Autorange Detector & Anti-alias Filter

In this case, all 160 resolution bandwidths are digitally implemented.
However, there is some analog circuitry prior to the ADC, starting with
several stages of down conversion, followed by a pair of single-pole prefilters
(one an LC filter, the other crystal-based). A prefilter helps prevent succeeding
stages from contributing third-order distortion in the same way a prefilter
would in an analog IF. In addition, it enables dynamic range extension via
autoranging. The output of the single-pole prefilter is routed to the autorange
detector and the anti-alias filter.

As with any FFT-based IF architecture, the anti-alias filter is required to
prevent aliasing (the folding of out-of-band signals into the ADC sampled
data). This filter has many poles, and thus has substantial group delay.
Even a very fast rising RF burst, downconverted to the IF frequency, will
experience a delay of more than three cycles of the ADC clock (30 MHz)
through the anti-alias filter. The delay allows time for an impending large
signal to be recognized before it overloads the ADC. The logic circuitry
controlling the autorange detector will decrease the gain in front of the ADC
before a large signal reaches it, thus preventing clipping. If the signal envelope
remains small for a long time, the autoranging circuit increases the gain,
reducing the effective noise at the input. The digital gain after the ADC is
also changed to compensate for the analog gain in front of it. The result is a
floating point ADC with very wide dynamic range when autoranging is
enabled in swept mode.

Figure 3-3. Autoranging keeps ADC noise close to carrier and lower than LO noise
or RBW filter response

Figure 3-3 illustrates the sweeping behavior of the PSA analyzer. The
single-pole prefilter allows the gain to be turned up high when the analyzer
is tuned far from the carrier. As the carrier gets closer, the gain falls and the
ADC quantization noise rises. The noise level will depend on the signal level
frequency separation from the carrier, so it looks like a step-shaped phase
noise. However, phase noise is different from this autoranging noise. Phase
noise cannot be avoided in a spectrum analyzer. However, reducing the
prefilter width can reduce autoranging noise at most frequency offsets from
the carrier. Since the prefilter width is approximately 2.5 times the RBW,
reducing the RBW reduces the autoranging noise.