Direct Digital Simultaneous Phase-Amplitude Noise and Allan Deviation Measurement System

In this paper, we present a direct digital measurement system capable of simultaneously measuring phase noise, amplitude noise, and Allan deviation with and without cross-correlation. The residual phase noise of the single-channel system achieves $\mathscr {L}\left ({{1 \text {Hz}}}\right)~ \textrm {=} -143 \text {dBc/Hz}$ for a 10 MHz input signal and an Allan deviation noise floor of $3.2 \times 10^{-15}$ at 1 second averaging time ( $\tau $ ). The system’s performance improves as expected with cross-correlation, resulting in an average-limited residual white noise floor of −185 dBc/Hz after only a few minutes of averaging, an improvement of 30 dB compared to a single-channel system. It also reaches an average limited flicker phase noise floor of $\mathscr {L}\left ({{1 \text {Hz}}}\right) ~\textrm {=} -160 \text {dBc/Hz}$ within two days, with an Allan deviation of $5 \times 10^{-16}$ @ $\tau ~\textrm {=}1 \text {second}$ . To our knowledge, this represents the lowest noise performance ever reported for a digital measurement system. Our solution is based on a pair of high-performance analog-to-digital converters and a single system-on-a-chip (SoC) with multiple processors and a field programmable gate array (FPGA). The architecture allows for processing all data samples in real-time without dead-time between calculation frames, enabling the fastest averaging possible during cross-correlation.