[AMRadio] Monitoring Modulation


Donald Chester k4kyv at charter.net
Fri Nov 15 13:48:53 EST 2013


From: W2XJ <w2xj at w2xj.net>

>>... I always trust a scope before a mod monitor. Properly clipped negative
peaks are easy to see and the absolute value of positives are not important.
In broadcast it is common practice to modulate as much positives as the TX
can take and in amateur operation a peak power measurement is the only thing
that really counts. >>

I don't fully agree. 

In broadcasting, the positive peaks are limited to 125% positive by FCC
rules, and FCC inspectors are extremely picky with broadcasters. There is no
such limitation in amateur operation. If there were, then SSB would be
illegal, since there is no carrier at all and therefore ANY modulation would
be "in excess of 125%".

With AM, you NEVER want to reduce modulation percentage to limit "peak
power"; the total sideband power of a full-carrier AM signal is low enough
as it is.  What  "really counts" is whether or not the positive peaks are
flat-topping as a result of exceeding the modulation capability of the
transmitter. Flat-topped positive peaks generate precisely the same kind of
splatter and distortion as does overmodulation in the negative direction.
Modulate as much as you can without  exceeding 100% in  the negative
direction OR seeing flat-topping on positive peaks.  I always try to have
enough modulation capability built into the transmitter to allow me to
modulate to just shy of 100% negative, and then let the positive peaks go
where they may with the natural waveform of my voice, without any danger of
flat-topping. If you are that concerned about what the power level hits on
an occasional voice peak, then for God's sake, reduce the carrier level and
make sure you are still fully modulating what carrier you have; don't run a
hefty carrier that is less than fully modulated as described above.

Don k4kyv


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