A "MEAN" Meter - Part 3

The Challenge:
To accurately measure the net mean or average voltage value of a high frequency wave form using a common and inexpensive digital voltmeter set on "DC Volts".

Many would not believe it could be done, and they would not try it.

For their benefit and for all others that enjoy learning new ways to make measurements, this is for you.

In Part 3 I challenge the scope and meter a little more with a frequency swept sine wave between 250kHz and 1MHz. The modulation rate is about 100 to 200Hz. Without heavy averaging applied to the input signal, the scope's "mean' voltage measurement is not useful. The meter out-performs the scope's limited capability in handling this aperiodic wave form.

Regards,
.99

Category:

Science & Technology

Tags:

• mean
• meter
• 03

[poynt99]

You're welcome ShereKhan, and thanks for your comments.

I was using a Tektronix TDS784A 1GHz storage scope.

[mdzacharias]

IMO the 'scope is simply able to sample at a much higher rate. The multimeter is just slow, that's all.

Multimeter Junkie

[poynt99]

@mdzacharias

Yes, this is precisely what is happening. But remember that the scope is then taking these many samples and doing the math in real time to come up with an average. The meters do this inherently, and they do a much better job of it.

.99

[TinselKoala]

Nice job as usual.

It looks like the scope is doing averaging not by taking some numerical value from the vertical scale of the signal -- which would correspond to an average voltage like the meter displays -- but rather is averaging entire screens of waveforms, then applying the numerical "average" to that resulting average waveform.

So the waveform averaging function is different from the numerical voltage average. The numerical average now only sees the one cycle at center freq.

[poynt99]

Thanks TK.

Yeah the two averaging functions are different.. The averaging function I applied was to the acquisition of the samples. The scope uses the no. of samples specified and averages them. Normally the scope only acquires and uses the first sample.

Clearly, although very useful, the MEAN and CycMEAN functions have their useful limits. I don't foresee the need to ever take the mean value of a FM wave form imo. It was an interesting stress test for both though.

[ShereKhan77]

i grew up watching my grandfather BUILD and USE a room full of wonderfull equiptment the screaming of squelch and watching ociloscops he bult was awesome.as a matter of fact i think its why i became a fan of digital music.its all about the machines its played on and the squelch of the vocals comming in.those were the days.thanks .99! are you using a program oscilloscope?

[OUTRUTH]

I died of an overdose of boredom before reaching the end. Send the body snatchers.