ANALYSIS OF SELF-NOISE IN A 4TH POWER CLOCK REGENERATOR

Bandwidth-efficient PAM systems require the use of signaling waveforms that are band-limited to near the Nyquist bandwidth. The popular squaring-type clock recovery circuit does not perform well for such systems due to excessive self-noise-to-tone power ratio. A fourth-power circuit is investigated as an alternative to the squaring circuit. A prior work has established that average jitter power in a clock recovery circuit is a function of the self-noise powers in phase with the desired recovered clock and in phase quadrature as well as the cross-power spectrum between these two components. This paper describes a method to compute these spectral components for the fourth-power clock recovery technique for PAM and QAM modulation cases. Comparison is made with the squaring-type recovery technique using cosine rolloff Nyquist pulses on BPSK mode, at several excess bandwidth factors (alpha). It was found that for alpha less than 0.4, the fourth-power circuit gives less time jitter than the squarer for a noiseless channel. The method for self-noise spectrum calculation is verified by computer simulation. A prefilter that completely eliminates time jitter in the fourth-power clock recovery circuit is described.