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).
- Kirt Blattenberger