Abstract: In a communication receiver, a first mixing circuit, a first band-pass filter responsive to the output of the first mixing circuit, a second mixing circuit responsive to the output of the first band-pass filter, a second band-pass filter with a different center frequency from that of the first band-pass filter responsive to the output of the second mixing circuit, a first local oscillator, the output of which is applied to the first mixing circuit and a second local oscillator, the output of which is applied to the second mixing circuit. Various kinds of interference signals are eliminated by changing the output frequencies of the first and second local oscillators to thereby vary the effective bandwidth and center frequency of the signals which can pass through the receiver and thus eliminate interference. In addition, a third mixing circuit is responsive to the output of the second band-pass filter.

Abstract: This invention provides a frequency-synthesizer type transceiver having a voltage-controlled oscillator and means for comparing the phases of mixed frequency, whereby the voltage-controlled oscillator is regulated by signals resulting from the phase comparison to generate a controlled frequency serving as the local oscillator frequency for transmitting and receiving. The frequency-synthesizer type transceiver further includes a means for selectively switching from the one carrier frequency to the other to differentiate the transmitting and receiving frequencies.