Frequency response of DMM's - Page 1
Forum section: electronics / testgear

author: HKJ
author_rank: Super Contributor
timestamp: May 18, 2021, 11:04:57 am
content: I am comparing a couple of high-end DMMs (Article coming later) and wondered about their frequency response. This is easily checked with my TestController and I decided to check all the meters within easy reach from my bench (That includes bench meters):http://lygte-info.dk/info/DMMFrequencyResponse%20UK.htmlI am lazy, i.e. the test only include meters that connect to TestController and could be tested automatic.All test are run from 50 to 500kHz at 1V.And this time I included both Fluke 189 and Gossen Energy.

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 18, 2021, 11:33:10 am
content: So either my Fluke 189 or 289 is good enough to use on audio equipment. Thanks.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 18, 2021, 11:41:29 am
content: The frequency response of the DMMs in AC mode can depend on the amplitude: there may be a slew rate limit in some cases, effecting a voltage rather close to the full scale. At the low end the analog RMS to DC converters tend to get slower.

author: HKJ
author_rank: Super Contributor
timestamp: May 18, 2021, 12:04:25 pm
content: Quote from: Kleinstein on May 18, 2021, 11:41:29 amA significant factor is the compensation of the input divider, it is a roughly 10Mohm resistor going into a amplifier or ADC with some input capacity. At 100kHz a 1pF capacitor has 1.6Mohm, i.e. without some sort of compensation across the input resistor you are not going to see anything nearly linear.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 18, 2021, 06:03:52 pm
content: Quote from: Kleinstein on May 18, 2021, 11:41:29 amShould be easy enough to repeat the test at a few levels.

author: HKJ
author_rank: Super Contributor
timestamp: May 18, 2021, 06:19:48 pm
content: Quote from: joeqsmith on May 18, 2021, 06:03:52 pmIt is, but I do not want to test all meters again, if there are suggestion of a specific meter that may show these problems I can test it at other levels.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 18, 2021, 11:26:51 pm
content: By far, the fastest RMS meter I have is a recent dumpster dive vintage Fluke 8506A that needed some work. It's a bit of an oddball with the thermal RMS converter but note how the 3dB BW is effected by the range.

author: Trader
author_rank: Frequent Contributor
timestamp: May 18, 2021, 11:49:02 pm
content: My Analog VOM can go up 1.2MHz

author: Caliaxy
author_rank: Frequent Contributor
timestamp: May 19, 2021, 12:31:40 am
content: DMM6500 should go up to 300KHz. Hmm...

author: joeqsmith
author_rank: Super Contributor
timestamp: May 19, 2021, 02:06:45 am
content: Quote from: Trader on May 18, 2021, 11:49:02 pmMy old analog meter isn't sensitive enough to detect 1VAC. It's 3dB point appears around 10KHz. For fun I tried one of the free meters from Harbor Freight. Again, it's not sensitive enough to detect 1V and has a 3dB point around 58KHz.We were playing around not too long ago with some detector probes, attempting to work below 100mV. The first homemade probe I showed was characterized to a GHz. Seems like using an external detector and not RMS is a bit outside the scope but then again if I wanted to look at a fast signal with a DMM, this is what I would be doing.https://www.eevblog.com/forum/rf-microwave/measuring-frequency/msg3431804/#msg3431804The AC meter function on my vintage Fluke 97 is spec'ed up to 1 MHz.

author: Trader
author_rank: Frequent Contributor
timestamp: May 19, 2021, 03:33:36 am
content: A simple and cheap ($65) handheld scope can measure VRMS up to a couple of MHz, why a Bench (and expensive $$$) DMM can't do the same?

author: Caliaxy
author_rank: Frequent Contributor
timestamp: May 19, 2021, 03:48:37 am
content: Quote from: Trader on May 19, 2021, 03:33:36 amMaybe because they trade off bandwidth for precision? The immense majority of scopes have 8 bit ADCs, i.e. they equate to 256 count meters...

