Theoretical study of the interferometric bragg-cell spectrum analyser

Abstract

The paper reports on a theoretical study of the two-Bragg-cell interferometric spectrum analyser system. This architecture offers an improvement in radio frequency (RF) dynamic range compared to that of the conventional power spectrum analyser. Bulkwave shear mode lithium niobate Bragg cells are assumed for the study together with shot noise limited detection by avalanche photodiodes. A theoretical model is presented which enables the temporal history of the intermediate frequency (IF) output on each channel of the detector array to be predicted. A time-domain model is employed in which the instantaneous Fourier transforms of the (Gaussian weighted) signal and reference waveforms are evaluated at intervals much less than the IF period. These are then coherently combined to simulate the heterodyne detection. A computer program based on this theory provides a realistic simulation of pulse responses, ringing and delay effects in the IF filter, image and sidelobe levels, and IF breakthrough due to the reference waveform. Results are presented for chirp, pseudonoise and Gaussian noise reference waveforms. It is concluded that for the detection of RF pulses with durations as short as 100 ns, an instantaneous dynamic range of 50–55 dB relative to rms noise should be achievable, for simultaneous signals.