Suppression of Radio Interference

the maximum possible reduction of the signal distortion caused by radio interference. Suppression methods take advantage of differences between the signal and the interference; these differences may be in shape, particularly with respect to frequency, spectral distribution and duration, in amplitude and power levels, in the time when the signal and interference reach the radio-receiver input, in the direction from which the signal and interference reach the antenna, and in their polarizations. The greater these differences, the greater the possibilities, in principle, for the suppression of the interference.

If information can be obtained at the receiving location about all the parameters of the interference at any moment of time, the interference can then be suppressed with any degree of accuracy. To this end, oscillations are generated in the radio receiver that have the same shape and magnitude as the interference oscillations but the opposite polarity. When these oscillations are fed into the main circuit of the radio receiver, the interference is canceled. In practice, a combination of interference signals of different types (the noise current) enters the input of the radio receiver; the receiver’s intrinsic noise is unavoidably added to this. The parameters of the noise current usually vary randomly. The cancellation method of suppressing radio interference proves effective only when the dominant interference is additive and has parameters that vary regularly with time. This method is used, for example, to suppress certain kinds of industrial and atmospheric interference.

In those cases where it is impossible to cancel the interference effectively, other methods are used. Relatively simple ones include frequency filtering, time selection, and amplitude control, which are based respectively on the differences between the signal and the interference in frequency, time of arrival, and intensity level. More complex methods include correlation reception and the use of synchronous detectors. The use of receiving antennas with a narrow directivity pattern is best when the signal and the interference reach the antenna from different directions.