Narrow-band filter having a variable center frequency

A narrow-band filter supplied through an input terminal with an input signal having a predetermined frequency for filtering said input signal to produce a filtered signal through an output terminal and comprising:

• first through n-th low-pass filters having the same filter characteristic which are connected in parallel to one another, where n represents a positive integer greater than unity;
• switching means connected to said first through said n-th low-pass filters for time divisionally connecting said first through said n-th low-pass filters between said input and said output terminals, said first through said n-th low-pass filters time divisionally filtering said input signal for first through n-th filtering durations, respectively, and producing first through n-th time division filtered signals, respectively, collectively as said filtered signal;
• and
• control means connected to said switching means and supplied with a frequency designation signal designating said predetermined frequency for controlling said switching means to vary at least one of said first through said n-th filtering durations in accordance with said frequency designation signal, such that said narrow-band filter has center frequency which varies in accordance with said at least one of said first through said n-th filtering durations.