Patent ID: 7587182
Filing Date: 2009-09-08
Classification: H04B

Abstract:
1. A receiver input circuit connected between a receiving antenna and a high frequency amplifier, comprising: a four-terminal R-∞ type low-pass filter having an input termination resistance indicative of a specific resistance value R and an output termination resistance indicative of an infinite resistance value; and an LC parallel tuning circuit connected to one output terminal of the four-terminal R-∞ type low-pass filter via a small-capacitance type coupling capacitor element, wherein the four-terminal R-∞ type low-pass filter has an inductor element and a first capacitor element series-connected between one input and output terminals thereof, and a second capacitor element shunt-connected between one and other output terminals thereof, wherein a series-connected portion of the first capacitor element and the second capacitor element corresponds to one in which a capacitor element shunt-connected between one and other output terminals of the four-terminal R-∞ type low-pass filter is divided into two, and a total capacitance value thereof is selected equal to a capacitance value of the shunt-connected capacitor element, wherein the LC parallel tuning circuit has a tuning capacitor element constituted of a first variable capacitance type capacitor element, wherein the small-capacitance type coupling capacitor element is constituted of a second variable capacitance type capacitor element, and wherein when a tuning frequency of the LC parallel tuning circuit is changed by adjusting the capacitance of the first variable capacitance type capacitor element, a capacitance value of the second variable capacitance type capacitor element is changed in conjunction with the adjustment to the capacitance thereof.