Patent GB-629718-A (GB)
Title: Improvements in or relating to arrangements for measuring the transmission equivalents of electric communication channels
IPC: H04J1/16
Filed: 1947-09-08
Abstract: 629,718. Measuring channel transmission equivalent. STANDARD TELEPHONES & CABLES, Ltd., GAMMIE, J., and JACOB. SEN, B. B. Sept. 8, 1947, No. 24650. [Class 40 (v)] A measuring arrangement for determining the level of a test wave having a given frequency. in the presence of other waves occupying a specified band of frequencies divided into a plurality of sub-bands of equal width, comprises a modulator for selecting the whole of the subband containing the test wave, a demodulator for selecting the test wave from the sub-band and means for indicating the level of the test wave. The invention is described with reference to a multichannel carrier current communication system having 720 channels each occupying a band 4 kc/s. wide. The channels are divided. into 60 basic groups each containing 12 channels and covering a frequency range of 60 to 108 kc/s. The 60 basic groups are divided into 12 supergroups each containing 5 basic groups and covering a frequency range of 312 to 552 kc/s. The 12 supergroups are transferred to the range 60 to 3036 kc/s., the first being transferred downwards to the range 60 to 300 kc/s., the second being unchanged in frequency range, and the others being transformed upwards. In order to test any given channel a wave of frequency 800 c/s. is injected at a specified level into the signal frequency input of the channel. The measuring arrangement picks out any desired one of the 720 channels without interrupting the service on any of the other channels and is arranged to avoid the transmission of any frequencies back into the system which are within the band covered by the system. The transformer 8 is bridged across the incoming line and feeds the modulator 10 through a variable attenuator 9. The modulator is supplied from oscillator 11 with a carrier wave of one of twelve fixed frequencies producing a lower side-band from the desired supergroup, the mid-band frequency of which is 3408 kc/s., the side-band being selected' by a band-pass filter 12 and passed to the demodulator 13. The carrier frequency for the demodulator is supplied as follows. The oscillator 11 supplies a modulator 17 which is also fed with the appropriate one of the supergroup carriers of the system from selector switch 19, the resultant lower side-band having a fre. quency of 2976 kc/s. and being fed to a further modulator 21 through band-pass filter 20. The carrier for modulator 21 comes from an adjustable oscillator 22 set to pick out a particular one of the 60 channels of the supergroup selected by oscillator 11. The upper side-band from modulator 21 is selected by the band-pass filter 23 and provides the appropriate carrier to cause the demodulator 13 to give an output of the test wave at 1.2 kc/s. which is amplified in circuit 15 and fed to a level indicator 16. For selecting the signals of supergroup 2, switch 18 is operated to feed the modulator 21 directly from oscillator 11, and filter 23 then selects the lower side-band from the output of the modulator 21. For using the arrangement for terminating instead of bridging measurements, transformer 8 may be provided with a terminating resistance which may be switched across its primary, or a switch may connect a suitable tapping on the primary to the line, Figs. 3 and 4 (not shown). -