RECEIVER INPUT CIRCUIT

Abstract

The present invention provides a receiver input circuit capable of maintaining impedance matching with an antenna feeder wire within all frequency bands to be used and constituting parallel resonant circuits in multi-stage form without using additional circuit portions. The receiver input circuit includes a constant resistance branching filter, a coupling inductor and a tuning circuit. The constant resistance branching filter comprises a low-pass filter and a high-pass filter having termination resistors connected thereto. The low-pass filter and the high-pass filter respectively have equal cut-off frequencies selected to frequencies slightly lower than those lying in a used frequency band and include input ends connected in common to an input terminal of the constant resistance branching filter connected to the antenna feeder wire. The tuning circuit has a parallel resonant circuit constituted of a tuning inductor and a variable capacitance diode. The coupling inductor is connected between a midtap of an inductor of the high-pass filter and an input end of the parallel resonant circuit. An output end of the parallel resonant circuit is connected to a high-frequency circuit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The organization and manner of the structure and operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in connection with the accompanying drawings, wherein like reference numerals identify like elements in which:

FIG. 1 shows a first embodiment of a receiver input circuit according to the present invention and is a circuit diagram illustrating a circuit configuration thereof;

FIG. 2 is a characteristic diagram showing one example of frequency selectivity obtained in the receiver input circuit illustrated in FIG. 1;

FIG. 3 depicts a second embodiment of a receiver input circuit according to the present invention and is a circuit diagram showing a circuit configuration thereof;

FIG. 4 is a characteristic diagram showing one example of frequency selectivity obtained in the receiver input circuit illustrated in FIG. 3;

FIG. 5 shows a third embodiment of a receiver input circuit according to the present invention and is a circuit diagram showing a circuit configuration thereof; and

FIG. 6 is a characteristic diagram showing one example of frequency selectivity obtained in the receiver input circuit illustrated in FIG. 5.

Claims

1. A receiver input circuit connected between an antenna feeder wire and a high-frequency circuit, comprising: a constant resistance branching filter;a tuning circuit; anda coupling inductor that couples the constant resistance branching filter and the tuning circuit to each other,wherein the constant resistance branching filter comprises a low-pass filter and a high-pass filter both having termination resistors connected thereto,wherein the low-pass filter and the high-pass filter respectively have cut-off frequencies equal to each other, which are selected to frequencies slightly lower than those in a used frequency band, and have input ends respectively connected so as to share an input terminal of the constant resistance branching filter, which is connected to the antenna feeder wire,wherein the tuning circuit has a parallel resonant circuit comprising a tuning inductor and a variable capacitance diode,wherein the coupling inductor is connected between a midtap of an inductor constituting the high-pass filter and an input end of the parallel resonant circuit, andwherein an output end of the parallel resonant circuit is connected to the high-frequency circuit.

2. The receiver input circuit according to claim 1, wherein the tuning circuit is constituted of a single parallel resonant circuit.

3. The receiver input circuit according to claim 1, wherein the tuning circuit comprises a first-stage parallel resonant circuit and a second-stage parallel resonant circuit connected in tandem, wherein a ground end of variable capacitance diode of the first-stage parallel resonant circuit and a ground end of a variable capacitance diode of the second-stage parallel resonant circuit are connected in common, andwherein a commonly-connected point of both ground ends is connected to ground through a high-capacity capacitor having impedance nearly zero to a used frequency.

4. The receiver input circuit according to claim 1, wherein the tuning circuit comprises first-stage, second-stage and third-stage parallel resonant circuits connected in tandem, wherein a ground end of a variable capacitance diode of the first-stage parallel resonant circuit and a ground end of a variable capacitance diode of the second-stage parallel resonant circuit are connected in common,wherein a commonly-connected point of the ground ends of both variable capacitance diodes is connected to ground through a first high-capacity capacitor having impedance nearly zero to a used frequency,wherein a ground end of a tuning inductor of the second-stage parallel resonant circuit and a ground end of a variable capacitance diode of the third-stage parallel resonant circuit are connected in common, andwherein a commonly-connected point of the ground ends of both the tuning inductor and the variable capacitance diode is connected to ground through a second-capacity capacitor having impedance nearly zero to a used frequency.