to 6GHz you would obviously need to have a decent cal kit and you would also need to know how to correct for your test fixture. I suspect the older PNA can support a 2 port model of your test fixture and you can tell it to use this model internally. This means the VNA will spit out s-parameters that are corrected to the pins of the resistor. Your test fixture looks very big and also very long to me... not where I'd want to be starting from.

author: G0HZU
author_rank: Super Contributor
timestamp: July 18, 2021, 09:10:54 pm
content: I don't have a 1206 test fixture here but I did manage to find a 100k 1206 resistor and I knocked together a very crude 1206 sized 2 port fixture using SMA PCB connectors.This is far from ideal but I tried measuring it in the 2 port fixture using a VNA with a homemade SMA cal kit and an Ecal kit.The comparison is below. My ancient homebrew SMA kit does quite well here although I did exploit the 'unknown through' feature and the 'fixture simulator' feature in the VNA itself in order to get this level of performance. I've not cheated and applied smoothing on the trace data.I'm not really that comfortable using a homebrew SMA cal kit above 2 or 3GHz but the results in this case are quite good all the way up to 6GHz. I did generate Cx and Lx corrections for the SMA cal kit some years ago and these obviously help a bit once up above 3GHz.

author: rf-messkopf
author_rank: Regular Contributor
timestamp: July 18, 2021, 09:15:09 pm
content: Quote from: G0HZU on July 18, 2021, 08:36:51 pmIf you want your cal planes somewhere in a planar circuit, TRL calibration is a good way to go. You could include include the requisite cal standards on your test fixture pcb. The problem is that you need a tightly controlled board impedance as the impedance of the line standard determines your system impedance. Another problem is that TRL has a lover frequency limit, determined by the length of the line standard. There is however a variant of TRL, called TRM (thru-reflect-match), that extends TRL to low frequencies. Modern four-receiver VNAs have that implemented in firmware.I did some experiments with that a while ago:https://www.mariohellmich.de/projects/trl-cal/img/trl-fixture.jpg. See alsohttps://www.mariohellmich.de/projects/trl-cal/trl-cal.html.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 18, 2021, 09:22:22 pm
content: I have a set of standards that I can use if I need to calibrate the system. If they are decent or not is subjective. Poor Mario had to put up with me while I attempted to sort them out.For my home hobby use, good enough.

author: G0HZU
author_rank: Super Contributor
timestamp: July 18, 2021, 09:50:15 pm
content: Quote from: rf-messkopf on July 18, 2021, 09:15:09 pmInteresting stuff, thanks!To demonstrate how easy it is to get wiggly data if the VNA isn't calibrated very well I tried putting together a simulation to demonstrate the window of uncertainty if the uncorrected VNA port VSWR is as poor as 1.3:1 across 30kHz to 6GHz. This tests a crude 100k resistor model that has 0.05pF parallel capacitance. This is shown below and it shows the area that any trace wiggle could reside within if the VNA ports aren't calibrated/corrected properly.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 19, 2021, 01:00:45 am
content: No doubt it's easy to get wiggly lines. No simulation required. I suspect OP didn't feel the need to cal it to see the general trend they were looking for but you would need to ask them. For me, that was certainly the case.There was a link above showing simple back to back SMAs for similar use. For lower frequency use, I mount the parts to a PCB. The two large fixtures make it convenient to change out parts but obviously there is a down side. As I said, not normally a problem though.I have not heard anymore from Jeroen. Maybe someone else will attempt to run it on their 8753D to see it if the problem was caused by normalization. These seem fairly common now.When looking to replace my 1970s HP8754A, I thought about getting the E or ES model but ended up with the PNA. For the most part I like it.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 20, 2021, 02:07:49 am
content: As long as the PNA was warmed up, ran a cal and soldered a 1206 into one of the small fixtures and ran it.Fullcal3: looking at the original data I collected, the data from CERN and tonight's sweep.Fullcal2: tonight's data. RBW was set to200(?)1kHz.I suspect the tail is the fixture. Open and short standards are home made. Loads are purchased and sorted. Borrowed a couple of sets of standards to characterize them (where Mario helped out).***Correct RBW

