Measuring Equivalent Series Resistance at higher frequencies - Page 1
Forum section: electronics / projects

author: luky315
author_rank: Regular Contributor
timestamp: January 30, 2014, 09:02:06 pm
content: What is the most precise method (better than +-3%) to measure the Equivalent Series Resistance (ESR) from HF Caps (<1nF) and Inductors (<=1µF) in the Frequency range from 10MHz to 50MHz?I need to qualify a bunch of components (I've experiments to run there is research to be done...) and the available equipment is not suitable for this kind of measurement.

author: G0HZU
author_rank: Super Contributor
timestamp: January 30, 2014, 09:11:38 pm
content: The (old) HP4191A from HP is a decent impedance analyser for use over the range of 1MHz to 1000MHz. However, it won't be able to measure a very low ESR to 3% accuracy on some caps if the ratio of reactance to resistance is very high.Why do you need 3% accuracy for ESR measurements?

author: The Electrician
author_rank: Frequent Contributor
timestamp: January 30, 2014, 10:42:52 pm
content: The Agilent 4294 would do it:http://www.home.agilent.com/en/pd-1000000858%3Aepsg%3Apro-pn-4294A/precision-impedance-analyzer-40-hz-to-110-mhz?nid=-33831.536879654.00&cc=US&lc=engYou can rent them for a lot less than the purchase price, fortunately!

author: dannyf
author_rank: Super Contributor
timestamp: January 30, 2014, 10:50:10 pm
content: QuoteIt is fairly unusual to need to do that.QuoteA (milli)ohm meter will do.QuoteA bridge would be the easiest approach.QuoteThen what is "this kind of measurement"? If your measurement is so unique, it is quite possible that no equipment readily available can do it.

author: G0HZU
author_rank: Super Contributor
timestamp: January 30, 2014, 11:46:10 pm
content: QuoteI'm not so sure about that... A bit like the HP4191A I think it will achieve much worse than 3% accuracy for ESR for hi Q components up at 10MHz-50MHz. I'd expect it to be able to measure mag really accurately but the real component of the impedance will be very difficult to measure reliably because it is so small in relation to the imaginary part.I can't think of a valid reason to be able to measure ESR this accurately for typical LC components in this frequency range anyway.

author: The Electrician
author_rank: Frequent Contributor
timestamp: January 31, 2014, 01:10:43 am
content: Of course, that depends on what is meant by High Q. The OP didn't say what he means by that. I probably assumed he wasn't going to be using components with Q too high.The older HP Q meter (4342A) could give 10% accuracy with Q up to 300, and 20% accuracy with Q up to 1000.I've often wondered how extremely high Q capacitors are measured. You can find in the old Bureau of Standards publications measurements of air dielectric capacitors with Q up in the tens of thousands. The techniques they used were cumbersome.

author: Richard Head
author_rank: Frequent Contributor
timestamp: January 31, 2014, 06:30:47 am
content: I've often wondered how extremely high Q capacitors are measured. You can find in the old Bureau of Standards publications measurements of air dielectric capacitors with Q up in the tens of thousands. The techniques they used were cumbersome.I expect that if you shorted out the capacitor with a broad copper strip and connected it to a network analyser (or SA with TG) with very short leads you could find the self resonant frequency.The copper strap would ensure that the self inductance of the cap would dominate the total inductance.Using the cursors to give you the -3dB points will yield the bandwidth and Q = Fo/BW.Dick

author: luky315
author_rank: Regular Contributor
timestamp: January 31, 2014, 07:41:55 am
content: I know that the measurement is a bit unusual, but we need them as sensors in this frequency range. Q is not extremely high. I tried measuring Q and calculating the ESR, but the VNAs from my University are not precise enought. Personally I would prefer to build a nice piece of Equipment, I have time, but not the budget to buy new Equipment (University...)

author: KJDS
author_rank: Super Contributor
timestamp: January 31, 2014, 10:18:10 am
content: When you were using the VNA, which method were you using to measure Q?

