| 摘要 |
A television tuning system employs a frequency synthesizer system for establishing the tuning of the receiver. A programmable frequency divider counter is connected between the output of a reference oscillator and a phase comparator to which the output of the local oscillator in the tuner also is applied. The phase comparator output provides a tuning voltage for controlling the tuning of the local oscillator. A microprocessor is used to control the count of the programmable frequency divider and initially to set a count corresponding to the selected channel in a counter connected between the output of the local oscillator and the phase comparator. The AFT discriminator signal is used to sense the presence or absence of a properly tuned condition and to control the operation of the microprocessor for establishing the count in the programmable frequency divider counter. For a localized search, the microprocessor may drive the programmable frequency divider counter to a predetermined maximum count and then step back a fixed number of counts to a lower count and resume counting in the original direction; so that even if the AFT discriminator output is in an ambiguous condition, proper tuning can be achieved. However, if the AFT discriminator output produces the correct tuning direction information initially, the programmable frequency divider is adjusted step by step to the appropriate larger or smaller count that corresponds to correct tuning.
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