Digital Filters & Residual FM

This allows us to calculate the filter rejection:

Thus, the 1 kHz resolution bandwidth filter does resolve the smaller signal.
This is illustrated in Figure 2-10.

Figure 2-10. The 3 kHz filter ( top trace) does not resolve smaller signal;
reducing the resolution bandwidth to 1 kHz ( bottom trace) does

Digital filters
Some spectrum analyzers use digital techniques to realize their resolution
bandwidth filters. Digital filters can provide important benefits, such
as dramatically improved bandwidth selectivity. The Agilent PSA Series
spectrum analyzers implement all resolution bandwidths digitally. Other
analyzers, such as the Agilent ESA-E Series, take a hybrid approach, using
analog filters for the wider bandwidths and digital filters for bandwidths of
300 Hz and below. Refer to Chapter 3 for more information on digital filters.

Residual FM
Filter bandwidth is not the only factor that affects the resolution of a
spectrum analyzer. The stability of the LOs in the analyzer, particularly the
first LO, also affects resolution. The first LO is typically a YIG-tuned oscillator
( tuning somewhere in the 3 to 7 GHz range) . In early spectrum analyzer
designs, these oscillators had residual FM of 1 kHz or more. This instability
was transferred to any mixing products resulting from the LO and incoming
signals, and it was not possible to determine whether the input signal or the
LO was the source of this instability.

The minimum resolution bandwidth is determined, at least in part, by the
stability of the first LO. Analyzers where no steps are taken to improve upon
the inherent residual FM of the YIG oscillators typically have a minimum
bandwidth of 1 kHz. However, modern analyzers have dramatically improved
residual FM. For example, Agilent PSA Series analyzers have residual FM of
1 to 4 Hz and ESA Series analyzers have 2 to 8 Hz residual FM. This allows
bandwidths as low as 1 Hz. So any instability we see on a spectrum analyzer
today is due to the incoming signal.