The start and stop frequency changes the range of frequency offsets from the carrier where measurements are collected. Increasing the start or stop range (for example, reducing the start frequency from 1kHz to 1Hz) provides more information about the phase noise of the measured signal at the expense of longer measurement times. For applications where close-in phase noise measurements are not needed, the start offset can be increased to greatly improve measurement speed.

Note that the maximum stop frequency the equipment can measure can be limited by the carrier frequency. For example, the Agilent E5052B supports a stop frequency up to 40MHz for carrier signals in the 99MHz - 1.5GHz band, but the Agilent E5052B limits the maximum stop frequency to 20MHz for carriers in the 39MHz - 101MHz band.