Variable inductance LC resonant circuit and radio receiver using the same

Abstract

The present invention provides a means for improving the sensitivity and selectivity of a car radio receiver. The variable inductance LC resonant circuit comprises: a amplifier 53 having enough high input impedance and enough low output impedance, a inductive element 51 connected a terminal to the input of said amplifier 53 and the other terminal to the output terminal of said amplifier 53, and a capacitive element 52 connected a terminal to the input terminal of said amplifier 53 and the other terminal to the ground. The proposed technique alters the parallel resonant frequency by varying an equivalent inductance 51, 53 seen from the condenser 52 side, wherein the equivalent inductance 51, 53 varies associated with the gain of said amplifier depending on the frequency control voltage from the PLL synthesizer to the terminal 54.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an equivalent circuit of an antenna.

FIG. 2 shows a coil switching scheme of the prior art.

FIG. 3 show a typical front end of a radio receiver of the prior art.

FIG. 4 shows a typical property of an antenna stage of the prior art.

FIG. 5 shows an embodiment of a variable inductance LC resonant circuit of the present invention.

FIG. 6 shows an equivalent circuit of the variable inductance LC resonant circuit of the present invention.

FIG. 7 shows an equivalent circuit of the circuit shown in FIG. 6.

FIG. 8 shows an equivalent circuit of the resonant circuit shown in FIG. 6 with an external load.

FIG. 9 shows a feedback path of the variable inductance LC resonant circuit of the present invention.

FIG. 10 shows a Nyquist locus of the variable inductance LC resonant circuit of the present invention.

FIG. 11 shows an embodiment of a variable gain amplifier used in the variable inductance LC resonant circuit of the present invention.

FIG. 12 shows an embodiment of a pre-amplifier included in the variable gain amplifier used in the variable inductance LC resonant circuit of the present invention.

FIG. 13 shows an embodiment of a post-amplifier included in the variable gain amplifier used in the variable inductance LC resonant circuit of the present invention.

FIG. 14 shows an example of the variable range of the variable inductance LC resonant circuit of the present invention.

FIG. 15 shows the other embodiment of the variable inductance LC resonant circuit of the present invention.

FIG. 16 shows embodiments of both a tap coupling and secondary coil coupling for the use of the variable inductance LC resonant circuit of the present invention as a tuning circuit.

FIG. 17 shows an embodiment of an oscillator with the variable inductance LC resonant circuit of the present invention.

FIG. 18 shows an embodiment of sharing the pre-amplifier of the variable inductance LC resonant circuit of the present invention with a RF amplifier having AGC function.

FIG. 19 shows an embodiment of sharing the pre-amplifier of the variable inductance LC resonant circuit of the present invention with a RF mixer.

FIG. 20 shows a typical radio receiver of the prior art.

FIG. 21 shows an embodiment of a radio receiver of the present invention.

Claims

1. A variable inductance LC resonant circuit comprising: a amplifier having enough high input impedance and enough low output impedance, a inductive element connected a terminal to the input of said amplifier and the other terminal to the output terminal of said amplifier, and a capacitive element connected a terminal to the input terminal of said amplifier and the other terminal to the ground;wherein resonant frequency of said resonant circuit is variable by changing the gain of said amplifier less than +1.

2. A radio receiver using the variable inductance LC resonant circuit of claim 1.