Abstract: In an antenna duplexer, one antenna is used by switching transmission signals and reception signals having different frequencies. The antenna duplexer includes a high-frequency switching circuit and a band-pass filter. The high-frequency switching circuit supplies the transmission signals from a transmission part to the antenna. The band-pass filter outputs the reception signals outputted from the antenna to the predetermined receiving circuit. In the band-pass filter, the frequency characteristic is selected and set to impress the frequency of the transmission signals, and simultaneously the reflected wave generated at the input end so that impedance increases at the frequency band of the transmission signals.

Abstract: A radio communication apparatus comprises a transmitting circuit, a receiving circuit, an amplifier, a control circuit and a detecting circuit. The receiving circuit receives a signal from an opposite station such as a base station. The transmitting circuit transmits a signal to the opposite station. The amplifier amplifies the signal to be transmitted from the transmitting circuit. The control circuit controls the level of the transmission signal outputted from the amplifier. The detecting circuit detects the reception state in response to the reception level or the error rate of the signal transmitted from the opposite station. And the control circuit selectively switches, in response to the detection signal obtained from the detecting circuit, the output level of the transmission signal obtained via the amplifier from a high-level mode to a low-level mode when the reception state of the signal transmitted from the opposite station satisfies a preset value.

Abstract: An antenna circuit includes a ferrite core around which a main coil and a negative feedback coil are wound, and a field effect transistor (FET) with its source grounded, and with a voltage induced in the main coil being applied across the gate and the source of the FET to thereby form an aperiodic circuit and the drain of the FET being connected through the negative feedback coil to a load.

Abstract: A double superheterodyne receiver has a frontend stage for receiving an RF signal, a first phase-locked loop (PLL) local oscillator circuit for generating a first local oscillator signal, a first mixer for mixing the RF signal with the first local oscillator signal to produce a first IF signal, a second PLL local oscillator circuit for generating a second local oscillator signal, a second mixer for mixing the first IF signal with the second local oscillator signal to produce a second IF signal, a detector for detecting information carried on the second IF signal and control circuitry for controlling the dividing ratios of programmable frequency dividers respectively included in the first and second PLL local oscillator circuits. To tune the receiver to a desired RF frequency, the control circuitry, which can include a micro-computer, selects the dividing ratio N.sub.1 of the first PLL local oscillator circuit and the dividing ratio N.sub.

Abstract: A voltage converter for boosting the voltage and/or inverting the polarity of a DC voltage source having positive and negative terminals includes at least one series circuit of a capacitor and a diode connected through the latter with one of the voltage source terminals while the other end of the series circuit is alternately connected with the positive and negative voltage source terminals through a respective switching circuit, and an output is derived from the connection point between the capacitor and diode of the series circuit. Such output may be applied, for example, through a peak-value rectifier, to one side of a load having its other side connected to the voltage source terminal opposite to that connected with the diode of the series circuit so that a voltage two times that of the source is applied to the load. If the diode of the series circuit is connected with the negative terminal of the voltage source, the polarity of the voltage applied to the load is inverted.