Closed-loop autocorrelator and its application to frequency discrimination

A new closed-loop autocorrelation system is proposed in the paper. It is based on a conventional autocorrelator; however, the fixed delay is replaced by a variable delay line and the autocorrelator output is fed back to control the delay. The variable delay can be realised by a SAW voltage-controlled variable delay line (VCDL). Adetailed analysis of the loop is given. It is shown that for some special inputs, the loop performs as a frequency-measuring system, as an instantaneous frequency measurement (IFM) or as a wideband linear FM discriminator. It is further shown that, in the presence of noise, the loop behaves similarly to a frequency modulation feedback (FMFB) loop. Therefore, the output SNR can be improved (in comparison with conventional FM discriminators) by appropriately choosing the loop parameters. Experimental results of a system implemented using an SAW VCDL are presented and are shown to support the analysis.