A new lock indicator circuit for microwave and millimeter-wave phase locked loops

A circuit useful as a lock detector in microwave phase-locked loop (PLL) systems has been developed. This circuit avoids the quadrature phase detector or coherent amplitude detector commonly used as a lock indicator in PLLS, thereby reducing the microwave circuitry and components. It is based on the properties of the phase-error signal coming from the phase detector; a frequency-voltage conversion is performed on it in a low-frequency (secondary) PLL, the input to which is the output of the phase detector in the main (microwave) PLL. The secondary voltage-controlled oscillator (VCO) control signal gives, after a comparison, a logic level related to the lock condition in the main (microwave) PLL. This circuit has been used with success in microwave phase-locked oscillators (PLOs) in which the phase detection was made at 290 MHz, 2.55 GHz, 27 GHz and 29.7 GHz