author: joeqsmith
author_rank: Super Contributor
timestamp: July 22, 2021, 12:43:08 pm
content: Like the original paper, I added included a simple RC for the 50fF//1Meg along with a 40fF and 60fF.Window0 is showing the original CERN data.Window2 is showing the original CERN data along with the first data I collected. Zoomed in 20MHz - 2GHz to better show the fit. The 50fF set was turned off. Granted, if I had spent the time to calibrate the system the data would indeed not have near the ripple. But when you consider the time along with wear and tear on the standards, I don't see the point if you were looking for a ballpark number. I can only guess if that was what the OP had concluded.I wonder if a high quality, characterized jig had been used with a modern VNA that was fully calibrated, what sort of variance would you measure across resistor brands and lots. Again, I'm surprised how close the data came out.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 24, 2021, 03:21:42 pm
content: I attempted to build up a little better fixture to see what effects it has on the upward trend above 2GHz. The PNA was calibrated w/ homemade standards and the RBW was set to 1kHz.Fullcal4_fixtureII_1Meg_1kRBW: Comparing the new fixture with the previous and data from CERN. Also shown is a perfect 50fF capacitor.Fullcal4_fixtureII_1Meg_1kRBW_10MHz_6GHz_normalized: To better show the errors, I normalized the three datasets to the calculated 50fF and zoomed into the 10MHz-6GHz range.As suspected, the fixture does appear to be the cause of the upward trend. Using homemade standards and jigs, I wouldn't put much stock in the data.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 24, 2021, 04:47:52 pm
content: Shown with the ideal 1Meg in parallel with a 60fF and 40fF capacitors. Comparing the uncalibrated data (normalized only) from my quick change coaxial fixture, the data from CERN and the most recent data with the PNA fully calibrated using my coplanar fixture with the resistor soldered in place. The three data sets are again normalized to an ideal 50fF capacitor.Looking at the normalized data from the quick change fixture, I can tell it's roughly 50fF as the original author stated. Other's need to take it to another level before coming up with an answer. There's a thread where a member was talking about cal standards and measuring SWR and I was using rough numbers as being good enough.https://www.eevblog.com/forum/testgear/nanovna-v24-sma-cal-load/msg3360916/#msg3360916

author: joeqsmith
author_rank: Super Contributor
timestamp: July 24, 2021, 08:02:01 pm
content: For fun, I thought I would repeat the test using both the original NanoVNA and the V2Plus4. I used the coplanar fixture and just normalized the data. Compared with the normalized data using the quick change fixture and the data from CERN.Also shown, all four data sets normalized to a perfect 50fF with a sweep range of 20MHz to 1GHz. Both low cost VNAs performed better than I would have expected.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 25, 2021, 01:03:33 am
content: For fun, I soldered a 100k onto the coplanar fixture. When looking at the data from CERN, their 100k seems to be a flyer (Capture1). I then normalized both data sets to the ideal 50fF.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: July 31, 2021, 12:57:45 am
content: From a time in the past, I found a very good model of SMT 1206 resistor is the resistance in series with a 1 or 1.5 nH inductor, all shunted by a .2 or .3 pF capacitor. This model seems to work well up to about 3-6 GHz.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 07, 2021, 06:30:13 pm
content: Quote from: Joel_Dunsmore on July 31, 2021, 12:57:45 amI would guess that the problem is not your model but my home made cal standards and test fixture. Or, I screwed up the math. Lots of possible mistakes. Feel free to repeat the test using your own math.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 07, 2021, 11:11:15 pm
content: I was curious about using even higher value 1206 resistors.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 07, 2021, 11:27:40 pm
content: I was also wondering how the 0603 package would compare to the 1206. To speed the collection, I went back to the coaxial quick change fixture. They seem very similar.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 07, 2021, 11:30:48 pm
content: Neat!0805 should be different, having a different aspect ratio.TIm

author: joeqsmith
author_rank: Super Contributor
timestamp: August 07, 2021, 11:44:31 pm
content: Just how much fun can you possibly have with a few resistors anyway?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2021, 05:18:40 am
content: Quote from: joeqsmith on August 07, 2021, 11:44:31 pmI used to think not much, but you're beginning to make me question that conclusion...Or you're just mental.Tim

author: David Hess
author_rank: Super Contributor
timestamp: August 08, 2021, 05:19:07 pm
content: Quote from: joeqsmith on August 07, 2021, 11:44:31 pmIn precision and high impedance circuits, plenty!

