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In the Agilent ESA Series, why does the Marker Frequency Counter accuracy specification exclude RBWs less than 1KHz even though the counter can be used in these narrower RBWs?

When using RBWs less than 1KHz, the ESA is performing FFTs on the IF signal. However, Marker Count is counting the 21.4 MHz IF in a 1 kHz bandwidth, and does not have the discrimination of the FFT of the IF signal. As a result, if there is only one signal present, then the marker count will work properly in RBWs <= 300 Hz. If there are two or more signals present within 1kHz, then the counter will count incorrectly.