All-digital transmission systems require clock recovery before decoding can commence. Audio examples include compact disks (CDs), DAT, NICAM stereo sound, and all other M-ary systems. The equivalent problem in nondigital communications is the recovery of a phase-coherent carrier in the presence of noise and signal amplitude variation. The fundamental problem of recovering the carrier from a double-sideband-suppressed carrier (DSSC) signal is investigated. Using a novel circuit technique called the amplitude-locked loop (ALL) to replace the hard limiter, coherent carrier recovery is demonstrated down to carrier-to-noise ratios of 0 dB and lower. The implications for improved digital clock recovery circuitry are equally as important.
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