Frequency discriminator producing at least one of two pulse sequences representing in average the result of frequency discrimination

摘要

In a frequency discriminator for a first and a second input signal of a first and a second frequency, a phase detector produces first and second rectangular pulse sequences. When the first input signal leads the second by 0 DEG through 180 DEG , the first pulse sequence takes a predetermined peak value with an instantaneous frequency corresponding to the phase difference while the second one continuously has the peak value. When the first input signal lags behind the second by 0 DEG through 180 DEG , the second pulse sequence takes the peak value with the instantaneous frequency while the first one continuously has the peak value. A low-pass filter rejects components of the first pulse sequence which are higher than the first and second frequencies. A high-pass filter has a prescribed cutoff frequency for rejecting lower-frequency components of the low-pass output to produce an output pulse sequence representing in average the difference between the first and second frequencies when the first frequency is higher than the second. Alternatively or additionally, a similar set of a low-pass and a high-pass filter produces a like output pulse sequence when the first frequency is lower than the second. At least one of the output pulse sequences is used as a discriminator output signal either per se or after conversion to a substantially d.c. signal. The frequency discrimination characteristics depend on the prescribed cutoff frequency. In an automatic frequency control (AFC) loop, both d.c. signals may be supplied to a voltage controlled oscillator through a differential circuit.