Patent GB-993585-A (GB)
Title: Repeater for multiplex communication system
IPC: H04J1/08
Filed: 1963-03-22
Abstract: 993,585. Radio signalling; frequency modulation. STANDARD TELEPHONES & CABLES Ltd. March 22, 1963 [March 23, 1962], No. 11481/63. Headings H3R and H4L. A repeater in a frequency division multiplex system is provided with means for dropping and inserting channel signals without demodulation and remodulation of the incoming channel signals. The arrangement is a modification of that disclosed in Specification 927,570. As shown in Fig. 1, the incoming signals are changed to an intermediate frequency #t in a conventional receiver 4 and supplied to a phase comparator 18, the output of the comparator being supplied via a band pass filter 19 to frequency modulate an oscillator 20 of frequency fi and via a band reject filter 22 to frequency modulate an oscillator 23 of frequency f 2 . The filter 19 passes only the signals to be utilized at the repeater and the filter 22 passes only the remainder of the signals for re-transmission. The outputs of the oscillators 20, 23 are supplied to a mixer 21 supplying an output of mean frequency # 1  + # 2  = #t to the phase comparator 18. Assuming that the I.F. signal from the amplifier 10 is unmodulated, there will be no output from the phase comparator but at the first instant that frequency modulation is present a resultant signal is produced from the comparator which is proportional to the modulating signal or baseband. During the next instant of time the signal from amplifier 10 will be compared with that from mixer 21 due to an earlier modulation signal so that the resultant signal from comparator 18 follows the frequency variation of the signal from amplifier 10. The channels to be dropped may be obtained directly from the filter 19 and supplied to a device 24 or may be taken from the output of the oscillator 20 and supplied via a frequency discriminator 26 and filter 27 to a utilization device. The output f 2  of the oscillator 17 is supplied via a mixer 33 to a conventional transmitter 5 for re-transmission. Channels corresponding to those dropped may be inserted via mixer 33 from a source 35 which frequency modulates a signal from an oscillator 34. In a modification, Fig. 2 (not shown) a further similar phase control network is included between lines A-A and B-B, Fig. 1 and signals to be inserted are added directly to the signals from the band reject filter. In a further modification, Fig. 3 (not shown) one of the oscillators in the second phase control network is dispensed with. Fig. 4 shows an alternative to Fig. 1 in which frequency rather than phase variation effects the control. The output fi from the I.F. amplifier 10, Fig. 1, is supplied to a mixer 57 together with the output # 0 =# 1 +#t from a mixer 56. The output #o-#t of the mixer 57 is limited at 58 and supplied to a frequency discriminator 59 providing a control signal when the frequency of its input signal deviates from a pre-determined value to control the frequencies # 1 , #t of the oscillators 52, 54 respectively so that they follow any frequency change of the signal #t from the amplifier 10. As before, the bandpass filter 53 is arranged to pass only those signals which are to be dropped and band reject filter 55 only the remainder of the signals. The channels to be dropped are supplied via a frequency discriminator 26 to a utilization device 24 and the channels to be inserted from a source 35 are added to the outgoing signals at 60.