GB1287375A - A device for transmission of information at varying frequency - Google Patents

Abstract

1287375 Radio communication system PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 18 Dec 1970 [23 Dec 1969] 60252/70 Heading H4L In a secret communication system of type wherein the frequency of transmitted carrier is changed in steps according to a repeated pattern and the received signal is heterodyned down to a constant frequency IF signal by mixing with a similarly frequency changing local signal, ground reflections &c. of the signal cause the phase of the IF signal to vary with the frequency. To overcome this, the If phase for each of the n frequency steps is detected and stored in a respective position is a memory having n positions, one for each frequency step. The next time a particular frequency step is reached, the IF signal is phase corrected by means of a variable delay under the control of the output of the position of the memory containing the phase information of the same preceding frequency step. The transmission system, Fig. 1, comprises a 10 Mc/s. oscillator 1, having antiphase outputs selected by switch 2 and key 12 to give phase modulation carriage of information at 1-2 bits/sec. The 10 Mc/s. carrier is mixed at 3 with an n frequency stepped signal (10-20 Mc/s.) from frequency synthesizer 4 under the control of driving oscillator 5 and programme selection device 6. The steps are at 100 per second. A filter 8 and amplifier 9 process the carried before transmission. At the receiving system, Fig. 2, the local oscillator produces an n stepped frequency waveform by means 24, 25, 26, 27, respectively similar to means 4, 5, 6, 7 but with frequency is the range (20-30 Mc/s. + IF). An IF signal at constant frequency is thus produced by mixer 23 and fed, via variable phase-shifter 28, narrow band filter 29 and amplitude limiter 30, to one output of a phase detector/mixer 31. An IF stable oscillator 32 feeds the second input of mixer 31 such that it gives an output indicative of the phase difference for each frequency step. This output is changed to digital form in A/D converter 34 and fed, via digital adder 35 to an n position shift register 36 shifted in synchronism with the frequency stepping. As a phase value for the rth step in the pth pattern is read out of the register, the phase value for the instant rth step is the (p+ 1)<SP>th</SP> pattern is being fed into the register, and the two values are combined at 35. Thus the pth pattern values are used to control the phase shifter 28 during the passage therethrough of the (p+1)<SP>th</SP> pattern. The output of the mixer 31 is thus tended to zero. The phase reversals caused by the informations key are extracted by means 37. The amplitude of the IF signal may also change from step to step and this is counteracted by a similar zero control system, Fig. 3 (not shown), using an A.V.C. amplifier instead of phase shifter 28, and a subtractor instead of mixer 31, the whole control system substituting for limiter 30.