Total posts : 45366
The efficiency tests I performed with the Wenzel circuit output modified to the SSTRAN test circuit were all done without modulation. I looked for maximum efficiency using only the carrier. After posting my results for data obtained only with the carrier, I continued my investigation by checking how well the circuit works when modulating the circuit when it was set for maximum efficency (41%). With this efficiency setting, the top half of the modulation waveform was clipped off, and the circuit was clearly incapable of transmitting intelligence without severe distortion. At the maximum efficiency setting, the 46 ohm load resistance was transformed by the L network to 1060 ohms at the collector of the final stage of the transmitter. 1060 ohms resistance was necessary for maximum efficiency. In order to get the modulation to work, I had to reduce the load resistance, and therefore the efficiency. When I adjusted the L network for a collector load resistance of 725 ohms, I was able to get 74% modulation before the top of the modulation waveform began clipping, but the efficiency was reduced to 28%. By setting the L network for a collector load resistance of 620 ohms, I was able to get 100% modulation, but the circuit efficiency was reduced to 24%.
This particular circuit configuration is unsuitable for being modulated by connecting a modulation transformer to the collector; so nothing much can be done with this circuit to improve its efficiency beyond about 24%.
After thinking about it for a while, it became apparent to me why transconductance modulation does not work when the transmitter is set for maximum efficiency. The Wenzel generates a square wave current waveform in the final stage, nominally 20 mA p-p. Since the voltage from collector to emitter in the final stage is nominally 10 VDC, The maximum RF waveform at the collector of the final stage is nominally 20 V p-p. No more voltage range is available at the collector without clipping. Maximum efficiency occurs when the collector voltage range is maximum. When trying to use transconductance modulation while the transmitter is already set up for a 20 V p-p voltage swing, the top half of the modulation waveform is clipped off. This is exactly what I observed when I tried to modulate the transmitter when it was set up for the maximum 41% efficiency. In order to be able to modulate the carrier, it is necessary to reduce the carrier level so that the positive modulation peaks will not be clipped. Reducing the carrier level reduces efficiency.
If it were possible to modulate the collector, this problem would not occur. The positive peaks of the modulator would increase the collector to emitter voltage to prevent the clipping on positive peaks that occurs with a transconductance modulator. As I already mentioned, however, adding a collector modulator to the Wenzel circuit (after removing the audio input to the transconductance modulator) would not work. To use a collector modulator, the transmitter would have to be completely redesigned, with a class C circuit as the final stage.
Now, we see that, using the Wenzel circuit modified for the SSTRAN output as a baseline, we can get as much as a 6dB improvement in signal strength if we are able to transform nearly all of the allowed 100mW input power to the final stage to RF power. This would make an effort to come up with a really efficient Part 15 AM transmitter design seem more worthwhile.