Reflection-Type Bi-Phase Amplitude Modulator with Improved Performance Using Feedback

The analogue reflection-type attenuator using FETs as variable resistance reflection terminations is widely used as a bi-phase amplitude modulator for digital communication. However, FET parasitics cause considerable performance degradation in terms of amplitude and phase error in the signal constellation. In this paper, it is shown for the first time that simple resistive feedback can be applied to the standard modulator to improve performance dramatically. The analysis of the technique is presented, along with practical results for a 900 MHz demonstration circuit.