Chapter 19 - Improving Receiver Performance in a High EMI Environment

Abstract

Several strategies can improve the electrostatic interference and noise performance of a radio communications receiver. Some methods are available for after-market use, while others are practical only when designing a new receiver. One method is to reduce the signal level applied to the input of the receiver at all frequencies. Another method is to prevent the offending signal or signals from ever reaching the receiver's front end. This goal can be achieved by using any of several bandpass filters, high-pass filters, low-pass filters, or tunable filters (as appropriate) ahead of the receiver's front end. The mixer in a newly designed receiver project should have a high dynamic range regardless of whether an RF amplifier is used. Popular with some designers is the double-balanced switching mixer. The chapter includes an example for the usage and architecture of receiver. Some modern receivers are equipped with one or more switchable attenuators in the front end. Some receivers also include an RF gain control that operates in the same manner. The idea behind the attenuator is to reduce all the signals to the front end enough to drop the overall energy in the circuit to below the level that can be accommodated without either overload or intermodulation at significant levels. One of the best EMI solutions is to filter out the offending signals before they hit the receiver front end in a manner that affects the desired signals only minimally. A wave trap is a tuned circuit that causes a specific frequency to be rejected. Two forms are used: series tuned and parallel tuned.