Lou,
The way spur reject works is by mixing the incoming RF using two unique combinations of local oscillator and sometimes a different IF frequency. The results are compared and a signal not present in both is rejected as probable spurious.
In essence, a signal that is present for the entire sweep (or, for large spans, the time it takes to sweep 21.4 MHz) will not be rejected or hidden. We show our internal clock noise, rather than hiding it and risking missing an actual signal.
If a signal is pulsed or rapidly frequency hopping, spur reject may reject the actual signal, which is why for these types of signals, spur reject mode must be disabled. To analyze these types of signals, the BB60C is strongly recommended, although if you know the frequency of the signal, the SA44B in real-time mode may work also.
