New method of phase noise and intermodulation distortion reduction in high-order QAM systems

Modern digital communication systems, such as microwave and millimeter wave point-to-point radios and local multipoint distribution system (LMDS) equipment now employ high-order quadrature amplitude modulation (QAM). In addition to the well known effect of additive white Gaussian noise (AWGN) on a QAM system's performance, phase noise and 3rd order intermodulation distortion (IMD) can also be major contributors to system performance degradation. This paper presents a simple analog distortion cancellation method which consists of a low-frequency feedback circuit connected between the FET output (drain) and the input (gate). This low-frequency feedback integrated with active bias is used to reduce phase noise and 3rd order IMD product by approximately 7 dB.