[AMRadio] Another mod monitor angle


CL in NC mjcal77 at yahoo.com
Fri Nov 15 16:59:35 EST 2013


Back in the 'olden' days where the average affordable scope had a BW of 1 MC,  The FAA used a 25 - 500MC mod meter which was the equivalent of a direct conversion receiver with basically 2 outputs.  One was about 20KC, really a beat note offset by mod meter tuning,  and fed to the vertical amp of the scope, the other was detected audio that went to the horiz. amp.  The end result was a nice trapezoid pattern, which, when you are wanting to maintain a textbook 100% on peaks, can't be beat, and is also the best way to determine mod % with voice modulation since the sharp point at the apex of the trap pattern is easy to evaluate.  The waveform display can lead to personal opinions as to the Emin point and usually makes for an inconsistent determination of percent mod while looking at voice.  This external unit was not as accurate as hooking the horiz. to the modulator to help in ID'ing phase problems, but it would reveal some issues with improper audio. 
 Now, in the modern era, if you have the cash, this little unit:  

http://www.waynekerrtest.com/global/html/products/modulationanalyzers/257.htm 

will do the same thing automatically through the air of via a pick up. Outputs on the back feed the scope, for trap or waveform display, the meter is only useful for a test tone, it is not a peak reading, unless they improved the unit over the last several year.

Charlie in NC


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