Total posts : 45366
Thanks to an email I received, it is better to use the more common name “Controlled Carrier AM Modulation” rather than progressive modulation. A Google search for controlled carrier modulation returns many more hits. I noticed a number of ham transmitters have used this. I saw Drake, Knight and Heathkit. There are likely others. I also saw a Varian 250kW shortwave broadcast transmitter that uses controlled carrier. The impetus seems to be to cut the carrier power down when there is little or no audio so as to reduce stress on the output tubes.
Neal pointed out that I was misleading when I said “In normal AM modulation, the power is constant.” I was thinking of the input power to the final RF stage, not the total radiated power consisting of carrier power plus sideband power. The input to the final is indeed constant over all modulation levels from 0% to 100% modulation. The input power to the final determines the carrier power.
In another email to me, it was pointed out that the AVC in AM radio receivers may cause strange and potentially annoying results when aggressive or poorly designed controlled carrier modulation is used. As an example, he Knight T-60 apparently ran at low carrier power until the audio input level passed a certain threshold. Then is switched to high power. This could cause an annoying pop or click in the receiver when it switched back and forth unless the microphone level was set carefully.
Even if the power supply voltage is varied continuously to follow the audio, there still could be artifacts due to the receiver AVC. Receivers typically have a slow AVC time constant, on the order of maybe .5 to 1 second. AVC is intended to level the volume for strong stations vs. weak stations and to level out fading. You can imagine what would happen when audio suddenly goes from low to very high. As the AVC lags in catching up to the increased carrier power, the volume will increase and then fall back to normal for up to 1 second.
In Part 15 transmitters, we aren’t concerned about reducing stress on the output stage. In our case, controlled carrier modulation could be used to increase the carrier power above 100mW in such a way that it continuously follows the audio level, and ensures that the carrier power will always increase, as required, so that audio peaks will never exceed 100% modulation. As I said before, there is no theoretical limit to the maximum full modulation carrier power vs. the idle 100mW level.
A decent Part 15 design might limit the highest carrier power to several hundred mW to reduce chances of FCC actions and to minimize the AVC artifacts.