192ConceptsResolving Closely Spaced SignalsResolving Closely Spaced SignalsResolving signals of equal amplitudeTwo equal-amplitude input signals that are close in frequency can appear as a singlesignal trace on the analyzer display. Responding to a single-frequency signal, aswept-tuned analyzer traces out the shape of the selected internal IF (intermediatefrequency) filter (typically referred to as the resolution bandwidth or RBW filter). Asyou change the filter bandwidth, you change the width of the displayed response. If awide filter is used and two equal-amplitude input signals are close enough infrequency, then the two signals will appear as one signal. If a narrow enough filter isused, the two input signals can be discriminated and appear as separate peaks. Thus,signal resolution is determined by the IF filters inside the analyzer.The bandwidth of the IF filter tells us how close together equal amplitude signals canbe and still be distinguished from each other. The resolution bandwidth functionselects an IF filter setting for a measurement. Typically, resolution bandwidth isdefined as the 3 dB bandwidth of the filter. However, resolution bandwidth may alsobe defined as the 6 dB or impulse bandwidth of the filter.Generally, to resolve two signals of equal amplitude, the resolution bandwidth mustbe less than or equal to the frequency separation of the two signals. If the bandwidthis equal to the separation and the video bandwidth is less than the resolutionbandwidth, a dip of approximately 3 dB is seen between the peaks of the two equalsignals, and it is clear that more than one signal is present.For Signal Analyzers in swept mode, sweep time is automatically set to a value thatis inversely proportional to the square of the resolution bandwidth (1/BW 2),to keepthe analyzer measurement calibrated. So, if the resolution bandwidth is reduced by afactor of 10, the sweep time is increased by a factor of 100 when sweep time andbandwidth settings are coupled. For the shortest measurement times, use the widestresolution bandwidth that still permits discrimination of all desired signals. Sweeptime is also a function of which detector is in use, peak and normal detectors sweepas fast or more quickly than sample or average detectors. The analyzer allows RBWselections up to 8 MHz in 1, 3, 10 steps and it has the flexibility to fine tune RBWs inincrements of 10% for a total of 160 RBW settings.For best sweep times and keeping the analyzer calibrated set the sweep time(Sweep/Control, Sweep Time) to Auto, and the sweep type (Sweep/Control,Sweep Setup, Sweep Type) to Auto. Use the widest resolution bandwidth and thenarrowest span that still permits resolution of all desired signals.Resolving small signals hidden by large signalsWhen dealing with the resolution of signals that are close together and not equal inamplitude, you must consider the shape of the IF filter of the analyzer, as well as its3 dB bandwidth. (See “Resolving signals of equal amplitude” on page 192 formore information.) The shape of a filter is defined by the selectivity, which is theratio of the 60 dB bandwidth to the 3 dB bandwidth. If a small signal is too close to alarger signal, the smaller signal can be hidden by the skirt of the larger signal.
