Greetings Simon:
When making a measurement
to determine conformance to a particular specification
(GSM, for example), the spec normally dictates the
bandwidth in which it is to be measured. Otherwise,
you have to rely on experience and knowledge of
how the RBW (resolution bandwidth) and VBW (video
bandwidth) affect the displayed signal. Most spectrum
analyzers come with a user’s guide that provides
some insight into how to set the instrument. There
are some good test notes on the RF Cafe “Test Notes”
page (1), including the classic from HP (now Agilent).
The subject is too broad to cover here, so please
read some of the notes on that page. One rule of
thumb to remember is that if a signal amplitude
varies as you change the RBW, then it has some bandwidth
to it (not a pure tome as with a synthesizer), and
if it does vary with RBW, then it has some bandwidth
(either noise or signal width).
(1)
https://www.rfcafe.com/references/test-notes.htm
- Kirt Blattenberger