Microwave Armstrong Amplitude Modulator

We describe a microwave amplitude modulator capable of operating at high modulation rates (limited by mixer IF port bandwidth) with minimal residual phase modulation. The modulator is based on Armstrong's idea to produce any modulated signal by varying the phase shift between the carrier and modulation sidebands. We conducted our experiments at 11 GHz. The frequency of the modulation signal was below 1 MHz to measure the residual phase modulation with the digital phase detector. We showed that the phase noise of the microwave pilot signal was the main factor limiting the minimal detectable level of the residual phase modulation.