Simultaneous digital measurement of phase and amplitude noise

In this article, we describe the simultaneous digital measurement of both phase (PM) and amplitude (AM) noise of VHF and microwave sources by the direct digitization of the signal-under-test. Our measurement approach takes advantage of a commercially-available, high-dynamic-range analog-to-digital converter driven by a high-performance clock to digitize the signal-under-test with high fidelity. Following digitization, phase and amplitude fluctuations are extracted and converted to PM and AM noise spectra. Measurement of microwave signals is accomplished by the inclusion of a specially-designed low-noise down-converter to translate the signal frequency to the VHF regime while introducing minimal additional signal noise. Measurements made on this system are shown to be in good agreement with those obtained using a conventional heterodyne mixer system. In addition to speeding the characterization of RF sources by simultaneously measuring both PM and AM signal characteristics, the digital noise measurement approach allows the direct measurement of the PM- and AM-noise spectra of more complex signals, such as pulsed CW waveforms, in both the VHF and microwave regimes.