Abstract: An improved Phase-Locked Loop (PLL) radio receiver circuit includes a phase detector, either an active loop filter or a set of parallel, alternately selectable loop filters, and a Voltage-Controlled Oscillator (VCO) whose input is connected to the output of the loop filter. The output of the VCO is fed to frequency divider. The output of the frequency divider and a reference frequency signal form the inputs to the phase detector/comparator. Toggling among the loop filters occurs such that the loop limit frequency, defined by a loop amplification of 1, is altered by this toggling without significantly changing the gradient of the amplification to the frequency, in a frequency range which includes the limit frequency.

Abstract: FM radio receivers, particularly in moving vehicles, are subject to intermittent adjacent-channel interference. It is known to detect such interference and suppress it by narrowing bandwidth in the Intermediate Frequency (IF) signal path. However, it is desirable to again broaden the bandwidth once the interference has subsided. An improved system, which automatically and dynamically adjusts the bandwidth, employs first and second signal level evaluation circuits (11, 12), a comparison stage (13) which compares their outputs, and a variable-bandwidth filter stage (6, 14) controlled by the comparison stage. The comparison stage is programmed with a set of signal difference threshold values and corresponding filter bandwidth values, and controls the filter stage (6, 14) to adjust the bandwidth to the appropriate value, indicated by which signal difference threshold value has been exceeded. In a preferred embodiment, a second IF filter (15) does "double duty" as part of the variable-bandwidth filter stage.

Abstract: A mobile radio receiver, for example, a car radio, needs a circuit to detect and suppress adjacent-channel interference. This can be done by using bandpass filters (14) of differing bandwidth. The present invention dynamically adjusts bandwidth to an optimum value by continuously comparing (13) a first signal level, upstream of the bandpass filter, to a second signal level, downstream of the bandpass filter, to derive a difference value representative of adjacent-channel interference, then uses an electronically controlled selector switch (6) to select a particular filter (14.sub.N) whose bandwidth is sufficiently narrow to cut out the interfering broadcast signal. A table associating each magnitude range of interference with a particular filter bandwidth assures that bandwidth is broadened as interference diminishes, thereby assuring sufficient reception signal strength.