Sweep time

In any case, phase noise becomes the ultimate limitation in an analyzer's
ability to resolve signals of unequal amplitude. As shown in Figure 2-13,
we may have determined that we can resolve two signals based on the 3 dB
bandwidth and selectivity, only to find that the phase noise covers up the
smaller signal.

Sweep time

Analog resolution filters
If resolution were the only criterion on which we judged a spectrum analyzer,
we might design our analyzer with the narrowest possible resolution (IF)
filter and let it go at that. But resolution affects sweep time, and we care
very much about sweep time. Sweep time directly affects how long it takes
to complete a measurement.

Resolution comes into play because the IF filters are band-limited circuits
that require finit times to charge and discharge. If the mixing products are
swept through them too quickly, there will be a loss of displayed amplitude
as shown in Figure 2-14. (See Envelope detector, later in this chapter, for
another approach to IF response time.) If we think about how long a mixing
product stays in the passband of the IF filter, that time is directly proportional
to bandwidth and inversely proportional to the sweep in Hz per unit time, or:

where RBW = resolution bandwidth and
ST = sweep time.

Figure 2-13. Phase noise can prevent resolution of unequal signals

Figure 2-14. Sweeping an analyzer too fast causes a drop in displayed amplitude and a shift in indicated frequency