Patent GB-1502254-A (GB)
Title: Spread spectrum multiple access modulation system receivers
IPC: H04B1/16
Filed: 1971-07-28
Abstract: 1502254 Receiving SIEMENS AG 28 July 1971 [30 Sept 1970] 35549/71 Heading H4L An SSMA superheterodyne receiver comprises a first means, such as a narrow bandwidth band pass filter 16, to extract from the IF signal, at a centre frequency, a first signal component, a second means to extract from the IF signal one or more other signal components whose frequency is outside the bandwidth of the first signal component, and means for comparing the signal power of the first and other signal(s) to produce an output control signal if coincidence between the codes of a received signal and a receiver-generated signal is present. The received RF signal passes from an antenna (1) (Fig. 1, not shown), through a superhet. mixing stage (2) a variable-gain amplifier (5), a multiplier (6) and a band pass filter 8 whose bandwidth is small compared with the IF bandwidth. The output from the filter 8 is supplied two parallel transmission paths. In one embodiment one path comprises a band pass filter 16 whose bandwidth is narrow compared with that of the IF signal, but whose centre frequency corresponds with the frequency of the effective signal carrier in the IF signal; the other path comprises a band pass filter 19 of the same bandwidth as filter 16, but of differing pass band, located within the bandwidth of the IF signal. Each path further comprises a rectifier 17 or 20 and a low-pass filter 18 or 21; one path also includes an inverter 22 and, optionally a measuring instrument 25. Outputs from the paths are fed to a summing element 23 and thence to an analyser 24. In the event that the latter detects a positive signal, this produces a character for code phase coincidence which is supplied as a control signal to circuits 14, 7. As long as the code phase coincidence condition has not been established, the power distribution at the output of the band pass filter 8 is almost constant. Consequently, the output signal powers from filters 16 and 19 are the same. However, when code phase coincidence is established, there will be a steep rise in power within the effective signal range, i.e. that range assigned only to filter 16. Thus, the outputs of the filters 16 and 19 differ and a positive signal appears at the output of the analyser 24. Instrument 25 can be used to indicate the signal-to-noise ratio in the associated transmission path. In another embodiment the filter 19 is replaced by a filter 28, identical to the filter 16, preceded by a modulator 27. In this arrangement two frequency sub-components, symmetrical about the pass band of the filter 16, are provided. In both embodiments the bandwidth of the filter 18 is variable so that it is reduced when the output from the analyser indicates phase coincidence, but is otherwise wider. The circuits described are independent of any fluctuations in the level of the IF signal (due to varying transmitter usage of the system) since both transmission paths experience the same fluctuations and the difference, only, of the outputs from these paths is used to obtain the code phase coincidence criterion.