author: luky315
author_rank: Regular Contributor
timestamp: January 31, 2014, 10:46:35 am
content: I used the "3dB down" method. But I'm not intrerested at Q. I need the ESR at various frequencies below the self resonance frequency of the DUT.

author: The Electrician
author_rank: Frequent Contributor
timestamp: January 31, 2014, 01:21:21 pm
content: Quote from: luky315 on January 31, 2014, 07:41:55 amHow about some numbers? What is the Q?

author: The Electrician
author_rank: Frequent Contributor
timestamp: January 31, 2014, 02:08:33 pm
content: Quote from: Richard Head on January 31, 2014, 06:30:47 amOne generally wants to use the capacitor at a frequency well below its SRF. Your suggested procedure may find the Q at its SRF, but what is wanted is the Q at a substantially lower frequency.The capacitors in the paper I referred to were air variables.

author: G0HZU
author_rank: Super Contributor
timestamp: January 31, 2014, 02:58:51 pm
content: A typical ceramic cap used at HF will have a very high Q in the region 500-2000 but of course it all depends on the cap value and the frequency it is tested at. So with such a high ratio of reactance to resistance it makes it very difficult to measure the ESR at each test frequency with any great precision (certainly not to +/-3%)Achieving +/-10% accuracy for ESR with a component with moderately high Q would be very impressive IMO. Assuming the OP doesn't want to go to the cost/complexity of the test setup used by the people who make these capacitors then I'd suggest you aim for something like +/- 25% accuracy (but maybe expect to get +/-50% accuracy) for capacitor ESR if you have access to typical RF test gear found in an RF lab.As typical inductors used in this frequency range will have a lower Q than this then it's obviously a bit easier to measure the Q and the ESR of typical inductors but even then you will struggle to get very good accuracy on anything with a decent Q.

author: The Electrician
author_rank: Frequent Contributor
timestamp: January 31, 2014, 09:47:58 pm
content: I must admit that I wasn't thinking of Qs as high as 1000!Perhaps the OP can simply buy suitable capacitors with known (more or less) ESR:http://atceramics.com/Capacitors/3/Multilayer-CapacitorsThere I see a 1 nF cap with an ESR of 10 milliohms at 30 MHz; that's a Q of 530. American Technical Ceramics uses the complex test setup you allude to:http://atceramics.com/Userfiles/esrlosses_appnote.pdfOn a side note, the HP 4191 is a VNA type instrument, that measures reflection coefficient and calculates everything else (such as ESR) from that. They claim a lower "practical" limit on ESR of 100 milliohms. The HP4294 is an I-V type instrument with a lower practical limit on ESR of 3 milliohms, but that is in the instrument's "sweet spot", which 30 Mhz and 5 milliohms is not. The 4294 can do much better for low ESRs in its sweet spot than the 4191.As I said above, the OP's best option may be to buy caps with "known" ESR. In fact, he might be able to get the factory to carefully measure a cap, and then use that as a reference. He could use that cap in a resonant circuit to determine Q of inductors.Or, he could set up a measurement arrangement like ATCs:Quote from: luky315 on January 31, 2014, 07:41:55 am

author: G0HZU
author_rank: Super Contributor
timestamp: January 31, 2014, 10:55:36 pm
content: QuoteWell I guess I'm looking at the problem from the other end of the OP's spec requirements. eg testing a 100pF cap down at 10MHz where the Q could be way over 1000 for a typical/reasonable ceramic capacitor.

author: G0HZU
author_rank: Super Contributor
timestamp: January 31, 2014, 11:16:03 pm
content: I dug out a couple of cheapo 1nF 0805 caps (Murata?) and measured them on my old HP VNA here.I've had to use a low bandwidth in the VNA and some averaging to get the noise down but this shows the limits of my VNA even with the 1nF cap. The trace is very noisy and the absolute figure may not be accurate either.But to add a degree of confidence I also tested two 1nF caps in series and the plot below shows I get 0.04R typical for one cap and about 0.08R typical for two in series. The two noisy traces are the ESR for the single 1nF cap and for the pair in series. The other traces are for the reactance in each case.Down at 10MHz the Q of the cap is nearly 400 (according to the VNA but there will be a fair bit of uncertainty about this measurement!).