author: Trader
author_rank: Frequent Contributor
timestamp: May 19, 2021, 04:24:19 am
content: Quote from: joeqsmith on May 19, 2021, 02:06:45 amhttps://www.eevblog.com/forum/repair/calibrating-a-hp-3400a-rms-voltmeter/msg2787932/#msg2787932The HP 3400A is a true RMS responding AC meter using a pair of internal thermocouples.Voltage range is 1mV to 300V and frequency response is 5Hz to 10MHz.Accuracy is specified as 1% over most of the frequency range.Meter movement is calibrated to better than 0.5%

author: bdunham7
author_rank: Super Contributor
timestamp: May 19, 2021, 04:36:04 am
content: Quote from: Trader on May 19, 2021, 04:24:19 amThose were very expensive in their day, with a special vacuum tube and a pair of thermocouple sensors. With their 1% accuracy on a good day, you may as well just use a DSO--either way you have the equivalent of a 2 digit meter. Modern TRMS DMMs don't go to 10MHz because they are typically 10 times more accurate or better and the two goals aren't compatible.

author: HKJ
author_rank: Super Contributor
timestamp: May 19, 2021, 12:49:07 pm
content: I tried 3 voltage on a meter, here I used the mV range making it possible to run at full input, half input and a low level.Here is comparison of the volt range and the mV range:As expected there is a difference between the V and mV range, there is also a small level depend variation in frequency response.

author: Caliaxy
author_rank: Frequent Contributor
timestamp: May 19, 2021, 05:37:38 pm
content: I'm impressed how sharply the voltage readings drop after 300 KHz in the two Keysight bench meters. That must be deliberate (a digital filter?). I'm wondering why. Maybe to avoid displaying seemingly valid but inaccurate ACV readings at those out-of-spec frequencies?

author: paschulke2
author_rank: Contributor
timestamp: May 19, 2021, 05:40:13 pm
content: Here's the frequency response of a Fluke 87V for input voltages ranging from 20mVrms to almost 2Vrms. "dbV" was calculated from the voltage displayed by the 87V.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 19, 2021, 05:43:25 pm
content: Quote from: Trader on May 19, 2021, 04:24:19 amI think that may have been the first RMS meters I ever saw or used.

author: radiolistener
author_rank: Super Contributor
timestamp: May 19, 2021, 05:45:22 pm
content: Here is my Brymen BM867S and UNI-T UT210E frequency response.Both working stable from 10 Hz, below that measurement is jumping and not stable.UT210E works up to 1 kHz with almost no change and above 1 kHz it starts decay.BM867S works up to 20 kHz with almost no change and after that it starts slowly increasing.

author: Trader
author_rank: Frequent Contributor
timestamp: May 19, 2021, 11:17:21 pm
content: Quote from: joeqsmith on May 19, 2021, 05:43:25 pmMaybe the HP 400Ehttps://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/msg2340645/#msg2340645They are basically the same:https://www.eevblog.com/forum/testgear/hp-400e-vs-hp-3400a-meter/

author: joeqsmith
author_rank: Super Contributor
timestamp: May 19, 2021, 11:55:22 pm
content: Quote from: Trader on May 19, 2021, 11:17:21 pmHard to say. That's been many years ago, when our high end scope was an Tektonix 78xx or 79xx mainframe, our fastest computer was a VAX 11/7xx and we were prototyping on breadboards. Our DMMs at the time were the Fluke 8600A. I think the reason I used that old HP meter was the BW on the Flukes wasn't so great and the Tektronix scope was always at Tek being repaired. The good old days.**While I had my old analog meter out, I looked at the manual for my first DMM, the Fluke 8000A.

