Also, from: "Fundamentals of Spectrum Analysis"; Christoph Rauscher

Digital IF filters

Narrow bandwidths can best be implemented with the aid of digital signal processing. In contrast to analog filters, ideal Gaussian filters can be realized.

Much better selectivity (SF = 4.6) can be achieved using digital filters instead of analog filters at an acceptable circuit cost.

Analog filters consisting of five individual circuits, for instance, have a shape factor of about 10, whereas a digitally implemented ideal Gaussian filter exhibits a shape factor of 4.6.

Moreover, digital filters feature temperature stability, are free of aging effects and do not require adjustment. Therefore they feature a higher accuracy regarding bandwidth.

The transient response of digital filters is defined and known. Using suitable correction factors, digital filters allow shorter sweep times than analog filters of the same bandwidth.

The IF signal after the IF amplifier must first be sampled by an A/D converter.

To comply with the sampling theorem, the bandwidth of the IF signal must be limited by analog prefilters prior to sampling.

This band limiting takes place before the IF amplifier so that intermodulation products can be avoided, as was the case for analog filters.

The bandwidth of the prefilter is variable, so depending on the set digital resolution bandwidth, a very small bandwidth can be selected.

The digital IF filter provides for limiting the noise bandwidth prior to envelope detection.