Anti-alias filtering apparatus for frequency domain measurements

Abstract

A filtering apparatus permits alias-free frequency domain measurement in any of a plurality of measurement bandwidths. The input signal is first passed through one of a plurality of low-pass filters. The appropriate filter is selected by a controller to provide a cut-off frequency at least as great as the selected measurement bandwidth. An analog-to-digital convertor ('ADC') samples the filtered input signal at a sample rate (f.sub.s) set by the controller to be at least twice the measurement bandwidth. The digitized samples are then processed by a digital low-pass filter (i.e. a finite impulse response filter) to further reduce the bandwidth of the input signal to the measurement bandwidth. Operation of the digital filter is controlled by a number of filter coefficients provided by the controller as a function of the desired measurement bandwidth. A memory is provided between the ADC and the digital filter. The samples output by the ADC can be processed in real time by the digital filter if the sample rate is sufficiently low, or the samples can be stored in the memory for subsequent processing by the digital filter if a higher sample rate is required. The number of samples can be reduced by adding a decimator which outputs only one sample from each series of K samples received from the digital filter, provided f.sub.s /K is at least twice the measurement bandwidth.