Telecommunications – Receiver or analog modulated signal frequency converter – Carrier and single sideband
Patent
1993-06-01
1997-02-11
Faile, Andrew
Telecommunications
Receiver or analog modulated signal frequency converter
Carrier and single sideband
455296, H04B 106
Patent
active
056031106
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates generally to double-sideband telecommunication signals and, more particularly, to interference elimination in such signals.
BACKGROUND OF THE INVENTION
It is known in principle that low frequency base signals of several origins can be converted into higher frequency signals by means of carrier frequencies. Such higher frequency signals can be transmitted via a common transmission path (either wire or wireless) in the form of a wide band transmission line or as a frequency band of a radio transmission.
With signals so converted, a mix of the higher frequency signals occurs in the transmission path. At the end of the transmission path, each individual higher frequency signal must be filtered out before it can again be converted (by means of the carrier frequency allocated to it) into the original low-frequency base signal. Such processes have to take place without substantially impairing the "intelligibility" of the signal and without significant "cross talk" between the individual channels.
During conversion, two sidebands of the same channel are created (a lower and an upper sideband), one each on either side of the carrier frequency. Either or both sidebands can be transmitted. And the carrier itself can be transmitted or such carrier can be suppressed on the transmission side. When suppressed, a new carrier of the same frequency has to be re-created at the receiving side, in order to re-convert the respective signal into the low-frequency base signal.
The aforementioned single-sideband transmission has been proven to be successful in utilizing the full "width" of the available transmission band of the common transmission path and yet spacing the channels as closely as possible, thereby obtaining as many channels as possible in the available band. However, it requires higher expenditure for high quality filtering circuits at the end of the transmission path, in order to filter out the only sideband and to regain the original base signal without loss of information. In addition and in the case of single-sideband reception, the demodulation of the signal cannot be performed through simple rectification, as in the case of double-sideband oscillations. Rather, demodulation is by multiplying the high-frequency signal and the intermediate-frequency signal by an oscillation, the frequency of which must be exactly equal to the frequency of the carrier.
The technology involved in double-sideband receivers, which is comparatively simple, is believed to be the reason why single-sideband technology has not found favor for use with the internationally-recognized wave plans, especially in the area of short, medium and long wave radio transmission. Moreover, the double-sideband transmission often has an advantage for the owners of single-sideband receivers. If one sideband is subject to interference from an adjacent signal, the receiver user can switch reception to the other sideband, providing the latter is substantially interference-free. This is sometimes referred to as diversity reception. Therefore, double-sideband transmission arrangements continue to exist.
However, compared to the conditions in the case of single-sideband transmission, there is a requirement to reduce the number of channels in the frequency band of the transmission path in half or to narrow the individual channels with regard to their low-frequency band width. If, however, the same number of channels (such number often being the same as with single-sideband transmission at nearly equal band width) is transmitted, there will almost certainly be overlapping of those sidebands of two double-sideband oscillations which are adjacent to one another. That is, the upper sideband of one oscillating channel will overlap and interfere with the lower sideband of the next-higher oscillating channel of different origin.
With such two channels of different origin, it is not possible (or at least not easy) when receiving the signals to filter out at least one of the sidebands from the "mix" of overlapping sideban
REFERENCES:
patent: 2140912 (1938-12-01), Huber
patent: 4314376 (1982-02-01), Williams
patent: 4691375 (1987-09-01), McGeehan et al.
patent: 4992747 (1991-02-01), Myers
patent: 5179589 (1993-01-01), Nowicki
"Eliminating Adjacent-Channel Interference," by Taylor, Wireless World, Jul. 1977, vol. 83 No. 1499.
"AM Receivers Without Interference", Illingworth, Wireless World, vol. 87, (1981) Oct., No., 1549.
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