US Patent 10396974 Self-testing of a phase-locked loop using a pseudo-random noise

An apparatus includes signal control circuitry, a phase-locked loop (PLL), and a correlation circuit. The signal control circuitry provides a reference clock signal carrying pseudo-random phase noise and as derived from an application clock signal and pseudo-random noise. The PLL, responsive to the reference clock signal carrying the pseudo-random phase noise, provides an output signal that is related to the phase of the reference clock signal. The correlation circuit self-tests the PLL by cross-correlating a signal corresponding to the output signal from the phase detector with the pseudo-random noise and, in response, by assessing results of the cross-correlation relative to a known threshold indicative of a performance level of the PLL.