Author Topic: Multimeters cannot measure AM power signal  (Read 614 times)

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Offline worldwideglide

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Multimeters cannot measure AM power signal
« on: July 14, 2021, 12:48:32 pm »
Hello,

I am trying to get an RMS measurement of this waveform:

[attachimg=1]

It's amplitude modulated, fixed frequency @ 1.7MHz, and the amount of AM can vary. None of my multimeters like it (tried fluke and others). So far the only way I can get a measurement is with an oscilloscope on RMS setting, but this is not an ideal setup because we need to be able to take this measurement quickly and not have to set up a scope every time.

Is there an easy way to get an RMS measurement of a signal like this that doesn't involve a scope?

Cheers!


 

Offline TimFox

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Re: Multimeters cannot measure AM power signal
« Reply #1 on: July 14, 2021, 01:46:36 pm »
There are specialized RF voltmeters that can do a true rms voltage measurement, but is that the parameter you want for this measurement?  A common parameter for modulated RF waveforms is PEP = "peak envelope power", which is the mean power associated with the voltage at the maximum excursion of the voltage.  A peak-to-peak responding voltmeter (which can be built easily with two capacitors and two diodes)  is more appropriate for that measurement.
 
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Offline Someone

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Re: Multimeters cannot measure AM power signal
« Reply #2 on: July 15, 2021, 12:18:05 am »
rms of an RF signal, thermal power meter is your "cheap" and simple tool.
 
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Offline f4eru

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Re: Multimeters cannot measure AM power signal
« Reply #3 on: July 15, 2021, 10:04:01 pm »
https://www.google.de/search?q=rf+wattmeter

-> you have to convert back to voltage, but it's the correct tool.


Same but with a disply in volts :
https://www.google.de/search?q=rf+voltmeter
 
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Offline David Hess

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Re: Multimeters cannot measure AM power signal
« Reply #4 on: July 15, 2021, 11:26:20 pm »
A sampling RMS voltmeter can make that measurement.  In principle you can make one using a sample and hold of sufficient bandwidth and any RMS voltmeter.
 
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Offline worldwideglide

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Re: Multimeters cannot measure AM power signal
« Reply #5 on: July 19, 2021, 12:12:53 pm »
I should probably add that what I'm measuring is the voltage applied to a nebulizer (water vaporizer). The 1.7MHz signal is to cause vibration and the AM is to set magnitude of vapor output.

I found another post about this on the forum:

https://www.eevblog.com/forum/beginners/info-about-rf-voltmeter/#:~:text=Assuming%20that%20RF%20stands%20for,anything%20above%20about%201%20MHz.

Looks like the capacitor + diode method may be the best way to do this without buying something expensive
 


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