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.
1 . A continuously variable bandwidth filter comprising, frequency conversion means, local oscillator means adapted to control the frequency conversion means, a first filter network and a second filter network means, the first and second filter networks having the same bandwidth and the output of the first filter network being coupled to input of the second filter network by means of the frequency conversion means, characterised in that the first and second filter networks exhibit a generalised Chebyshev transfer function.
2 . A continuously variable bandwidth filter according to claim 1 , characterised in that the second filter network exhibits a pseudo high pass characteristic.
3 . A continuously variable bandwidth filter according to claim 2 , characterised in that the first filter network exhibits a pseudo-high-pass characteristic.
4 . A continuously variable bandwidth filter according to claim 2 , characterised in that the first filter network exhibits an elliptic low pass characteristic.
5 . A continuously variable bandwidth filter according to any preceding claim, wherein the frequency response overlap of the first and second filter networks produces a composite band-pass filter which is determined only by the steep band edge transition region of the individual filter networks.
6 . A continuously variable bandwidth filter according to any preceding claim, wherein the maximum pass-band of the first and second filter networks is selected to precisely fit the maximum band-pass bandwidth required by in a channelised radio communications system.
7 . A continuously variable bandwidth filter according to any preceding claim, wherein the frequency converting means comprises a heterodyne mixer.
8 . A continuously variable bandwidth filter according to any preceding claim, further comprising a second frequency conversion means for up or down conversion of the output signal of the second filter network to a suitable frequency transmission band.
9 . A continuously variable bandwidth filter according to any preceding claim, further comprising a second frequency conversion means for up or down conversion of the input signal to the first filter network to a suitable frequency transmission band.
10 . A satellite communications system including a plurality of continuously variable bandwidth filters according to any preceding claim.