Cost effective PDN impedance measurements? - Page 1
Forum section: electronics / testgear

author: Feynman
author_rank: Regular Contributor
timestamp: February 26, 2021, 07:37:22 am
content: I'm looking for a cost effective setup to measure the impedance of power delivery networks (PDN) on PCBs.Requirements for my PDNs are usually not very high. I have no big FPGAs or huge DDR busses switching, so I don't need particularly high accuracy. Most of the time I just have some mixed-signal micro-controller stuff. I just want to get a feeling where the anti-resonance peaks are and how accurate my SPICE simulation of the PDN is to avoid EMI problems at an early stage.Frequency range should be somewhere between a couple of kHz up to max. 1GHz so I can measure the voltage regulator and bulk caps on the lower frequencies, any plane-capacitor-resonances at higher frequencies and bypassing caps in between. Impedance range should go down to maybe 10 m\$\Omega\$ or less.I don't want to spend 20k$ for a VNA and 5k$ for probesBut I could invest something up to 5k$, e. g. for a spectrum analyzer or a low-end VNA.Has someone experience in such PDN measurements and any tips what equipment and accessories to use?Regards,feynman

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 26, 2021, 12:44:06 pm
content: NanoVNA is probably the best bet. As for probes, ...?!Tim

author: Pitrsek
author_rank: Regular Contributor
timestamp: February 26, 2021, 06:05:52 pm
content: Couple of Khz might be too high to see effects of PSU output impedance. Indeed you don't need VNA, SA with tracking generator will do the job as well. But it needs to go low enough. The most cost effective solution would be probably Signal Hound SA + tracking generator(cca 1600 usd). You do not need to go that high in frequency, the limiting factor will be package inductance(sometime described in ibis SI models). Even if you provide perfectly low and flat impedance from zero to infinity, package inductance will be the blocking point from certain frequency upwards. Decoupling will be provide by package and on die capacitors. The plane resonance is spatial issue, I've never needed to investigate those. Another very reasonably priced instrument would be Bode 100. You trade frequency range for much bigger versatility (psu stability measurements, filter transfer functions). I've been using Bode 100 for quite some years already. It is very nice tool with great SW. In all that time, it was only once that the bandwidth of Bode 100 was not enough for a PDN issue. Every time I check the prices this is the most affordable low frequency VNA.As for the probes - if your design allows, put two sma or mmcx connector, directly connected to power plane on the pcb. No probes needed, direct cable connection. Calibration is through only anyway. If there is no room, you can make probes from semi rigid cable/sma connector and sewing needles or pogo pins for test fixtures. Picotest have just released updated PDN probes, but they are IMHO way overpriced. If you do not have room on the pcb, you can take thin coax and solder it directly to capacitor pads. You have to remove the capacitor, otherwise the measurement will be influenced by the capacitor ripple current. Personally I prefer mmcx connector for all high speed test point, small, cheap and hands free probing.As for the accessories - for 10m and less, preamp will be handy, and you will need something to break the ground loop on the input(braid error). Either common mode choke or diff amp. Common mode choke - take the core with highest permeability, and run as many turns of coax through it. I used some cores from Vacuumschmelze. This did attenuated braid error to low hundreds Hz. If you need to go lower, diff amp is the way. I built one myself, text book 3 opamp version with some high speed opamps - it also provides 20dB of gain.For reading I'd recommend Istvan Novak, Steve Sandler and Omicron appnotes.

