The HP 11848A is the Phase Noise Interface for the HP 3048A Phase Noise Measurement System. The Interface supports several measurement techniques for phase noise and AM noise measurements. Inside the Interface are the phase detectors, amplifiers, filters, and switches necessary to measure phase noise over a frequency range of 5 MHz to 18 GHz. An input for an external phase detector outside that frequency range is also provided. The built-in sources allow the system to functionally check all of its signal handling circuits to insure proper operation before measurements are made.