What is low level AM modulator?

Answer 1

Historically, AM modulation has always been applied to the final power amplifier stage of the transmitter, just before it goes to the antenna. The reason for doing it this way is that once it's modulated, the composite signal can't be handled by any non-linear devices; any non-linearity generates new frequencies, which would have the effect of introducing audible distortion into the modulated signal.

The disadvantage of AM-modulating the final amplifier stage is that doing so requires audio power equal to 50% of the carrier power ... if you're running a 50 KW AM radio station, you need 25 KW ( ! ) of clean audio to modulate it with. You know what 100 watts of audio for your Home Theater or your car costs, so you can imagine what 25 KW of broadcast-quality audio, or even 5 KW, must cost.

Low level modulation means modulation applied to a low-power stage before the power amplifier boosts it to the full transmit power level. That's obviously a great benefit, because you don't need nearly as much audio power, so the audio stages are much less expensive. BUT ... it means that every stage after modulation must be a highly linear stage, in order to avoid distortion. So now, instead of a husky Class-C final power amplifier, it has to be a Class-A (I think maybe Class-AB also works). Class-A for a final means much lower efficiency, so it has to be bigger, and use more DC power supply than the old Class-C needed, in order to send the same power up to the antenna. So it's a cost trade-off.

Answer 2

Historically, AM modulation has always been applied to the final power amplifier stage of the transmitter, just before it goes to the antenna. The reason for doing it this way is that once it's modulated, the composite signal can't be handled by any non-linear devices; any non-linearity generates new frequencies, which would have the effect of introducing audible distortion into the modulated signal.

The disadvantage of AM-modulating the final amplifier stage is that doing so requires audio power equal to 50% of the carrier power ... if you're running a 50 KW AM radio station, you need 25 KW ( ! ) of clean audio to modulate it with. You know what 100 watts of audio for your Home Theater or your car costs, so you can imagine what 25 KW of broadcast-quality audio, or even 5 KW, must cost.

Low level modulation means modulation applied to a low-power stage before the power amplifier boosts it to the full transmit power level. That's obviously a great benefit, because you don't need nearly as much audio power, so the audio stages are much less expensive. BUT ... it means that every stage after modulation must be a highly linear stage, in order to avoid distortion. So now, instead of a husky Class-C final power amplifier, it has to be a Class-A (I think maybe Class-AB also works). Class-A for a final means much lower efficiency, so it has to be bigger, and use more DC power supply than the old Class-C needed, in order to send the same power up to the antenna. So it's a cost trade-off.