|[AMRadio] Monitoring Modulation|
bcarling at cfl.rr.com
Fri Nov 15 15:03:43 EST 2013
> On Nov 15, 2013, at 1:48 PM, "Donald Chester" <k4kyv at charter.net> wrote:
> 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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