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As I am settling on a design for a radio I need to figure out one thing:

Should I modulate on low power and boost it up with a post amp, or should I go on full power right out of the gate?

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  • $\begingroup$ This seems to have two questions, one of which may not be answerable ("needs clarity" is the current parlance). Can you edit it to just the one for which we might be able to offer "one best answer"? $\endgroup$ – Zeiss Ikon Jan 14 at 16:45
  • $\begingroup$ I fixed it @Zeiss Ikon $\endgroup$ – Ben Madison Jan 14 at 16:50
  • $\begingroup$ Nice. That looks like an answerable question now. $\endgroup$ – Zeiss Ikon Jan 14 at 16:55
  • $\begingroup$ What mode(s) do you intend to use with this? AM, CW, SSB or something else? That is important. $\endgroup$ – Mike Waters Jan 14 at 18:41
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    $\begingroup$ Ben, it would also be good to know what you consider "high power"; that's a very stretchable term in amateur radio, and the orders of magnitudes of power that you're planning to work with make all the difference to a radio design. Also, the same applies to the frequency range you're working with. $\endgroup$ – Marcus Müller Jan 14 at 21:08
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All modes @ Mike Waters

So, clearly, then your only choice is to modulate at low powers and then amplify the modulated signal:

Devices that affect a high-power signal do exist (for example, high-powered frequency mixers), but they are inherently somewhat mode-specific. (or very high in loss, which means you'd never want to use them at high powers.)

You might want to share your design so far in future questions!

For example, there might be a few exceptions to the above rule, but only for very specific power/frequency/flexibility/complexity/cost tradeoffs, which I don't think apply to you (and which to discuss out of blue air would be far too open-ended).

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Pretty much every design I've looked at in detail (I'm not a radio engineer or electronic designer, but I've looked at circuits with an eye to building them) does the modulation before any power amplification.

The way I understand it, the modulator can be less costly when its modulating a milliwatt signal than if you try to modulate, say, a hundred watts -- but the amplifier (if well designed) doesn't care if it's boosting a pure RF sine wave or a modulated signal. That means, for the simple case of an AM transmitter, you'd generate the RF in the oscillator stage, modulate the output of the oscillator, then amplify the result.

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