Spectrum analysers are extremely helpful tools for visualising signals in frequency space. This note will describe how to use settings to minimize error when performing peak measurements using a Rigol DSA Spectrum analyser.
Radio Frequency signals, especially continuous wave (CW) signals, have a finite frequency width. Spectrum analysers, like the Rigol DSA series, use the
Resolution Bandwidth Setting (RBW) to determine the bandwidth of the filter used analyse each resolution. For a fixed span, smaller RBW settings will equate to smaller frequency “steps” and finer frequency resolution measurement capabilities.
If the bandwidth of the source is less than the RBW setting of the spectrum analyser, the resultant spectrum analyser trace will have a finite width and a shape very much like a Gaussian or Bell curve as shown in Figure 1.
In Figures 1through 4, the input is a fixed 10MHz Sine wave. The only difference is the RBW setting on the DSA. As you can see, the width of the trace decreases as the RBW decreases. This is because the width of the RF input is less than the RBW value. We are effectively viewing the width of the RBW filter.
It’s also important to point out that the shape of the peak of the trace also changes. It goes from a “bubbled” shape with no real “peak” to a more pointed shape. Traces with “bubbled” shapes may have peak readings that vary because there is not a singular point that is has the highest amplitude.
To achieve high accuracy peak measurements, you should use the smallest RBW setting that you can. If you have varying peak values, try lowering the RBW value and retest.
Figure 1: 10MHz input, 300kHz span, RBW = 30kHz
Figure 2: 10MHz input, 300kHz span, RBW = 10kHz
Figure 3: 10MHz input, 300kHz span, RBW = 3kHz
Figure 4: 10MHz input, 300kHz span, RBW = 1kHz