author: joeqsmith
author_rank: Super Contributor
timestamp: August 09, 2021, 11:24:58 pm
content: Even with low impedances and standard parts, I have kept myself amused. Certainly seems a bit on the spectrum....I was hoping Joel would have added his own twist to the experiment or at least commented on G0HZU's response about the 8753D upswing that Jeroen observed. For some reason, I envision Joel having a home lab with every VNA HP ever made sitting in it or at least every flavor of the 8753.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: August 17, 2021, 03:05:19 am
content: Quote from: joeqsmith on August 09, 2021, 11:24:58 pmI have no home to go to, just always at work.No, I'm not so much of a hoarder (I was until the big fires in 2017 and lost a lot of gear in my lab to smoke damage). Now I'm down to only 4 PNAs, 2 PXI VNAs, a fieldfox and a streamline. and a quite old 8753D (A.K.A worlds best network analyzer).On the up-turn of the data at low frequencies in the measurements: I would suppose it might be mismatch due to the source match of the bridge starting to run out of steam. In the 8753, the bias network and blocking cap started to degrade the match pretty seriously at low frequencies, similarly with the PNA. And the impedance of the DUT measured is quite close to the edge of the Smith chart for hi-Z resistors, so source-match error is pretty significant. Oh, and my earlier comment was mistaken; I looked again and I see more on the order of 0.02-.05 pF for these SMT resistors. (0.2 pF was for a through-hole part I wrote about back in the day). BUT, the real thing about SMT parts is how they are positioned on a line. We did a big study pre 2000 for SMT libraries in ADS, and found that series and shunt mounting made a big difference in the model (most of this was for capacitors but holds for resistors as well). So to model your part properly you need to put in into a configuration that matches your use-case. For example for a resistor used in a Pi pad (shunt R, series R, shunt R) you need to build a shunt verison of your resistor model and a series version of your model, and very importantly, consider the pad configuration. For FR4 dual layer board at about 1.5 mm, the 50 ohm line will be wide enough for the resistor pad, but if you are using 4 or higher layer boards with RF lines on the inner layers, then the whole structure of the final-pad layout will have some strong affect on the model, and even more so if you use the resistor shunt to ground through a via. Even the pad around the via can make a difference, odd as it sounds. For example, two 100 ohm resistors in parallel will give a better match than one 50 ohm to ground, but the shunt inductance (which might limit the impedance at high frequency) is not 1/2 for two in parallel, due to coupling between the two resistors. So 2 resistors in a T-shape to ground is different than 2 resistors hung in parallel of the end of the line. It all becomes very 3-D ish.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 17, 2021, 12:09:56 pm
content: Quote from: Joel_Dunsmore on August 17, 2021, 03:05:19 amI knew it! I can only imagine what it was like before your having to clean house.Your mention of the large SMT library study sounds like what member niconiconi was asking.https://www.eevblog.com/forum/rf-microwave/can-a-siggensadirectional-coupler-be-used-as-a-poor-mans-scalar-analyzer/msg3605548/#msg3605548I was playing around with a few of the free EM simulation tools but didn't get very far with it. It's an interesting subject.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: August 18, 2021, 07:08:02 pm
content: Quote from: joeqsmith on August 17, 2021, 12:09:56 pmBefore the remodeling for the big fire recovery, I as assigned a double cube and 2 benches, it was one of the perks of being a Keysight Fellow (well no, it wasn't, but that was what the lab manager would tell other engineers when they asked why I got the big space; truth was he didn't want to deal with the trouble of making me get rid of all my stuff).After the fire there was a strict 1-cube rule, which since the cube next to me was not so occupied I laid claim to it and yes, the one next to that as well. So my equipment empire is being re-established.Quote from: joeqsmith on August 17, 2021, 12:09:56 pmIn fact I was reading that and posted a link to an SMT bridge from years ago. I'll need to repost to also comment on the original question of measuring the influence of footprints and other parasitic effects. Once you get above 1-2 GHz, you have to have rather precise phase to get a good estimate of reactive aspects such as excess shunt capacitance or series inductance. I don't think a scalar network analyzer would provide the needed information.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 19, 2021, 01:36:19 am
content: So no double cubes, but nothing about only one cube per customer. Cube etiquette.Does Keysight have internal auctions/giveaways for the engineers to help dispose of the fire damages and no longer used equipment?It does seem like they would need a VNA to make that measurement. I was interested in seeing if I could simulate what they had proposed and then test it using actual hardware. I suspect it would take more than my home hobby budget to pull it off.