On False-Lock Phenomena in Carrier Tracking Loops

The false-lock phenomena in carrier tracking loops for the suppressed carrier modulation signal, such as phase-shift keyed (PSK) signals, present serious problems to the design of digital communication systems. There are two kinds of false-lock phenomena: one is related to the loop delay time and the other is to the modulation frequency. This paper describes an analysis of the latter type obsered in the phase-lock loops (PLL's) comprising the remodulation or decision feedback. The mechanism of the false-lock phenomena is best described by using the minimum phase difference between the input modulated carrier and the voltage controlled oscillator (VC-O) output as a parameter. It is shown that the loop can be locked at the offset frequencies of
\pm(m/n)f_{c}
in both the sampled and nonsampled control systems, where
m/n
is a simple fraction and f
c
denotes the modulation frequency. The lock and capture ranges for these false-lock frequencies are derived.