Agilent Technologies E7401A, E7405A, E7402A, E7404A, E7403A Resolving Signals of Equal Amplitude

Making Basic Measurements

Resolving Signals of Equal Amplitude

Resolving Signals of Equal Amplitude

Two equal-amplitude input signals that are close in frequency can appear as a single signal trace on the analyzer display. Responding to a single-frequency signal, a swept-tuned analyzer traces out the shape of the selected internal IF (intermediate frequency) filter. As you change the filter bandwidth, you change the width of the displayed response. If a wide filter is used and two equal-amplitude input signals are close enough in frequency, then the two signals will appear as one signal. If a narrow enough filter is used, the two input signals can be discriminated and will 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 can be and still be distinguished from each other. The resolution bandwidth function selects an IF filter setting for a measurement. Typically, resolution bandwidth is defined as the 3 dB bandwidth of the filter. However, resolution bandwidth may also be defined as the 6 dB or impulse bandwidth of the filter.

Generally, to resolve two signals of equal amplitude, the resolution bandwidth must be less than or equal to the frequency separation of the two signals. If the bandwidth is equal to the separation and the video bandwidth is less than the resolution bandwidth, a dip of approximately 3 dB is seen between the peaks of the two equal signals, and it is clear that more than one signal is present. See Figure 1-7.

In order to keep the analyzer measurement calibrated, sweep time is automatically set to a value that is inversely proportional to the square

of the resolution bandwidth (1/BW2 for resolution bandwidths ≥ 1 kHz). So, if the resolution bandwidth is reduced by a factor of 10, the sweep time is increased by a factor of 100 when sweep time and bandwidth settings are coupled. Sweep time is also a function of the type of detection selected (peak detection is faster than sample or average detection). For the shortest measurement times, use the widest resolution bandwidth that still permits discrimination of all desired signals. Sweeptime is also a function of which Detector is in use, Peak detector sweeps more quickly than Sample or Average detector. The analyzer allows you to select from 10 Hz (or 1 Hz with Option 1D5) to

3 MHz resolution bandwidths in a 1, 3, 10 sequence and select a 5 MHz resolution bandwidth. In addition you can select the three CISPR bandwidths (200 Hz, 9 kHz, and 120 kHz) for maximum measurement flexibility.

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