| 摘要 |
RCA 68,346 TELEVISION FREQUENCY SYNTHESIZER FOR NONSTANDARD FREQUENCY CARRIERS A television receiver tuning apparatus includes a programmable phase locked loop (PLL) to synthesize local oscillator signals having frequencies appropriate to tune the receiver to standard frequency carriers respectively allocated to conventional channels. Local oscillator signals having frequencies appropriate to tune the receiver to nonstandard frequency carriers such as may be generated by a master antenna or cable television distribution system can also be provided. The PLL derives a first control voltage to control the frequency of the local oscillator signals having a range sufficient for a particular band to tune the receiver to all the channels in the band. During the reception of a nonstandard frequency carrier, a programmable divider associated with the PLL is incremented to change the local oscillator frequency in a predetermined range around the standard local oscillator frequency corresponding to the selected channel. The predetermined range is determined by the maximum expected deviation between respective nonstandard and standard frequency carriers. The frequency incremented local oscillator signal is mixed with the received signal to derive an intermediate frequency signal having a carrier incremented in a frequency range corresponding, in direct relationship, to the frequency range of the frequency incremented local oscillator signals. An automatic fine tuning (AFT) circuit including a frequency discriminator develops a discriminator signal representing the frequency deviation of the incremented intermediate frequency RCA 68,346 carrier from the standard picture carrier frequency (e.g., 45.75 MHz). The discriminator voltage is amplified to provide a second control voltage with approximately the same amplitude range as the first control voltage. When the amplitude of the second control voltage is changing in the right direction to allow AFT control and is substantially the same as the amplitude of the first control voltage for the presently selected channel, the voltage controlled local oscillator is decoupled from the first control voltage and is coupled to the second control voltage. - 1a |
