Resolving Small Signals Hidden by Large Signals Using the Resolution Bandwidth Function
When dealing with resolution of signals that are not equal in amplitude, you must consider the shape of the IF filter as well as its 3 dB bandwidth. The shape of the filter is defined by the shape factor, which is the ratio of the 60 dB bandwidth to the 3 dB bandwidth. (Generally, the IF lilters in this spectrum analyzer have shape factors of 15: 1 or less.) If a small signal is too close to a larger signal, the smaller signal can be hidden by the skirt of the larger signal. To view the smaller signal, you must select a resolution bandwidth such that k is less than a. See Figure
k < a
Figure
The separation between the two signals must be greater than half the filter width of the larger signal at the amplitude level of the smaller signal.
Example: Resolve two input signals with a frequency separation of 200 kHz and an amplitude separation of 60 dB.
1.To obtain two signals with a 200 kHz separation, connect the equipment as shown in the previous section, “Resolving Signals of Equal Amplitude Using the Resolution Bandwidth Function. ))
2.Set the center frequency to 300 MHz and the span to 2 MHz: press
FREQUENCY) 300 (MHz), then ISPAN] 2 @.
3.Set the source to 300.2 MHz, so that the signal is 200 kHz higher than the calibration signal. Set the amplitude of the signal to