Zero span
If the superheterodyne spectrum analyzer is tuned to a fixed frequency, the displayed trace is the envelope of the IF filtered signal.
This mode is called zero span because the instrument does not perform a sweep.
In this video three examples of zero span analysis are shown.
In the first example the input signal is a sinewave. In this case the output is a line.
In the second example the input signal is an AM and the RBW is narrower then the input signal bandwidth. In this case the IF filter selects only the carrier and, as a consequence, the displayed trace is a line.
Also in the last example the input signal is an AM signal but, in this case, the RBW is wider than the input signal bandwidth. This setting does not cut any frequency component of the input signal. As a consequence, the displayed trace is the envelope of the signal in time domain (i.e the modulating signal with an offset).