author: Feynman
author_rank: Regular Contributor
timestamp: February 28, 2021, 09:40:21 am
content: Quote from: Pitrsek on February 26, 2021, 06:05:52 pmMain priority for me is measuring bulk and decoupling capacitors. Measuring the voltage regulator would be a bonus, but its not required (most of my boards don't have their own voltage regulators).I'm currently looking at the FPC-1500 from R&S which has a tracking generator and a built in 20dB pre-amplifier. This instrument would be handy for other EMI pre-compliance measurements as well.What confuses me a little is the tracking generator specification in theR&S datasheet(page 6):"Frequency Range" is specified from 4kHz to X GHz (depending on the option), but "Output power" and "Frequency Response" are only specified down to 2 MHz.I'm wondering if this tracking generator might be useful for PDN impedance measurements.If the TG can't output anything below 2 MHz, this would be useless, of course. I'm guessing that the TG outputs frequencies below 2 MHz, but with unspecified performance. Is it maybe still possible to do PDN measurements in the lower frequencies by normalizing the measurements with a short between TG and SA input? Measurements down to 4 kHz would be adequate for my purposes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 28, 2021, 01:24:07 pm
content: Huh, well then I don't know where 1GHz+ comes from -- regular PDN stuff basically runs out at 100MHz or thereabouts, where pin inductance dominates, even for BGAs (which have onboard bypass for this reason). Which means probing is a heck of a lot easier, just, say, tack on a piece of coax to a convenient footprint and use a nice stack of ferrite beads on the cable to deal with ground-return inductance (which isn't perfect, remember there's trace/via inductance from the pad to the plane).And power supply (control loop) stuff rolls off in the 10s to 100s kHz, and most PDN action (cutoffs/resonances) in the low MHz, easy to probe.Planes can resonate into the GHz as suggested in the OP, but perhaps there is another way to measure this, besides direct measurement at component pads.How about... if it's just passive behavior of the PCB and network, excepting devices -- the PCB itself (stuffed with chip passives, but nothing bulky like tantalums or electrolytics; which could perhaps be substituted with resistors for the test?), could be placed inside a stripline jig, and any resonances in the board will register as notches in the stripline's passband.The stripline would be like, a modest width ribbon of copper foil/braid/PCB stock, strung between two BNC jacks inside a low metal box. Adjust the box height, strip width, and strip end taper, to get close to 50 ohms or whatever, either when loaded with a test item or unloaded, whatever the case may be. Obviously you can't account for any width/length of test board, you'll have to factor that out somehow. Anyway, slide the board around, under the stripline, watching how the passband changes. There should be a few dips/peaks corresponding to reflections off the leading and trailing edge of the board; these will change position as you rotate the board (thus making a longer diagonal chord across it, or spanning the long/short side if it be rectangular, etc.). As you slide the board lateral to the strip, you should see peaks and dips corresponding to PCB structures; any that may be particularly objectionable, in terms of radiated emissions, should show up as a sharp notch in the response.This will tend to miss resonances that aren't coupled well to external fields: say an internal trace makes a stub length, and only via's to a single pad on the surface. It's internal, so that stub is shielded by planes -- it could have a fairly high Q, or couple into internal waveguide modes*, and be missed in this test. Well, the argument is that it's not going to be an emissions problem; whether it's a functional problem, you'll have to make a special-case test, probably with whatever components connect to that structure. And hey, maybe it's intentional (e.g. buried stripline filters), who am I to judge?*Mind, these are very low impedance, due to the narrow spacing between planes and dielectric constant; and the Q isn't great, due to FR-4 being the dielectric. It does exist though, so for example it's a good excuse to stitch or bypass planes together reasonably frequently, and more-or-less randomly over the board rather than making a grid pattern (which selects waveguide modes). Or bypassing only locally at loads. Which, to be clear, a random pattern isn't going to prevent modes, it just randomizes them. Presumably that gives better statistics... but it would be cool to actually see a model proving whether that's actually helpful or what...So, I have no idea if this is actually a test anyone does, or has thought of before; the finished-product version is a TEM cell. This is just a localized partial-assembly level test, on the same premise.And, I'm thinking s12 (transmission) would be the way to go, but it really shouldn't be much different with the far end terminated, and reading just s11 (basically, backscattering).And, it could be done with pulses instead (TDR), in which case you might miss the high Q notches/peaks as a few pixels of ripple, but that might be valuable for other reasons, say checking continuity of planes, coupling or damping of signal traces, etc.Signal traces, right, you'd have to replace most components with representative driving resistances. Well, that's probably why this isn't a thing, huh...Tim

