Phase locked loop signal demodulator and squelch circuit

Claim 1

In a demodulator for receiving a frequency modulated (FM) signal having a modulating signal with a predetermined range of frequencies, said demodulator including a phase locked loop having a phase comparator, a low pass filter and a DC amplifier connected in series in a forward path and a current controlled oscillator (CCO) connected between the DC amplifier and the phase comparator in a feedback path, said phase locked loop for locking to the FM signal and recovering the modulating signal, said demodulator including improvement comprising: said CCO being configured as an emitter-coupled multivibrator having timing capacitor means and providing a first output signal; output signal utilization means; differential comparator means differentially coupled to the timing capacitor means of the CCO for providing a second output signal phase shifted by 90° from the first output signal of the CCO; signal detecting means including a quadrature phase comparator, filter means, and amplifier means connected in series, the quadrature phase comparator multiplying the FM signal and the second output signal from the differential comparator means and providing a third output signal, the filter means filtering out substantially all frequencies from, and passing the DC component of, the third output signal of the quadrature phase comparator to provide a DC indication signal, the amplifier means providing a predetermined amplification to the DC indication signal; and switching means coupled to the amplifier means for gating the recovered modulating signal from the phase locked loop to said output signal utilization means when the magnitude of the amplified DC indication signal is greater than a predetermined magnitude.