| 摘要 |
<p>A circuit monitors the period of separation between impulses and provides an output only when the period of separation between successive impulses is substantially the same as a predeterimed period. The impulses are applied to a monostable circuit which produces pulses of fixed duration in response. A switching circuit controlled by the pulses switches a binary counter to be driven by a low frequency oscillator output between pulses and by a high frequency oscillator output during pulses, the counter being reset to zero at the trailing edge of each pulse. A comparator determines whether, before reset, the counter reaches a count corresponding to the predetermined period and provides an output pulse to an AND circuit input when the count is reached. The AND circuit has another input connected to the monostable circuit output and thus provides an output pulse when the comparator output is coincident with a corresponding pulse from the monostable circuit. The high and low oscillator frequencies and the pulse duration of the monostable circuit are selected such that the counter will reach the predetermined count if the input impulse separation is substantially the same as the predetermined period.</p> |
