This invention relates to a narrow-band filter having a variable center frequency. Such a narrow-band filter is particularly useful for detecting a plurality of tone signals which have different frequencies.
In general, a narrow-band filter comprises an input terminal supplied with an input signal having a predetermined frequency, an output terminal, first through n-th low-pass filters which are connected in parallel to one another, where n represents a first positive integer greater than unity, a switching circuit connected to the first through the n-th low-pass filters, and a control circuit for controlling the switching circuit. The first through the n-th low-pass filters have the same filter characteristic and are collectively called an n-path filter. The control circuit delivers first through n-th switch driving signals to the switching circuit. Supplied with the first through the n-th switch driving signals, the switching circuit time divisionally connects the first through the n-th low-pass filters one by one between the input and the output terminals at a predetermined period Tp. As a result of switching operation of the switching circuit, the first through the n-th low-pass filters time divisionally filtering the input signal for first through n-th filtering durations, respectively. The first through the n-th filtering durations are equal to one another. In this event, the narrow-band filter has a single center frequency fc. Such a narrow-band filter is disclosed in Japanese Unexamined Patent Prepublication No. 16013/1985 (Tokkai Sho 60-16013).
In the meantime, a mobile radio telephone system of an AMPS type uses an SAT (Supervisory Audio Tone) signal in order to monitor a connection state of a radio channel. The SAT signal is carried by a radio signal and has one of first through third tone frequencies which are equal to 5970 (Hz), 6000 (Hz), and 6030 (Hz), respectively, and which are assigned to base stations. Let the mobile radio telephone system comprise first through third base stations which are assigned with the first through the third tone frequencies, respectively. When the first base station transmits the SAT signal having the first tone frequency, a mobile station receives the SAT signal transmitted from the first base station. When the mobile station detects the first tone frequency of the SAT signal, the mobile station transmits a transmission signal, as an answer back signal, having a frequency equal to the first tone frequency. On reception of the transmission signal having the frequency equal to the first tone frequency, the first base station confirms the fact that the radio channel is connected to the mobile station.
As apparent from the above description, the mobile station must discriminate a tone frequency among the first through the third tone frequencies. The narrow-band filter mentioned above is particularly useful for detecting the tone frequency. However, the mobile station must be provided with first through third narrow-band filters having first through third center frequencies which are equal to the first through the third tone frequencies, respectively. This is because the narrow-band filter detects a single tone frequency only. This means that the mobile station requires a receiver circuit of a large size for discriminating the tone frequency among the first through the third tone frequencies.