Effects of Amplitude and Timing Jitter on the Performance of Photonic Sigma-Delta Modulators

Photonic sigma-delta modulators can directly digitize wideband signals with high resolution directly atthe antenna. In our first-order, single-bit architecture, the antenna signal is applied to a pair of Mach-Zehnder interferometers and oversampled using a CALMAR 10 GS/s mode-locked laser (MLL) with a pulse width of 10 ps. The measurements of the MLL pulse-to-pulse sample time uncertainty (time jitter) and the laser pulse amplitude uncertainty (amplitude jitter) are described. Considering the jitter to be the result of non-uniform random sampling we show that a normal distribution is a good noise model for both jitter mechanisms. The sigma-delta modulator and the decimation filtering process are described. Using asynchronous spectral averaging of the reconstructed signal's magnitude spectrum, an expression for the noise floor without jitter is developed and compared to simulation results as a function of the oversampling ratio (OSR) and record length using a 100 MHz signal bandwidth. The noise floor is then evaluated as a function of the time jitter power and amplitude jitter power for several OSRs.