Patent GB-871356-A (GB)
Title: Improvements in or relating to carrier current communication systems incorporating repeaters
Inventor: FRANKTON WILLIAM JESSE
IPC: H04B17/40
Filed: 1959-02-05
Abstract: 871,356. Testing unattended repeaters. STANDARD TELEPHONES & CABLES Ltd. Jan. 29, 1960 [Feb. 5, 1959], No. 4061/59. Class 40 (4). In testing an unattended repeater of a 2-wire carrier system a first single-frequency wave of a frequency individual to the repeater concerned is transmitted from an attended station, together with a second single-frequency wave common to all repeaters. The first wave is passed through a frequency multiplier after amplification in the amplifier of the repeater concerned, a harmonic is selected which is again amplified in the amplifier and is then combined in a modulator with the second wave, a selected sideband being returned to the attended station. If desired, the amplified harmonic of the first wave may be modulated by a band of repeater noise, by an harmonic of the second wave produced by the amplifier, or by an intermodulation product produced by the amplifier instead of by the second wave, in order to test the amplifier noise level or its non- linearity. The invention is described as applied to a 120-channel system using the band 60 to 552 kc/s. for West-East transmission and 672 to 1164 kc/s. for East-West, each direction employing two super-groups of 60 channels at 4 kc/s. spacing with a gap of 12 kc/s. between the two super-groups in each case. Each repeater (Fig. 2) comprises a single amplifier 17 which is used for both transmission directions, being connected to the West via low-pass filter 19 and high-pass filter 20 and to the East by lowpass filter 23 and high-pass filter 22. As shown, the amplifier 17 lies between two hybrid coils 24 and 25 which are of the " skew " type, giving a loss from the junction of filters 19 and 22 to the amplifier input, and from the amplifier output to the junction of filters 20 and 23 of approximately ¢ dB. in each case, while the loss from conductor 29 to the amplifier input and from the amplifier output to conductor 28 is of approximately 10 dB. in each case. The first test signal, of frequency appropriate to the repeater, lying within the range 302 to 310 kc/s. forming part of the gap between the two West- East super-groups, is applied at the West terminal station and passes via hybrid 25 to the amplifier 17, appearing at conductor 28 at about- 17 dBm., where it is selected by the crystal band-pass filter C1 tuned to the frequency in question and applied to the multiplier 30 at about - 21 dBm. Band-pass filter K1 selects the second harmonic K at about -33 dBm. and applies it via the impedance-matching transformer T1 and the hybrid 25 to the amplifier input. The frequency K is in the range 604 to 620 kc/s., which is in the dead space between the go and return bands, and appears at the output of the amplifier at a relatively high level as the amplifier characteristic favours the higher frequencies. It is selected from the hybrid 24 at point 31 by the filter K2 and, since filters 20 and 23 both present a high impedance at this frequency, appears at the output of K2 at the high level of about + 3 dBm., which is adequate for application to the modulator M as a carrier wave. The second test signal of frequency 408 kc/s. transmitted from the West terminal emerges from the frequency-selective bridge 21 at point 32 at about -17 dBm. and is passed via resistor 33 and the narrow band-pass filter S1 to the signal input of modulator M at about- 40 dBm. The resulting upper sideband in the range 1012 to 1028 kc/s. is selected by filter R1 and passes through hybrid 25 to the main transmission path through amplifier 17, hybrid 24, filter 20 and network 18 and over the cable 3 to the West terminal. The level at the output of filter R1 is about - 50 dBm. which is adequate for measurement on a transmission measuring set at the terminal. Noise test.-The filter N1 selects a band of frequencies 100 kc/s. wide centred on 1632 kc/s. If transmission of the 408 kc/s. test signal is not taking place, the modulator M receives a band of noise 100 kc/s. wide selected from the output of amplifier 17 at about - 69 dBm. This level is about 30 dB. below that of the 408 kc/s. signal when it is present and hence has no noticeable effect on measurements made with that test signal. In the absence of that test signal the noise band modulates the amplified harmonic of the first signal and the lower sideband in the range 1012 to 1028 kc/s. is selected by filter R1 and returned to the West terminal. As R1 has a pass band only 16 kc/s. wide, the effective noise level is reduced by 8 dB. As the part of the spectrum from which the noise is selected is well above the highest frequency transmitted the cable is not equalized at that part of the spectrum so that noise generated by adjacent repeaters will not be present to any measurable extent and stop filters are not required. Non-linearity test.-A test signal of 544 kc/s, may be sent from the West terminal at a relatively high level, the third harmonic 1632 kc/s. passing via filter N1, as did the noise, and being measured at the West terminal in the same manner. Alternatively two or more test signals may be sent and a selected intermodulation product may be measured. Specification 656,188 is referred to.