Bandwidth limiting circuit with variable bandwidth

A bandwidth limiting circuit with a variable bandwidth comprising:

• an input terminal for receiving an input signal to be limited in bandwidth;
• an output terminal for outputting a bandwidth limited output signal;
• bandpass filter means, which includes a surface acoustic wave filter with a fixed passing bandwidth connected between said input terminal and said output terminal for limiting the bandwidth of the input signal to output the same;
• first resonance circuit means connected to said bandpass filter means, said first resonance circuit means receiving a control voltage and having a resonance frequency varying in proportion to the control voltage, said resonance frequency having a frequency near the upper limiting frequency of the passing band of said bandpass filter means;
• and
• second resonance circuit means connected to said bandpass filter means, said second resonance circuit means receiving the control voltage and having a resonance frequency varying reverse to the varying direction of said first resonance circuit means in proportion to the control voltage, said resonance frequency having a frequency near the lower limiting frequency of the passing band of the bandpass filter means;
• wherein said first resonance circuit means includes an upper limiting side LC series resonance circuit having at least one variable capacity diode as a capacity element, said variable capacity diode varying its capacity value in response to said control voltage, and
• said second resonance circuit means includes a lower limiting side LC series resonance circuit having at least one variable capacity diode as a capacity element, said variable capacity diode being connected reverse to a direction of connection of said variable capacity diode in said upper limiting side LC series resonance circuit, and varying its capacity value in response to said control voltage.

An indoor unit of a satellite broadcasting receiver comprising:

• means for receiving a first intermediate frequency signal;
• local oscillator means for generating an oscillation signal;
• mixer means for mixing the first intermediate frequency signal received by said receiving means and the oscillation signal generated by said local oscillator means to obtain a second intermediate frequency signal;
• a control terminal for receiving a control voltage;
• bandwidth limiting means, having a variable bandwidth, for limiting the second intermediate frequency signal to a desired bandwidth, said bandwidth limiting means including:
• an input terminal for receiving the second intermediate frequency signal obtained by said mixer means;
• an output terminal for outputting the second intermediate frequency signal limited in its bandwidth;
• a bandpass filter with a fixed passing bandwidth connecting between said input terminal and said output terminal for limiting the bandwidth of the input second intermediate frequency signal to output the same;
• a first resonance circuit connected to said bandpass filter and said control terminal, said first resonance circuit receiving said control voltage and having a trapping frequency near one of the upper and lower limiting frequencies of the passing band of said bandpass filter, said tripping frequency varying in proportion to the control voltage;
• and
• a second resonance circuit connected to said bandpass filter and said control terminal, said second resonance circuit receiving the control voltage and having a trapping frequency near the other of the upper and lower limiting frequencies of the passing band of said bandpass filter, said trapping frequency varying reverse to the varying direction of said first resonance circuit in proportion to the control voltage;
• and
• demodulating means for demodulating the second intermediate frequency signal limited in its bandwidth by said bandwidth limiting means to obtain a video signal.