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True RMS digital meter

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Sure just use a frequency to cal the meters that is in the better accuracy portion of the meter. i.e. 400Hz
 
If there are no frequency response adjustments for the meter circuit, calibrating at one mid band frequency is all that's required. That one frequency should be a frequency should be one that your calibration equipment can verify (400Hz, 1% on the Fluke 115). If there are frequency response tweaks that can be made on the meter circuit you are calibrating, then you might want to have a flat amplitude response oscillator.
A function generator can be a good source for flat, wide frequency, output, but you don't want to use one to calibrate distortion analyzers. at least not for adjusting a distortion analyzers nulling performance.

A low distortion, wide frequency, and leveled oscillator are generally more expensive.

Kind of a rule for calibration equipment is, the reference equipment should be quite a bit better (better performing) than the equipment it is used to calibrate.
 
Jon_Logan said:
If there are no frequency response adjustments for the meter circuit, calibrating at one mid band frequency is all that's required. That one frequency should be a frequency should be one that your calibration equipment can verify (400Hz, 1% on the Fluke 115).
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The though being I'd really like a True RMS DVM that's accurate to 20kHz or even 100kHz to measure voltage across an 8 ohm load for power output tests. Shouldn't have sold my VTVM for $40, but it would need to be calibrated anyway.


-Gregory
 
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A true RMS meter isn't really necessary anyway if measuring sinusoidal waves, is it?

The usually small additional cost typically makes it a no brainer, but it's a technical point because I think there is a lot of assumption that one must have a true RMS meter to get accurate measurement. That may or may not be true depending on what you're measuring.
 
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Gregory said:
The though being I'd really like a True RMS DVM that's accurate to 20kHz or 100kHz to measure voltage across an 8 ohm load for power output tests. Shouldn't have some my VTVM for $40, but it would need to be calibrated anyway.
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Just FYI, you do NOT want to be pushing frequencies of more than 20KHz through any power amp at levels above, say, 1/2 the rated power (and I'd suggest that you limit such tests to a few watts).

The manufacturer's standardized test for power bandwidth is performed at -3db below rated output power (1/2 power). But even this can cause problems at frequencies above 20KHz for the output Boucherot cell on some amplifiers.
 
Yeah, I've also seen some amps get ugly running high power/high frequencies, besides burning the HF load.

Some output stages go into a common mode conduction, where both PNP's and NPN's are on at the same time. I don't mean bias current either. The VariAC current meter starts getting scary.
 
True r.m.s. meters came out to deal with modern frequency drives and the like for electric motors.
If you have a scope, you can convert the peak-to-peak values to r.m.s. for any of the standard waveforms.
Usually your "standard" should be at least 4 times as accurate as your "test instrument".
 
EchoWars said:
Just FYI, you do NOT want to be pushing frequencies of more than 20KHz through any power amp at levels above, say, 1/2 the rated power (and I'd suggest that you limit such tests to a few watts).

The manufacturer's standardized test for power bandwidth is performed at -3db below rated output power (1/2 power). But even this can cause problems at frequencies above 20KHz for the output Boucherot cell on some amplifiers.
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No need to worry.. I won't be driving the amp with any frequency above 20 kHz. For sine wave the RMS is 0.707 x peak value, but I don't have an o-scope. I have TWO cheapy DVMs sold by Sears of Chinese origin, and you can see in the "specs" they are NOT accurate, especially above 60 Hz.

What I DO need is an accurate DVM from DC to 20 kHz for SINE WAVE measurements. Scopes are ok but a DIGITAL READOUT is better!! Can anyone suggest an accurate DVM even if it's from abroad?


-Gregory
 
As is a usual question, what is your budget for the meter?

A Fluke 87 is around $400 and it's 20k, 2%+20 counts.

If you're going to be calculating power to compare against specs (or even just relative between units), extreme accuracy is of question anyway IMO, unless you're going to also get a distortion meter so you can see where you're at relative to distortion. IOW, comparing power from various units at unknown levels of distortion isn't particularly accurate method regardless of how accurate the instruments themselves are.
 
My everyday meter at work is a 88. I have a 190-202 scopemeter but it lives on my bench and does not get used that much.

For home I have an old 27fm that I bought back in 90. It went t back to fluke in 01 for a cal but is still going.

I would take a look at some of the older ones. Make sure and test/check the Ma/A side as I have seen a few that are toast. send it back to Fluke to have it calibrated and there ya go.

I have a couple of cheap meters but other than the basics I do not trust them much. I keep one in the camper just in case.
 
Just for shits and grins I tested one of those el-cheapo $12 meters against our calibration standard (not sure of the degree of accuracy of the standard). To your point, the meter had only basic functions of AC/DC volts, current, and resistance, but bottom line is it was pretty much spot on and plenty accurate enough for everyday basic needs. YMMV, and not necessarily advocating the cheap ones but for a loaner or emergency use they do work.
 
I've been seriously considering buying a GOOD digital meter now. Must admit that while shopping at the nearby Sears Tool department I saw a Fluke 117? which is no long there, nor can be found on the sears dot com site searching for true RMS or even Fluke. Heck if a descent meter can be purchased at Sears then I'm game. Any suggestions? they have lots of models available.

Again.. what is a 'keeper' Fluke model that doesn't run 500 skins? what about the BK Precision 2831E bench model ~300 skins? The 287 and 289 are just a bit much for my needs.


-Gregory
 
An old HP 400E voltmeter will do what you want, with an analog meter. Measures AC RMS Volts 10 Hz to 10 MHz. Ones on ebay usually need the internal rechargeable batteries replaced though.

A Fluke 87-V will measure RMS AC voltage accurately to 20KHz, and it will measure the frequency as well. Used ones go for around $150-250. This would definitely be a more reliable and hassle-free option than the HP 400E. The Fluke 117 is only spec'd to 1 KHz AC.

I think what you'd really want though is a source/measurement combo device. An HP 8903A or 8903B will generate a very clean sinewave from 20 Hz to 100 KHz, and measure the distortion, level, and frequency of the input signal. You can use just the input to measure arbitrary frequency/level/distortion of any signals. These usually go for around $500 or more. There are other similar devices from other brands too.
 
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