Tunable input trap circuit and image trap circuit

An image trap circuit, which is inserted in an input stage of an FM tuner circuit, for removing an image frequency component included in a received signal and then outputting the received signal to a circuit at a later stage, said image trap circuit comprising:

• a parallel oscillation circuit in which a pair of variable capacitance diodes are connected in series such that cathodes thereof are connected to each other at a junction portion and in which an inductance is connected in parallel to the variable capacitance diodes between anodes of the variable capacitance diodes at both ends thereof;
• a voltage applying device for applying a voltage to the junction portion so as to vary capacitance values of the variable capacitance diodes on the basis of reverse voltage characteristics of the variable capacitance diodes respectively;
• and
• a variable capacitor element connected between the junction portion and a ground,
• wherein a voltage variable range of said voltage applying device is set in correspondence with a frequency band of the image frequency component, and
• the variable capacitor element is not connected to the anodes of the variable capacitance diodes.

A trap circuit for removing an undesired component from an input signal having both the undesired component and a desired component, the trap circuit comprising:

• an oscillation circuit comprising two variable capacitance diodes connected in series such that the cathodes thereof are connected at a junction portion, and an inductance connected in parallel to the series-connected variable capacitance diodes;
• a voltage applying device for applying voltages to the junction portion to vary the capacitances of the variable capacitance diodes;
• and
• a variable capacitance element which provides a path from the junction portion to ground that does not pass through the voltage applying device.

A trap circuit for removing an undesired component from an input signal having both the undesired component and a desired component, the trap circuit comprising:

• an oscillation circuit comprising two variable capacitance diodes connected in series such that the cathodes thereof are connected at a junction portion, and an inductance connected in parallel to the series-connected variable capacitance diodes;
• a voltage applying device for applying voltages to the junction portion to vary the capacitances of the variable capacitance diodes;
• and
• a variable capacitance element connected between the junction portion and ground,
• wherein adjustment of the variable capacitance element adjusts the frequency of the undesired component removed by the trap circuit while maintaining the desired component at substantially the same signal level, and
• the variable capacitance element is not connected to the anodes of the variable capacitance diodes.