author: Fungus
author_rank: Super Contributor
timestamp: May 20, 2021, 06:13:58 am
content: Quote from: paschulke2 on May 19, 2021, 05:40:13 pmNot bad for a meter with a -3dB point at 1kHz:The manual only goes to 20kHz. What exactly is going on?(Yeah, we know, "Flukes are awesome, dude!!!" but it's hard to believe Fluke wouldn't mention it if the response is a flat line until over 100kHz)

author: paschulke2
author_rank: Contributor
timestamp: May 20, 2021, 06:41:06 am
content: Quote from: Fungus on May 20, 2021, 06:13:58 amThis is simply badly written by the marketing/advertising department. The spec is 20kHz (2%+20digits) and -3dB @ 1kHzwiththe low pass filter.Quote from: Fungus on May 20, 2021, 06:13:58 amThe manual states 2%+20 digits error at 20kHz. 2% equals approx. 0.17dB error. 3dB equals approx. 29% error.Quote from: Fungus on May 20, 2021, 06:13:58 amNot one of the lines is "flat until over 100kHz". You did have a look at the scale, didn't you? You did realize that this diagram mainly had the purpose of showing the level dependency of the RMS converter's frequency response?

author: Fungus
author_rank: Super Contributor
timestamp: May 20, 2021, 06:52:04 am
content: Quote from: paschulke2 on May 20, 2021, 06:41:06 amMaybe so, that's why I posted the manual page as well. The manual clearly only goes to 20kHz.Quote from: paschulke2 on May 20, 2021, 06:41:06 amThe line at 1.98V is pretty darn flat.Quote from: paschulke2 on May 20, 2021, 06:41:06 amAnd my point was that it seems weird that nobody at Fluke's marketing/advertising department would ever mention that it can go over 20kHz, no problem. Other Fluke meters/manuals certainly mention it, eg. the Fluke 87 IV:

author: Caliaxy
author_rank: Frequent Contributor
timestamp: May 20, 2021, 02:17:45 pm
content: Quote from: Fungus on May 20, 2021, 06:52:04 amThat's because the frequency response is reasonably linear at all levels up to (only) 20KHz. They are Fluke, not FNIRSI.Quote from: Fungus on May 20, 2021, 06:52:04 amI quickly checked a few datapoints from paschulke2's curves on my 87V and can confirm that 2Vrms at 100 KHz are shown as 1.71Vrms (-1.4dB). Not bad, indeed.Quote from: Fungus on May 20, 2021, 06:52:04 amThat's *exactly* their marketing strategy: buy a Fluke 87IV for VAC measurements over 20KHz to complement your 87VJoke aside, I wouldn't measure anything over 20KHz with 87V (I have other meters for that), but it's good to know that's an option (with paschulke2's curves in mind).

author: PartialDischarge
author_rank: Super Contributor
timestamp: May 20, 2021, 02:46:25 pm
content: Quote from: Trader on May 18, 2021, 11:49:02 pmMeh, my HP 410C goes up to 700MHz. Also is has 100MOhm input for volts measurements, and in current measurements it beats all modern DMMs in terms of burden voltage.Also, the scale in the meter was printed individually for each unit, giving it a extremely good precision

author: Fungus
author_rank: Super Contributor
timestamp: May 20, 2021, 03:06:01 pm
content: Quote from: Caliaxy on May 20, 2021, 02:17:45 pmWhich is precisely why they should document it.eg. A lot of people are looking for a multimeter for audio work but if you read the paper specs the 87V doesn't look suitable but in reality it looks like it might be (based on a sample size of one). Fluke could be capitalizing on that instead of keeping the market for used 8060As and 189s alive.nb. The 87 IV had no problem claiming "100kHz bandwidth!" even though the small print says there's 15% error above 20kHz on the lowest range and 8% on all the others.

author: Trader
author_rank: Frequent Contributor
timestamp: May 20, 2021, 03:47:40 pm
content: Quote from: MasterTech on May 20, 2021, 02:46:25 pmAwesome, I found this in another forum:"The big deal about the RF probe on the 410B/C is that it is rated to be within +/- 1 dB from 20 Hz to 700 MHz, and usable out to 3 GHz. I've used mine for L-band work (950 MHz to 1.9 GHz), and while accuracy did suffer a bit, it told me what I wanted to know."

