Frequency Channelization and Afterwards Processing

Abstract

Channelization is one of the most important operations in building digital electronic warfare (EW) receivers. The equivalent analog operation is the filter bank. Therefore digital channelization can be considered a digital filter bank. It can also be considered as an N-port network with one input and N - 1 outputs. An input signal will appear at a certain output according to its frequency. By measuring the outputs from the filter bank, the frequency of the input signal can be determined. The only practical approach to building a wideband digital EW receiver with today's technology is through channelization. A common method of performing channelization is by employing the fast Fourier transform (FFT). To build a receiver using FFT, the length and the overlap of the FFT are very important parameters. These parameters are related to the minimum pulse width (PW) and the frequency resolution, which determines the sensitivity of the receiver. The frequency information can be obtained from the outputs of the digital filters. In order to obtain the input frequency, the filter outputs must be further processed. The main objectives of a receiver are to determine the number of input signals and their frequencies. The circuit used to accomplish these goals is referred to as the encoder. The encoding circuit is the most difficult subsystem to design in an EW receiver and most research effort is spent on the encoder design. This is true for both digital and analog receivers. The main problems are to avoid the generation of false alarms and the detection of weak signals. In an analog filter bank, the shape of the filter is difficult to control, and it is difficult to build filters with uniform performance, such as bandwidth and ripple factor, therefore the encoder must accommodate this problem. The shape of each individual filter in a digital filter bank can be better controlled. As a result, the encoder should be slightly easier to design because it does not need to compensate for the filter differences. Because of the complexity of the encoder, its design will not be discussed in detail. The design of a specific digital filter bank will also be presented. This specific example is used to illustrate the design procedure while avoiding the unnecessary mathematical complexity of a general design. In this example, the concepts of polyphase filters and multirate operation are introduced. In order to understand these concepts, decimation and interpolation are discussed first.