| 摘要 |
A television tuning system has a ramp voltage source including an electrical current source and an electrical current sink coupled through selectively operable transmission gates for varying the voltage of a tuning capacitor to control a local oscillator for tuning independent of received signal. The local oscillator signal is mixed with a received signal to produce an intermediate frequency signal which is amplified in an IF amplifier. A frequency detector is coupled to the IF amplifier and produces an error signal dependent upon the frequency difference between a reference frequency and the IF signal. An additional current source and sink are coupled to the tuning capacitor through transmission gates controlled by the error signals. Differences greater than a predetermined amount selectively activate the latter transmission gates to provide substantially constant magnitude tuning voltage corrections irrespective of the amount of the difference.
|
