Frequency response of DMM's

[HKJ]

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.html

I 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.

[BeBuLamar]

So either my Fluke 189 or 289 is good enough to use on audio equipment. Thanks.

[Kleinstein]

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.

[HKJ]

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.

A 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.

[joeqsmith]

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.

Should be easy enough to repeat the test at a few levels. 

[HKJ]

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.

Should be easy enough to repeat the test at a few levels.

It 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.

[joeqsmith]

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.   

[Trader]

My Analog VOM can go up 1.2MHz 

[Caliaxy]

DMM6500 should go up to 300KHz. Hmm...

[joeqsmith]

My Analog VOM can go up 1.2MHz 

My 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/#msg3431804

The AC meter function on my vintage Fluke 97 is spec'ed up to 1 MHz.   

[Trader]

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?

[Caliaxy]

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?

Maybe because they trade off bandwidth for precision? The immense majority of scopes have 8 bit ADCs, i.e. they equate to 256 count meters...

[Trader]

My old analog meter isn't sensitive enough to detect 1VAC.  It's 3dB point appears around 10KHz.

The AC meter function on my vintage Fluke 97 is spec'ed up to 1 MHz.

https://www.eevblog.com/forum/repair/calibrating-a-hp-3400a-rms-voltmeter/msg2787932/#msg2787932

The 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%

[bdunham7]

The 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%

Those 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.

[HKJ]

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.

[Caliaxy]

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?

[paschulke2]

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.

[joeqsmith]

https://www.eevblog.com/forum/repair/calibrating-a-hp-3400a-rms-voltmeter/msg2787932/#msg2787932
The 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%

I think that may have been the first RMS meters I ever saw or used.   

[radiolistener]

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.

[Trader]

https://www.eevblog.com/forum/repair/calibrating-a-hp-3400a-rms-voltmeter/msg2787932/#msg2787932
The 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%

I think that may have been the first RMS meters I ever saw or used.   

Maybe the HP 400E https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/msg2340645/#msg2340645

They are basically the same: https://www.eevblog.com/forum/testgear/hp-400e-vs-hp-3400a-meter/

[joeqsmith]

https://www.eevblog.com/forum/repair/calibrating-a-hp-3400a-rms-voltmeter/msg2787932/#msg2787932
The 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%

I think that may have been the first RMS meters I ever saw or used.   

Maybe the HP 400E https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/msg2340645/#msg2340645

They are basically the same: https://www.eevblog.com/forum/testgear/hp-400e-vs-hp-3400a-meter/

Hard 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. 

[Fungus]

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.

Not 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)

[paschulke2]

Not bad for a meter with a -3dB point at 1kHz:

This is simply badly written by the marketing/advertising department. The spec is 20kHz (2%+20digits) and -3dB @ 1kHz with the low pass filter.

The manual only goes to 20kHz. What exactly is going on? 

The manual states 2%+20 digits error at 20kHz. 2% equals approx. 0.17dB error. 3dB equals approx. 29% error.

(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)

Not 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?

[Fungus]

Not bad for a meter with a -3dB point at 1kHz:

This is simply badly written by the marketing/advertising department. The spec is 20kHz (2%+20digits) and -3dB @ 1kHz with the low pass filter.

Maybe so, that's why I posted the manual page as well. The manual clearly only goes to 20kHz.

Not one of the lines is "flat until over 100kHz".

The line at 1.98V is pretty darn flat.

You did realize that this diagram mainly had the purpose of showing the level dependency...

And 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:

[Caliaxy]

[...] The manual clearly only goes to 20kHz.

That's because the frequency response is reasonably linear at all levels up to (only) 20KHz. They are Fluke, not FNIRSI.

Not one of the lines is "flat until over 100kHz".

The line at 1.98V is pretty darn flat.

I 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.

And 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:

That's *exactly* their marketing strategy: buy a Fluke 87IV for VAC measurements over 20KHz to complement your 87V   

Joke 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).