A selective call receiver (100) includes a receiver (1104) for receiving a signal. The receiver (104) includes a controlled filter (142) having a variable frequency response. A decoder (106) coupled to the controlled filter (142) of the receiver (104) decodes the received signal. The decoder (106) has a decoder clock for generating a decoder clock signal (122). A controlled oscillator (134) has an input filter (136) controlled by a control signal (132). The controlled oscillator (134) generates an output signal (140) in response to the control signal (132) for coupling to a comparator (124) which generates the control signal (126) from the decoder clock signal (122) and the output signal (140). The control signal (126) is further coupled to the controlled filter(142) for controlling the frequency response of the controlled filter (142) which filters the received signal.

A tuner for receiving signals broadcast on two spaced bands, each divided into several channels, has two fixed bandpass filters, one to pass all of the channels in the lower band between the lowest frequency f.sub.P of the lowest channel and the highest frequency f.sub.S of the highest channel of the lower band, and the other to pass all of the channels in the upper frequency band between the lowest frequency f.sub.T of the lowest channel and the highest frequency f.sub.W in the highest channel of the upper band. The bandwidth of each channel is f.sub.B. The tuner includes an amplifier and a third filter to which signals of either band are supplied by selective operation of a switch. The third filter has a cut-off frequency below the second harmonic of the lowest frequency of the upper band to simplify the avoidance of image interference from second harmonic signals of the band. A tunable local oscillator supplies a heterodyning signal to the mixer to convert any selected channel of the received signal to an intermediate frequency (i.f.) band, and image interference is avoided by setting the local oscillator frequency f.sub.n required to convert channel n to the i.f. at a frequency above that channel such that the lowest frequency f.sub.L of the i.f. band satisfies the following conditions: ##EQU1##

A plurality of parallel branch paths are connected in an intermediate frequency or high frequency amplifying circuit of a radio receiver for the purpose of switching the bandwidth. Between an intermediate point of each branch path and the ground is connected output electrodes of a transistor, and the base electrode of the transistor is selectively connected to a source of control voltage via a transfer switch to render the selected transistor conductive.

The demodulator combines a conventional frequency discriminator with a bandpass filter for removing the noise which is present from the band occupied at a given instant by the modulated signal. The center frequency of the bandpass filter is controlled in dependence on the instantaneous frequency of the modulated signal and its bandwidth b.sub.u is controlled in dependence on the instantaneous width of the band occupied by the modulated signal.

A digital communication system has a plurality of channels with distinct carrier frequencies and overlapping bands. Each receiver includes a filter having a time varying bandwidth. The received signal includes at least one carrier modulated by an information signal having a plurality of levels and transitions between the levels at predetermined intervals and components due to other channels. The receiver is synchronized to a selected one of the carriers. The received signal is translated to baseband and applied to the time varying filter which is set to a first bandwidth at the information signal transitions and reduced in the intervals between transitions. The first filter bandwidth accommodates the information signal transitions. Reducing the bandwidth between transitions limits interference components in the time varying filter output when the information signal level is sampled. The technique may be applied to various modulation schemes.