[AMRadio] Monitoring Modulation


W2XJ w2xj at w2xj.net
Fri Nov 15 15:09:19 EST 2013


The reality is that no broadcaster has ever been cited by the FCC for excessive positive modulation. 145% is common because that is where many transmitters built in the last 30 years run out of steam. These days the FCC has stopped looking at modulation completely, they just look for mask compliance. The 125% rule was only put in place in the 1970s because of the 5KW transmitters built for 200% or more supermod out penetrating 50 KW competitors who at the time were still running plate mod units for the most part that were not easily modified for supermod. These days most transmitters have pretty much equal performance regardless of the power level. 

On the rest I think we agree. 

Sent from my iPad

> On Nov 15, 2013, at 10:48 AM, "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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