| 摘要 |
A synchronous detecting system includes an automatic phase and frequency control system having a pair of multipliers coupled to a television tuner. A detector oscillator is also coupled to each of the multipliers. Means are provided to phase shift the oscillator signal at one multiplier such that quadrature beat signals are produced. The output of one of the multipliers is coupled directly to a third multiplier while the output of the other is coupled to a low pass filter. The filtered signal is amplitude limited and applied to the third multiplier and produces an information control signal. In two of the described embodiments, a second low pass filter couples the output of the third multiplier to the frequency control input of a voltage controlled detector oscillator. Two other embodiments are disclosed in which the tuner oscillator is controlled while the detector oscillator is maintained constant. The frequency and phase control system has two equal probability stable phase lock conditions. An inverter selectively operable responsive to the limiter information control assures the correct polarity of recovered information.
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