| 摘要 |
A double conversion tuning system for a high definition television receiver includes a microprocessor supplying first and second digital numbers to first and second local oscillators. The first local oscillator is controlled by a wide band phase locked loop that includes a programmable divider that receives the first digital number from the microprocessor and a prescaler that couples the first local oscillator signal to the programmable divider. The output of the programmable divider supplies one input of a phase detector, the other input of which is supplied with the divided down output of a crystal reference source. The output of the phase detector controls the first local oscillator frequency. The tuning of the first local oscillator is coarse and the first IF signal frequency may deviate within a predetermined range from a desired value. The second digital number from the microprocessor controls, via a D/A and varactor, the resonant frequency of a tunable dielectric resonator in the second local oscillator to adjust the second local oscillator frequency to compensate for the deviation in first IF signal frequency so that the second IF signal frequency remains constant.
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