>> Uh, afaik, the class C amplifier is THE reason for the infamous 30% AM
>> modulation usual in broadcast afaik.
>
> Could you elaborate on this?  I thought the legal AM modulation limit
> was, sensibly enough, 100%.  The little class C transmitter I built
> only put out a few watts, but could easily be driven to 100%
> modulation.  Where does the 30% come from?

When you have (+/-) 30% voltage modulation the power changes between 15%
and 100% peak. (power on an indle AM Tx is 50% peak power afair).
Increasing the index more will bring very little range gain, it will also
reduce the modulation index reserve under 15% which means that very ugly
distortions will happen the moment the index reaches 100% (clipping +
inversion). Also the power on one half of each modulation waveform may be
so low that background unintended radiation will be heard and demodulated
(esp. adjacent channel etc).

In the days when receivers were made of a garnet rock and a sewing needle
on a wooden stand this made perfectly good sense.

The good side is, that if you have a class C transmitter with collector
(or plate) modulation you will only need an AC source that supplies 60%
(pk-pk = +/-30%) of the supply voltage in modulation, in series with the
plate/collector source. This makes it more affordable, especially if you
are moving towards higher powers.

Peter

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.