[AMRadio] Monitoring Modulation Accurately

Steve WA1QIX wa1qix at piesky.com
Fri Nov 15 08:15:23 EST 2013

This topic has been talked about a lot over the years.

Using a scope to monitor your modulation gives you a good indication of the 
shape of your waveform, but the human eye is just not fast enough to catch 
the peaks nor can you really determine accurately your actual percentage of 
modulation unless you are using a very modern peak storing scope.

If you can see 100% negative modulation on a regular scope, chances are you 
are overmodulating very badly.

Measuring positive modulation on a scope without storage capabilities is, 
again, very difficult because the eye cannot see the fast peaks.

All real modulation monitors (as opposed to the CB "modulation meters") 
have a peak storage and display as part of the normal function of the 
instrumentation.  This is generally in the form of peak flashers, negative 
and positive.  Some more advanced instruments have a "peak hold" capability 
on the modulation percentage meters themselves.  This is really nice to have.

The other thing you get with a modulation monitor is a low distortion, high 
fidelity audio output.  This is very useful for listening off-air with 
headphones or for generating a recording of the demodulated signal.

Personally, I like a scope display but I *need* an accurate modulation 
monitor to show what's really going on and to give a high fidelity audio 
output for the headphones.

For many years, I used to run a modulation monitor and a scope at the same 
time.  Fortunately, with the advent of computers and modern displays, you 
can get both an envelope display and meters together (along with a whole 
lot of other features as well) in one unit.

Regards,  Steve

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