Application Note

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The RF sections of Analog FM and Digital Radios primarily differ in the required linearity. If the transmitter amplifiers and attendant switching functions are not extremely linear, AM to AM and AM to PM conversion can cause serious deterioration of bit error rate (BER) and an increase in intersymbol interference (ISI). Higher order QAM Systems such as 64-QAM and 256-QAM require even higher levels of linearity. This article discusses the linearity requirements for the RF sections of digital communications links and compares the low-distortion performance of silicon PIN diode and GaAs MESFET switches.

Introduction

The RF section of a digital communications link consists of the modulator, transmitter, channel medium, receiver and demodulator. The non-linearities and noise, inherent in these analog components of the link, are major causes of link performance degradation. Physical constraints on link inputs and outputs, imposed by limitations in the digital encoder and decoder, may lead to suboptimal operation; but, they produce relatively small degradation in link performance.

In this article, the overall performance parameters of the digital communications link are described. The nonlinear distortion characteristics of GaAs MESFETs and Silicon PIN diodes are discussed and related to the problem of optimizing the RF Transmitter linearity. A comparison of the intermodulation performance of Silicon PIN and GaAs MESFET Switches is given and particular system design considerations are listed to aid in the selection of a particular switch type for a specific system application. As an example, the intermodulation of a transmitter chain caused by an interfering station is discussed and some practical values of IP3 are given.

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•Application Note: Silicon PIN Diode And GaAs MESFET Switches And Their Effects On Linearity Of Digital Communications Systems