author: Pitrsek
author_rank: Regular Contributor
timestamp: February 28, 2021, 08:22:18 pm
content: That is the thing, most of the reasonable priced SA/VNA do not go low enough. Even if your current project does not have regulator onboard, I'd suggest that you have a setup that is able to measure it. If you can't measure output impedance you cant properly size the capacitors to match it and have flat PDN. Also in lot of cases you have a nasty resonance between the smps/ldo and bulk capacitance. All the datasheets with 22uf of capacitors on the output, the PDN profile is ugly.. They want to have nice load regulation numbers in datasheet, so there is a very strong feedback loop. With lot of gain, they need to roll off the feedback rather soon, so output impedance turns inductive. By the time you meet the impedance of the capacitor, the LDO/SMPS output impedance is purely inductive. L parallel with C, MLCC have very low ESR, the LDO as wel, Q can be rather high...If you did not specified "10 mΩ or less", I'd suggest trying out one of the new scopes with Bode functionality. Given the limits of the ADC, I'm not sure that it would be up to the task, but I have never tried that.Let me ask from the other side, what do you have at your disposal(scope,SA)?Is this for home lab, one man show or company environment?If it is for home lab, there are some older HP FRA/VNA that might fit the bill, they go higher than Bode 100, but are rather big.I'm looking at SA/VNA to complement my Bode 100 - mainly for EMC and some RF design stuff.

author: Pitrsek
author_rank: Regular Contributor
timestamp: March 05, 2021, 05:47:42 pm
content: https://www.signalintegrityjournal.com/articles/1976-a-low-cost-capacitor-characterization-systemSeems to fit the bill. surprisingly there is no braid error

author: precaud
author_rank: Frequent Contributor
timestamp: March 06, 2021, 01:46:06 pm
content: I agree with everything others have said. If you have room for it on your bench and can live with 200MHz freq limit, the HP 3577A is the best bang-per-buck for such measurements. As T3sl4co1l said in detail, for most situations, bulk capacitance dominates from ~20kHz and up. If you use that as your lower-freq limit, it simplifies the measurement setup greatly, as you don't need to power up the board to evaluate it, and hence you don't need to protect the VNA inputs from the DC. For most accurate results, I find it necessary to bring the data into a computer and do Open/Short/Load compensation on it. The Bode 100 has that built-in to its software.

author: precaud
author_rank: Frequent Contributor
timestamp: March 06, 2021, 02:03:15 pm
content: Hello Pitrsek, I hope you're well.Quote from: Pitrsek on February 28, 2021, 08:22:18 pmI have experimented with it, and for even 10 mΩ at higher freqs you need lots of speed + bits + averaging to approach the accuracy of a swept analyzer. 100MSa 10-bits is good to maybe 500kHz. 12 bits would get you to a few MHz, and 14-bits maybe 10MHz. OSL compensation is absolutely necessary. Above 10MHz higher sampling speeds are needed. A swept analyser ends up being easier to use, gives more repeatable results, and takes less time.

author: Feynman
author_rank: Regular Contributor
timestamp: March 06, 2021, 07:05:19 pm
content: Quote from: Pitrsek on February 28, 2021, 08:22:18 pm"10 mΩ or less" is not a necessity at all. An impedance range down to somewhere between 100 and 10 mΩ would be totally fine, as far as I can tell from my basic knowledge right now. I would love to have a Bode 100 in my lab, but I don't have much use for its versatility at the moment (I have a scopes, but no SA, yet).Right now, by far the most useful addition to my lab would be a SA I can perform some basic PDN impedance measurements with. So that's the route I'm looking into currently. Main goal is to verify that my choice of decoupling and bulk capacitor values make sense. Or, if they don't make any sense, to see where the anti-resonant peaks in my PDN are

author: Someone
author_rank: Super Contributor
timestamp: March 07, 2021, 03:29:05 am
content: Quote from: Pitrsek on February 26, 2021, 06:05:52 pmGood post all round, the accessories applies equally well to a scope or SA. I've had good results up to 10MHz using a signal generator + scope. Rather than probing the sourcing voltage, a current clamp in the injection current eliminates most of the errors introduced by the isolation choke/transformer/blocking cap etc. Isolated source and isolated measurement (both sides) really helps see down into the small details.Some of these methods are expanded on here:https://www.signalintegrityjournal.com/articles/1060-preamplifier-options-for-reducing-cable-braid-loop-errorQuote from: precaud on March 06, 2021, 02:03:15 pmI'm not sure bits is the right metric here, the signal noise (floor) is related to the impedance magnitude rather than frequency. 1mΩ and below is entirely possible without using a SA but the acquisition times grow quickly to get a narrow bandwidth and keep the noise down.Long Scope Capture (high-res/averaging mode optional) -> FFT -> magnitude/phaseStep and repeat for each frequency of interest and it gets slow to produce the full plot!