author: G0HZU
author_rank: Super Contributor
timestamp: January 31, 2014, 11:40:43 pm
content: I dug out a new 1nF ATC100B cap from my stash of ATC/microwave caps and measured it on the VNA across 10MHz to 60MHz.See below for the result compared to the cheapo cap.I would expect that the true ESR of the ATC cap will be closer to 0.01R rather than 0.02R. I could fit two in series but it's hard to fit them on the jig to prove this! They are in a huge SMD package!

author: The Electrician
author_rank: Frequent Contributor
timestamp: February 01, 2014, 12:49:43 am
content: I'm puzzled. Page 5 in this data sheet:http://atceramics.com/UserFiles/100b.pdfsuggests that ESR should change with frequency, yet your measurement shows an essentially constant ESR. Do you have any idea why that might be?Table 1 in the App note:http://atceramics.com/Userfiles/esrlosses_appnote.pdfindicates that the Rsd component of ESR should increase 10 times for every decade decrease in frequency, yet the graphs in the 100b data sheet show ESR decreasing with decreasing frequency. I wonder where the crossover frequency for the contributions of Rsd versus Rsm occurs for the 1 nF cap.

author: rbola35618
author_rank: Frequent Contributor
timestamp: February 01, 2014, 01:00:08 am
content: Bart, the designer of the Cleverscope, recently came out, or rather say, added LCR capabilities as well as a FRA (Frequency Response Analyzer) to the Cleverscope. I have one and it works very well.LCR analyzerFRA to plot gain and phaseRobert

author: G0HZU
author_rank: Super Contributor
timestamp: February 01, 2014, 01:00:57 am
content: I don't know why it's so flat (for all of them!) but the losses are made up of losses due to skin effect and losses in the dielectric and I don't know how this pans out over 10-60MHz.I did also try measuring a short strip of copper wire. it's about 20mm long and about 0.3mm diameter. You can see the result below. It does appear to show a predictable trend in ESR due to skin effect eg 4xF = 2xESR.TBH I'm really stretching the limits of this old VNA. I have a very precise and well proven cal kit with it but even so, these results can't be treated as particularly accurate I'm afraid...

author: G0HZU
author_rank: Super Contributor
timestamp: February 01, 2014, 01:44:53 am
content: I dug out a 100pF cap in the ATC 800b range and measured it across 100MHz to 1600MHz as per the ATC datasheet.Again, this is stretching the limits of my test setup but see below for the results.The results agree fairly well in the early to section where the ratio of resistance to reactance is low but you can see the data gets ripply and uncertain at the start and the finish of my results as the Q goes higher. Generally, my measurement is a bit high compared to ATC. If I had access to a megabucks 3.5mm cal kit I think the results would be slightly better but I don't really need this accuracy for the type of RF research work I do hereNote: I have to take great care in the measurement setup wrt the jig, the cal kit and the way I use the VNA to get this result. The mag of the reflection coefficient is something like 0.996 on my VNA so there will be a fair bit of uncertainty in these results despite the care I'm taking

author: The Electrician
author_rank: Frequent Contributor
timestamp: February 01, 2014, 10:41:47 am
content: Quote from: rbola35618 on February 01, 2014, 01:00:08 amThere's something wrong with Bart's software. In the first video at about 2:18, he says that the ESR goes negative. This is impossible; ESR never goes negative. If it did, we would have perpetual motion. A negative ESR would be a source of energy, and that can't happen with a passive component like a capacitor.

author: dannyf
author_rank: Super Contributor
timestamp: February 01, 2014, 12:03:08 pm
content: it can go negative, depending on how it is measured.

author: KJDS
author_rank: Super Contributor
timestamp: February 01, 2014, 12:18:36 pm
content: Quote from: dannyf on February 01, 2014, 12:03:08 pmThe ESR of a passive is positive. It's possible that the measurement could read negative. If the uncertainty is say 0.05ohms, and the value is 0.02ohms then you could validly measure -0.03ohms. The true value is, however always going to be positive.

