Modulation impact when amplifying a DSB signal?

Jim Bromley, K7JEB k7jeb at COX.NET
Sun Feb 15 17:48:12 EST 2009

kd4e wrote:
> In AM plate modulation, also screen or other AM modulation,

> I seem to recall the concern that as it is amplified in latter
> stages some of the power of that modulation is lost - thus
> the desire for the highest-level modulation possible -
> despite the bulk and cost of huge modulators.

No, if you follow the low-level-modulated stage with truly
linear amplification, 100% AM modulation comes out at
100%.  The problem is that 100% modulation peaks are
at 4 times the carrier power and many linears begin to
saturate before they get there [ie: become non-linear
amplifiers ;-)  ]  And the modulation "power" and fidelity
is impaired as a result.

But the big hassle for low-level AM followed by linear
amplification is that, although the efficiency on 100%
modulation peaks can approach 67%, the efficiency at
carrier level is much lower, typically 25%.  In AM
broadcasting, at the 50 kW level, this really runs up
the electric bill and burns up tubes in the transmitter.
Plate modulation pays for itself in both areas in that
application, but becomes less cost-effective for intermittent
uses such as amateur radio and ground-air communications.

> Is the same true of DSB or is the modulation level not affected
> in the same way by multiple stages of amplification following
> the modulated stage?

DSB with suppressed carrier is the same as SSB as far as
amplification is concerned.  There's no carrier, so no carrier-
level inefficiencies to contend with, just the issue of linearity
getting up to peak envelope power.

Also, generating DSB with a suppressed carrier might be
more easily done at the milliwatt level than in a high-power
balanced mixer stage.

> I am thinking of this in terms of a QRP DSB exciter using low plate
> voltage (125v) versus having to manage modulation at a higher power stage.

I'm a bit confused here.  Generating clean DSB with suppressed
carrier requires a fair amount of circuitry.  If you really want
AM (DSB plus carrier), why not modulate at the milliwatt level
with something like a cathode modulator or the output-transformer
scheme used in Clegg 22ers and follow it with class-A linear
stages up to the 5-watt level.

Jim Bromley, K7JEB

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