IMPROVEMENTS RELATING TO CHANNEL FILTERING IN RADIO COMMUNICATIONS SYSTEMS

A system which provides frequency conversion and continuously variable bandwidth control is implemented using first and second filter networks that exhibit a generalised Chebyshev transfer function. The first and second filter networks may comprise a pseudo-high-pass type filter in combination with a pseudo-low-pass type filter, or in a particularly efficient embodiment, a pseudo-high-pass type filter in combination with an elliptic low pass type filter. The effective frequency response overlap of first and second filter networks produces a composite band-pass filter response which is highly selective by nature and is determined only by the steep band edge transition region of the individual filter networks. The maximum pass-band of the first and second filter networks can be tailored to precisely fit the maximum band-pass bandwidth required by a channelised radio communications system. This is advantageous in that the individual filter pass-bands will be fully utilised and the single sided band edge transition slope is for a given number of components is optimised. This eliminates the requirement to increase the number of circuit components in order to achieve the desired selectivity.