Analysis of self-noise in a fourth-power clock regenerator

A method for computing the power spectra of the in-phase and quadrature components of self-noise in a fourth-power clock recovery scheme is presented. This method is shown to be effective if the symbol waveform is time-limited or can be so approximated. The method applies to baseband signaling schemes such as pulse-amplitude modulation and to modulated carrier schemes such as quadrature-amplitude modulation. Comparison is made with the squaring-type recovery technique using cosine rolloff Nyquist pulses on the binary-phase-shift-keyed mode, at several excess bandwidth factors ( alpha ). It is shown that when alpha is 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.>