Re: [IRCA] [NRC-AM] Over Mod on the increase?

["Jim Tonne" <Tonne@xxxxxxxxxxx>]

I have been watching this "overmodulation" thread and think that
there may be a problem with observation.

I suspect strongly that junky/bad/unwanted/splattery sidebands
are being observed and the problem is being immediately
(and incorrectly) blamed on overmodulation.

I suspect that the REAL problem is simply distortion in the
modulating circuitry in the transmitter, not actual overmodulation.
Just because a signal splatters over one or more adjacent
channels does not mean the station is overmodulating.
Distorted modulation (caused by a dead modulator tube, for 
example) can easily cause such trashy sidebands.  Knowing 
how little maintenance is being pulled on transmitters 
nowadays, this seems perfectly logical to me.  Or an RF
amplifier tube that has low emission and simply can't produce
the required positive peaks.   The list goes on.

To flush out the answer about whether the station is 
overmodulating or not, simply watch the output of your receiver
I.F. strip on a scope.   Don't look at the demodulated audio.
Look at the "RF" being applied to the demodulator.   I point out 
this little tidbit because even my Collins 51J4 has distortion on 
the negative peaks at modulation levels considerably below 
100% and most radios do the same thing.

And while I have your attention, you would do yourselves a
favor by giving a brief look at an appnote on this subject of
why demodulators distort.   To see it go here
 http://tonnesoftware.com/miscellany.html 
and click on Demodulator.  I suspect you will find the paper to
be an eye opener.

73,  JimT  W4ENE


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