author: The Electrician
author_rank: Frequent Contributor
timestamp: February 01, 2014, 12:53:17 pm
content: Quote from: dannyf on February 01, 2014, 12:03:08 pmIf a negative result is obtained, then the method by which it was measured is either incorrect, or very error prone.

author: dannyf
author_rank: Super Contributor
timestamp: February 01, 2014, 01:21:21 pm
content: Any method employed to measure anything can be incorrect and/or very error prone under some circumstances.Yet, we use them all the time.

author: The Electrician
author_rank: Frequent Contributor
timestamp: February 01, 2014, 03:09:36 pm
content: When you said "itcan go negative, depending on howitis measured.", it was unclear what you meant by "it". Apparently the red "it" refers to the result of a measurement, which could certainly be in error, even a negative value; this is obvious. The blue "it" would appear to refer to the actual value of the ESR, which is never negative. You're using one word, "it", to refer to two different things. A person reading your post could think you were saying that the actual ESR can be negative. I thought so, and apparently KJDS did also.My point was that the video at about 2:18 shows the ESR of the capacitor at low frequency being about 1 ohm and at the high frequency end it reaches a value of about -8 ohms. This is not just a small error; he's doing something wrong.Part of what he's doing wrong is using an ordinary resistor with long leads as his 50 ohm reference at frequencies nearing 100 MHz.Knowing that ESR can never be negative should be a clue that something's wrong when a result of -8 ohms is obtained for ESR.

author: dannyf
author_rank: Super Contributor
timestamp: February 01, 2014, 03:17:54 pm
content: QuoteIn general, resistance can be negative. It is questionable if the ESR of that particular capacitor is negative.Hope it helps.

author: KJDS
author_rank: Super Contributor
timestamp: February 01, 2014, 03:25:40 pm
content: Quote from: dannyf on February 01, 2014, 03:17:54 pmESR of a passive can never, ever, be negative. It isn't questionable if that cap has a negative ESR.Having said that, there are some semis that do exhibit bands of negative resistance, but that's not what we're discussing here.

author: G0HZU
author_rank: Super Contributor
timestamp: February 01, 2014, 07:13:45 pm
content: QuoteI just watched it and I agree that there are lots of technical errors in that video. The person presenting it clearly doesn't have any experience of component theory or practical testing/layout or even how a typical capacitor behaves as it approaches resonance.I learned some time ago that most hobby based tutorial videos involving RF theory/measurement are pretty dire. What's really bad about this one is that the presenter sounds confident about the results he is getting even though the results defy basic physics.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 01, 2014, 09:52:02 pm
content: Hey, defying the laws of physics is a tough job, but somebody has to do it.Long ago I got a nice HP 4815 vector impedance meter, good to 110 MHz. The first thing I learned was how much the test setup contributes to the measurement. In fact, it's almost impossible to design a test fixture that doesn't introduce errors you have to correct for. The second thing I learned was how little I really knew about the HF behavior of components.

author: The Electrician
author_rank: Frequent Contributor
timestamp: February 01, 2014, 10:10:19 pm
content: Quote from: G0HZU on February 01, 2014, 07:13:45 pmI just love the way he has the DUT just lying on the bench, with clip leads and scope probes connecting his box, a full lead length ordinary resistor as his 47 ohm reference and then does a sweep up to nearly 100 MHz! Sheesh! His setup with the BNCs for surface mount components is much better, but it's still not a proper jig.Quote from: G0HZU on February 01, 2014, 07:13:45 pmAfter re-watching it, I notice that at that same spot (about 2:18) he outright says:"If you look at the effective series resistance, it's gone negative....That tells us that it's gone inductive". He doesn't see the fact that ESR measures negative as a sign of a problem; he thinks it's ok, and furthermore allows us to infer that we're above the capacitor's SRF, where, apparently, we shouldexpectthe ESR to be negative.