| 摘要 |
An improved frequency sythesizer for duplex radios is described that includes a phase-locked loop coupled to a reference oscillator for developing a transmit carrier signal. The phase-locked loop includes a phase comparator, loop filter and voltage controlled oscillator in a forward path for generating a transmit and receive injection signal. A first mixer mixes the injection signal and an offset signal from an offset oscillator to generate the transmit carrier signal. A divider divides the transmit carrier signal by the pre-selected number to generate a feedback signal which is also coupled to the phase comparator. A second mixer mixes the injection signal with a receive carrier signal to generate an intermediate frequency signal, which is coupled to a receiver and demodulated. A duplexer couples the transmit carrier signal to an antenna and also couples the receive carrier signal from the antenna. Two techniques are described for modulating the transmit carrier signal with information signals, such as voice, tone, or data signals. According to one technique, both the reference oscillator and the offset oscillator are modulated. Since the modulations are cancelled by the phase detector, the injection signal is not modulated. According to the other technique, only the offset oscillator is modulated. Modulation of the error signal is cancelled by combining it in a summer with the integrated information signal provided by an integrator. The second modulation technique may be utilized when it is desired to lock the phase-locked loop to an external reference signal or when the reference oscillator cannot be modulated.
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