author: precaud
author_rank: Frequent Contributor
timestamp: March 07, 2021, 01:24:44 pm
content: Quote from: Someone on March 07, 2021, 03:29:05 amIt's not *the* metric, but if you tried it, I'm sure you would find as I did that more bits is absolutely important when resolving signals at or near the noise floor at higher freqs. Cleverscope found the same thing. And in many cases the # of bits sets the noise floor.

author: Someone
author_rank: Super Contributor
timestamp: March 07, 2021, 10:55:57 pm
content: Quote from: precaud on March 07, 2021, 01:24:44 pmTalking like I've never made such measurements? [ s]you sure are polite.Conflating frequency with signal level is a fundamental mistake. They should be clearly explained so that people who are less familiar can learn something and not end up with stupid "rules of thumb" they once heard, don't understand, but regardless follow blindly.Frequency; limited by frequency response of measuring tools/calibration/etc.Magnitude; limited by noise at the low end and maximum signal measurement/minimum source at the high end.That measurement signal range can be impacted by far more than just "bits" (often a marketing headline that is completely disconnected from the noise performance of the instrument). Rather than a lower noise single instrument adding a low noise amplifier at the measurement might be all thats needed to see the smaller signals, or a power amplifier for the source.There is oversimplification, misleading/distracting specs, and outright lies. You're heading off into the latter.

author: precaud
author_rank: Frequent Contributor
timestamp: March 08, 2021, 12:41:21 am
content: I'm not conflating anything, I was simply sharing my experience. I was going through this with Bart at Cleverscope to see if I really needed to get the 12- or 14-bit converter option. He may have a video posted that demos it. The takeaway was: Keep all else constant, and more bits will extend the accuracy of low-level measurements, such as made doing FFT-based FRA for very-low-impedance measurements, into higher frequencies. I ended up sticking with the 10-bit and getting a different instrument to do higher freqs.

author: Someone
author_rank: Super Contributor
timestamp: March 08, 2021, 12:58:18 am
content: Quote from: precaud on March 08, 2021, 12:41:21 amSo the takeaway was some specific Celeverscope products have different noise performance with different models, that also have a different number of bits. An arbitrary n bit acquisition system can have lower noise than another n+m bit, for the specific signal. The specific (pre)amplifier gain settings/ranges usually dominate those effects when compared across different products. You can't generalise your narrow experience to the wider range of products available.Comparing on bit depth is nonsense, the metric is equivalent noise level (input referred) considering/comparing/accounting for the bandwidths of that measurement in the different instruments (and how it may change with frequency!).

author: precaud
author_rank: Frequent Contributor
timestamp: March 08, 2021, 01:12:49 am
content: Whatever, dude. I'm not arguing theory. My recommendations to the OP stand.

author: Pitrsek
author_rank: Regular Contributor
timestamp: March 09, 2021, 08:35:37 pm
content: Quote from: Feynman on March 06, 2021, 07:05:19 pmThe very same parallel resonance peaks that you have between bulk and 100nF can be had between LDO/SMPS and bulk caps. The usual 9khz cutoff of affordable modern SA is a bit high for my taste for PDN measurements. Also the tracking generator starts a bit higher. For those reasons, I do not see much options. The mentioned signal hound is not without caveats:https://signalhound.com/support/forums/topic/discontinuity-issue-in-scalar-network-analysis-using-sa44b-tg44a/I must say that at VNWA 3 seems rather attractive.WRT to scope implementation - if we start to talk long averages, the vector average/cross correlation might help quite a bit. Ie. spiting the measurement signal in two and feeding tow channels. If you have 4ch. scope, the two channels are unused anyway. Theoretically you can get up to 20dB of dynamic range. provided that your acq. path is not highly correlated and you wait long enough. But for that one would have to calculate own fft to have access to complex spectra(I assume that fft export from scope is magnitude only).

