At the time of ordinary communication, up-mixer 108 combines a signal with a frequency f2 from frequency synthesizer 124 with I and Q signals for transmission that are quadrature modulated in quadrature modulator 105, down-mixer 115 combines the signal with the frequency f2 with a received signal, then only in the case of receiving a signal with a frequency fr' different from a frequency fr, frequency synthesizer 125 that generates a signal with a frequency f3 different from the frequency f2 is operated, and frequency synthesizer 125 and down-mixer 115 are connected with switch 127, so that down-mixer 115 combines the signal with the frequency f3 with a received signal. It is thereby possible to switch frequencies fast and with low current consumption in receiving a signal with a reception frequency other than a combination of transmission frequency and reception frequency specified in an ordinary radio system.

A circuit device provided in a video signal processing apparatus. The circuit device includes a tuner for tuning a television signal, an IF amplifier for demodulating the television signal, and an RF converter having an RF oscillator for modulating video and audio signals into a television signal in a VHF band. The tuner, IF amplifier and RF converter are accommodated in a shield case. Further, the circuit device includes a circuit including a resonance circuit having a coil and a capacitor for controlling an oscillation frequency of the RF oscillator and a switch for switching a parameter of the resonance circuit to vary the oscillation frequency.

A radio receiver with isolation between an antenna and a regenerative oscillator/detector. The isolation is provided by periodically (at the receiver's quench frequency) uncoupling the antenna from the regenerative oscillator/detector. The receiver preferably includes an antenna, a super-regenerative oscillator/detector, a clock that indicates the quench frequency, a timing circuit coupled to the clock, and a switch coupled to the antenna, to the oscillator/detector, and to the timing circuit. The timing circuit specifies a first part and a second part of each clock period. During the first part of each clock period, the switch at least partially couples the antenna to the oscillator/detector on the nominal reception frequency. During the second part of each clock period, the switch at least partially uncouples these two elements on the nominal reception frequency. The timing circuit determines the first and second parts of each clock period so that the first part corresponds to a phase of sensitivity for the oscillator/detector, and so that the second part corresponds to phases in which oscillations in the oscillator/detector rise in amplitude, and/or have an increased amplitude, and/or decay in amplitude. Thus, the inadvertent emission of electromagnetic radiation from the regenerative oscillator/detector at the nominal reception frequency is attenuated or suppressed at least during the second part of each clock period.

An analog cordless telephone has a handset and a base. The handset and/or the base of the telephone may include the following components. A phase locked loop (PLL) and a voltage controlled oscillator (VCO), adapted to be shared between a receive path and a transmit path. A first analog gate is adapted to gate a received audio signal on and off, in response to a clock signal. A second analog gate is adapted to gate a transmit audio signal on and off, in response to an inverted version of the clock signal. The transmit audio signal is enabled when the received audio signal is gated off, and the VCO is further adapted to be modulated by the enabled transmit audio signal to output a modulated RF signal.

A communications system (10) comprising a base station (12) for controlling a control channel signal and a plurality of traffic channels and a communications device (17) responsive to the control channel signal (16) and receptive to the plurality of traffic channels. The communications device comprises a tuneable filter (48), having an adjustable characteristic, which substantially filters all but a selected traffic channel of the plurality of traffic channels. The communications device further comprises a tuning circuit (24, 38, 40, 44, 46, 52) for adjusting the filter characteristic. The tuning circuit (24, 38, 40, 44, 46, 52) uses the control channel signal (16) as an accurate central frequency (F.sub.o) for a passband of the tuneable filter (48) and adjusts the filter characteristic to substantially a minimum insertion loss at the accurate central frequency, thereby substantially optimizing selectivity and sensitivity for the communications device (17).