author: Trader
author_rank: Frequent Contributor
timestamp: May 20, 2021, 03:50:45 pm
content: Quote from: Fungus on May 20, 2021, 03:06:01 pmI'm not sure about that:https://www.eevblog.com/forum/testgear/fluke-87-iii-267380/msg3422654/#msg3422654

author: Fungus
author_rank: Super Contributor
timestamp: May 20, 2021, 04:40:36 pm
content: Quote from: Trader on May 20, 2021, 03:50:45 pm< 3dB down at 125kHz isn't bad for a "20kHz" meter.Maybe Fluke was/is reserving the right to make it worse in the future.

author: Caliaxy
author_rank: Frequent Contributor
timestamp: May 20, 2021, 05:06:14 pm
content: Quote from: Fungus on May 20, 2021, 03:06:01 pmWell, then let's thank paschulke2 for bothering to check.Quote from: Fungus on May 20, 2021, 03:06:01 pmMake it two. As I said, mine behaves similarly.

author: Caliaxy
author_rank: Frequent Contributor
timestamp: May 20, 2021, 05:35:07 pm
content: Quote from: MasterTech on May 20, 2021, 02:46:25 pmYou mean 700 McyclesOne of the most interesting features of your vintage HP 410C (and of my vintage HP 400E, which I mentioned in a different thread) is that it has a DC output where it provides a 0 to 1V (0 to 1.5V in yours) voltage proportional to the displacement of the needle, regardless of the scale. Hook up there a DC digital voltmeter (any DMM with decent DC accuracy would do it) and you get a killer digital AC voltmeter (for AF or RF) with an impressive bandwidth for free. Not sure the digital reading improves the accuracy but it should improve the effective resolution (and eliminate the reading errors).

author: Kosmic
author_rank: Super Contributor
timestamp: May 20, 2021, 06:02:07 pm
content: Quote from: Caliaxy on May 20, 2021, 05:35:07 pmProbably simpler to get a probe that rectify AC to DC. Ex, my Coline M12DM4 probe. Goes from 100KHz to 750MHz and up to 50Vrms (47dBm) in amplitude. You plug it to any regular 10M\$\Omega\$ DC voltmeter and you get 1VDC per 1VAC.The fluke 85RF is similar.

author: PartialDischarge
author_rank: Super Contributor
timestamp: May 20, 2021, 06:41:40 pm
content: Quote from: Kosmic on May 20, 2021, 06:02:07 pmThat probe capacitance is higher I think. Anyway, the good thing about the 410C is that it is a multimeter with all the functions, I have no better tool to measure currents with precision sue to the low burden for example, and I have lots of meters.

author: Kosmic
author_rank: Super Contributor
timestamp: May 20, 2021, 06:55:18 pm
content: Quote from: MasterTech on May 20, 2021, 06:41:40 pmyes a bit higher. The M12DM4 is 5pF and the probe of the HP 410C 1.5pF.

author: Caliaxy
author_rank: Frequent Contributor
timestamp: May 20, 2021, 09:51:28 pm
content: Quote from: Kosmic on May 20, 2021, 06:02:07 pmYes - the passive rectifier probes are great for RF, but not so for AF.I have an 85RF, which is spec'd +/-0.5dB between 100KHz and 100MHz (and +/-3 dB up to 500 MHz), not recommended for actual measurements (other than relative) between 20KHz and 100KHz and not recommended at all under 20KHz. Interestingly, it seems to complement perfectly the AC frequency response of a Fluke 87V, in light of the curves mentioned above (in my 85RF leaflet, Fluke 87III is the recommended companion - no idea what the actual frequency response curve of 87III is).Other disadvantage of these probes is that, being diode rectifiers, they have a minimum voltage input (85RF doesn't work under 250 mV). In contrast, the HP analog meters are amplifiers. The lowest range of HP400E is 0-1mV, which is quite nice.

author: paschulke2
author_rank: Contributor
timestamp: May 21, 2021, 05:54:48 am
content: I still think that 100kHz for the Fluke 87V is too optimistic. My measurements were done with 20000 digits resolution in two ranges: 200mV and 2V. At 500mV in the 2V range, at 100kHz the response is down -5.6dB. IMHO the 20kHz spec for the 87V is correct.