A sub-sampling digital PM/AM noise measurement system

A digital phase/amplitude modulation (PM/AM) noise-measurement system (DNMS) implementing field-programmable gate array (FPGA)-based digital down-converters (DDCs), and 250 MHz analog-to-digital converters (ADCs) is reported. Performance in the first, baseband Nyquist region shows white phase-noise floors of less than -180 dBc/Hz. With proper pre-filtering of the input signals to prevent undesired aliasing, high-bandwidth track-and-hold amplifiers (THA) extend the operating range of the DNMS to microwave frequencies. Preliminary testing with an 18 GHz THA shows residual white phase-noise floors at 10 GHz of less than -160 dBc/Hz.