Patent GB-1001116-A (GB)
Title: Frequency-modulated supervisory circuit
Inventor: OSWALD THOMAS
IPC: H04B17/40
Filed: 1963-05-08
Abstract: 1,001,116. Supervising unattended repeaters. SUBMARINE CABLES Ltd. May 7, 1964 [May 8, 1963], No. 18191/63. Heading H4R. A submarine cable carrier telephone system includes a number of bi-directional repeaters which are identical in design each repeater having a test circuit which receives a common test signal transmitted from one terminal of the cable, converts the frequency of the test signal, and transmits the converted test signal back to the terminal, in which the test signal consists of a frequency modulated sine wave signal in which during a substantial portion of the modulation cycle the signal frequency changes linearly with time. The terminal equipment distinguishes the return signal from different repeaters by comparison of the instantaneous frequencies of the return signals which, due to the different delays due to the different loop lengths, are different for different repeaters. Fig. 1 shows a typical bi-directional repeater in which on the far side of the repeater from the terminal from which the test signal originates is a test circuit comprising a filter 4 which selects the test signal to apply it to a frequency doubler 5 the output from which is filtered in 6 and applied via a level setting attenuator 7 to the line 2 for backward transmission through the repeater to the test terminal. The frequency doubler could be replaced by a modulator to accomplish the frequency conversion from the send channel to the receive channel in the repeater. Figure 2 shows the test terminal apparatus in which the output of a timing oscillator 1, of e.g. 100 c.p.s., is squared in 2 and synchronizes a sawtooth generator 3 which frequency modulates an oscillator 5 via a reactance valve 4. The frequency sweep is applied via a level setting attenuator 7 and an amplifier 8 to the line to the repeaters. In addition the squared timing wave is applied via a variable delay circuit 9, which is set to a delay equal to the loop delay to the repeater it is desired to test, to a sawtooth generator and frequency modulator 10, 11, 12 similar to the units 3, 4, 5, save that the frequency of the f.m. signals differ at all times by a frequency of say 5 kc/s. This frequency modulated wave is frequency doubled in unit 13, similar to the frequency doubler in the repeaters, and the resulting signal is applied via an amplifier 14 to a modulator 15 as a carrier signal. The modulator 15 is also fed with the test signal returned from the repeaters and produces with the test signal from the repeater under test, selected by the setting of the delay 9, a constant frequency, e.g. 10 kc/s., which is selected by the narrow band filter 18, amplified in amplifier 19 and applied to an indicator or recorder 20 to give a measure of the transmission loss in the loop including the repeater under test. Alternative methods of generating the f.m. signal are suggested, e.g. synchronous motors driving variable capacitors, the delay between the test signal and the demodulating signal being produced by relative displacement of the motor or capacitor stators or the capacitor rotors, or one f.m. signal may be generated and recorded on a magnetic drum or stored in acoustic form in a mercury column or a wire, the two f.m. signals with appropriate delay being obtained by two relatively displaceable read-out heads, Fig. 4 (not shown). A further system, Fig. 5, utilizes only one f.m. signal the beginning of which is utilized as the test signal and the later part of the frequency sweep as the local oscillator for demodulating the returned test signal. In this case the requirement of linearity of sweep is more severe and therefore a feedback loop is provided over the oscillator by the linear discriminator 63. In this system the demodulated return signal at the output of filter 53 will have a frequency dependent on the delay time over the loop including the repeater under test. The particular frequency corresponding to the repeater under test is selected by the modulator 54 which, fed by the appropriate frequency from variable frequency oscillator 55, transposes the required signal to a frequency of 20 kc/s. which is applied via a high pass filter with a cut-off at 19À2 kc/s. to a modulator supplied with a carrier frequency of 19À5 kc/s. the required signal, at 500 c/s., being selected by band-pass filter 59 and applied to the indicator 62 via the amplifier 60 and rectifier 61. Gates 21 and 22 may be connected in the test signal channels in any of the above described embodiments to gate out the flyback portions of the sawtooth f.m. test signals. If a modulator is provided in the repeaters in place of the frequency doubler a similar modulator should be employed in place of the doubler 13 in the terminal equipment.