author: Pitrsek
author_rank: Regular Contributor
timestamp: March 09, 2021, 08:45:07 pm
content: In case you'd like to build something...https://www.golac.fr/network-analyzer-150mhz/

author: virtualparticles
author_rank: Regular Contributor
timestamp: March 09, 2021, 10:17:03 pm
content: Here is a decent article on the subject. The VNA used goes down to 9 kHz. If you check the PicoTest website, there is quite a bit of information about measuring the stability of an LDO.https://www.signalintegrityjournal.com/blogs/8-for-good-measure/post/1344-using-a-vna-for-power-plane-impedance-analysisThis article also shows how to make your own probes. Search for "Pogo Pins" to find the ground pins on this. You can make an on-board calibration kit which will work up to a couple of GHz, Open, Short, Load.Here is a Webinar done by Steve Sandler who is the guru on this subject:https://coppermountaintech.com/webinar-vna-impedance-measurement-for-power-distribution-networks-form-page/Cheers!Brian

author: jmw
author_rank: Frequent Contributor
timestamp: March 10, 2021, 07:47:07 am
content: Quote from: Feynman on February 28, 2021, 09:40:21 amThe TG lower limit on the FPC-1500 is 5 kHz. The 2 MHz figure is only when using the VNA mode; it can only do S11 measurements down to 2 MHz, so S21 measurements (scalar only, it can't see the phase of S21) get capped to this range.

author: Feynman
author_rank: Regular Contributor
timestamp: March 30, 2021, 11:17:21 am
content: Quote from: jmw on March 10, 2021, 07:47:07 amHmm. So is scalar S21 measurement down to 5 kHz possible or not?Isn't a TG measurement the same thing as a scalar S21-measurement?

author: jmw
author_rank: Frequent Contributor
timestamp: April 01, 2021, 05:20:40 am
content: Quote from: Feynman on March 30, 2021, 11:17:21 amYes, they are the same. The instrument can be run in "spectrum analyzer mode" (which can then be used with or without the TG) or "VNA mode", and the lower frequency limit is different, owing to the capabilities of the VSWR bridge. You can do scalar S21 measurements down to 5 kHz, you just have to be in the "spectrum analyzer mode".

author: Feynman
author_rank: Regular Contributor
timestamp: April 01, 2021, 05:40:21 am
content: Quote from: jmw on April 01, 2021, 05:20:40 amAh, okay. Got it nowThanks!

author: UnDerKetzer
author_rank: Newbie
timestamp: June 09, 2021, 12:36:46 am
content: Siglent SVA1015X

author: tautech
author_rank: Super Contributor
timestamp: June 09, 2021, 01:29:50 am
content: Quote from: UnDerKetzer on June 09, 2021, 12:36:46 amWelcome to the forum.Except for the SVA1015X in VNA mode is only rated to 10 MHz min frequency whereas SVA1032X is 100 KHz.They will operate below those frequencies but accuracy is not guaranteed.https://int.siglent.com/u_file/document/SVA1000X_DataSheet_DS0701X_E04B.pdf

author: joeqsmith
author_rank: Super Contributor
timestamp: June 09, 2021, 12:03:09 pm
content: Quote from: Feynman on February 26, 2021, 07:37:22 amMaybe one of the Signal hound products. The software interface is well documented. I have their BB60C and have written custom software for it.https://signalhound.com/products/usb-tg44a/https://signalhound.com/products/usb-sa44b/I have a few vintage VNAs and not one would meet your requirements. At least in the $5k price range, I am not sure what you could find on the used market.I have a NanoVNA V2+4 that has fairly decent performance above 300MHz but the firmware I have limits it to 50KHz. If they were to open this up, I am not sure how well it would perform.The NanoVNA I have falls short of your 2kHz lower end. There is firmware that will allow it to run down to 10kHz but mine will start to get fairly below 20kHz. You 10mOhm would really be pushing it. Performance above 300MHz is poor.I am in the process of putting together some software and peripherals for it including the DC blocks and transformer. You can find more details about that in the following two links:https://www.eevblog.com/forum/rf-microwave/nanovna-custom-software/msg3578610/#msg3578610https://www.eevblog.com/forum/rf-microwave/impedance-measurement-with-vna-using-series-shuntseries-through-methods/The software to make these measurements is not yet available. However, I did attempt to measure the ESR of a popcorn RF capacitor using the current software with both the NanoVNA and the V2Plus4. There's a little background starting about 5min in, but the actual parts I wanted to show starts about 11mins. This may give you some idea how poor the performance of these low cost VNAs would be. Keep in mind that first gen one was $50 and I think the V2+4 was about double that. From that standpoint, I'm very impressed with their performance.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 28, 2021, 05:55:56 am
content: Here is the entire setup.