1. Field of the Invention
The present invention relates to a frequency synthesizer for use in a transmitter/receiver. More specifically, the present invention relates to a frequency synthesizer for use in a transmitter/receiver adapted to be capable of providing an oscillation frequency signal of various desired frequencies.
2. Description of the Prior Art
A digital frequency synthesizer has been proposed and in practical use. Such a frequency synthesizer is much more advantageous in that it can provide a much more stabilized oscillation frequency. A typical frequency synthesizer employs a phase locked loop, which is often simply referred to as "PLL."
A frequency synthesizer employing a phase locked loop usually comprises a voltage controlled oscillator the oscillation frequency of which is controllable as a function of an output voltage, as low pass filtered, obtainable from a phase detector, which is adapted to compare the phase or the frequency of the output from a reference oscillator and the phase or the frequency of an output from a programmable frequency divider adapted to frequency divide the output frequency from the said voltage controlled oscillator at the frequency division rate which is adapted to be variable as a function of a control signal. Automatic scanning of the oscillation frequency of the output from the said voltage controlled oscillator is effected by varying the said control signal and thus the frequency division rate of the programmable frequency divider.
Such a frequency synthesizer has been utilized by way of a frequency signal generator in a citizens band transceiver, for example. However, since in a citizens band transceiver the frequency of the received signal in the receiving mode is in any one frequency band within the frequency range of 29.965 MHz through 27.255 MHz depending on the channel of the received signal, the oscillation frequency of the frequency signal generator should also be changed within the frequency range of 37.66 MHz through 37.92 MHz depending on the receiving channel in order to obtain a constant intermediate frequency of 10.695 MHz. On the other hand, in the transmitting mode, the oscillation frequency of the frequency signal generator should be changed within the frequency range of 26.965 MHz through 27.255 MHz depending on the channel in which a modulated carrier wave is to be transmitted. A typical approach for changing the frequency variable range depending on the transmitting and receiving modes in such a frequency signal generator employing a frequency synthesizer comprises two reference oscillators, which are typically crystal oscillators. One of them is a reference oscillator to be included basically in a frequency synthesizer for providing a reference signal of the frequency of 26.965 MHz to be phase detected and the other oscillator is a reference oscillator to be used only on the occasion of reception for providing a reference frequency of 10.695 MHz to be mixed with the output from a voltage controlled oscillator to be used by way of a local oscillator.
As described previously, a typical prior art frequency synthesizer to be used in a transceiver comprises at least two crystal oscillators. Since provision of one such crystal oscillator entails an increase of cost of approximately ten percent of the whole apparatus, reduction of cost in this connection is of great interest. If any scheme can be implemented which is capable of skillfully selecting the transmitting and receiving modes with a less number of crystal oscillators, then it would be much advantageous from the stand point of cost, even if a configuration of the circuit concerned becomes complicated, insofar as the circuit is implemented by an integrated circuit.