All-digital IF

In Chapter 2, we did a filter skirt selectivity calculation for two signals
spaced 4 kHz apart, using a 3 kHz analog filter. Let's repeat that calculation
using digital filters. A good model of the selectivity of digital filters is a
near-Gaussian model:

At an offset of 4 kHz, the 3 kHz digital filter is down 24.1 dB compared
to the analog filter which was only down 14.8 dB. Because of its superior
selectivity, the digital filter can resolve more closely spaced signals.

The all-digital IF
The Agilent PSA Series spectrum analyzers have, for the first time, combined
several digital techniques to achieve the all-digital IF. The all-digital IF brings
a wealth of advantages to the user. The combination of FFT analysis for
narrow spans and swept analysis for wider spans optimizes sweeps for the
fastest possible measurements. Architecturally, the ADC is moved closer
to the input port, a move made possible by improvements to the A-to-D
converters and other digital hardware. Let's begin by taking a look at the
block diagram of the all-digital IF in the PSA spectrum analyzer, as shown
in Figure 3-2.

Figure 3-2. Block diagram of the all-digital IF in the Agilent PSA Series