Patent GB-292077-A (GB)
Title: Improvements in carrier frequency signalling systems
IPC: H03C1/60
Filed: 1928-06-08
Abstract: 292,077. Electrical Research Products, Inc., (Assignees of Nyquist, H.). June 11, 1937, [Convention date]. Multiplex signalling. - In multiplex carrierwave line telegraphy one side-band is suppressed in each channel, and the system is designed to obviate the distortion produced by the lack of sharpness in cut-off of the filters. Low frequency signals from the telegraphic sender 14 are freed from harmonics of frequency higher than fm (where fm may, for instance, be 80 cycles per second) by a low pass filter 15, and are combined in a modulator 17 with carrier current of frequency fc (which may, for instance, be 1048.5 cycles) derived from a generator 18. The product, comprising side-bands from fc to (fc + fm) and from fc to (#c - #m) respectively, is sent to a band pass filter 13 designed to admit to the line 20 approximately only the second of the two sidebands to the line 20. At the receiving station, Fig. 2, this side-band is selected by the band pass filter 22 and applied through a phaseadjuster 23 to a homodyne detector 26, whence the demodulated signal (comprising components between 0 and fm) passes through a low pass filter 32 to a telegraph receiving instrument 33. Local oscillations of the carrier frequency fc are supplied to the detector from a local generator 28 through a phase adjuster 29. Several similar pairs of transmitters and receivers may be employed for multiplex working, the generators 18, 18<1> - - - at the transmitter being driven. off a single shaft which is synchronized with a shaft driving the corresponding generators 28, 28<1> - - -, at the receiver. The synchronizing current of frequency fo is derived from a machine 18<11> and applied through filters and through the line to a corresponding machine 28<11> at the receiver, these machines being on the same shafts as the generators 18, 18<1> - - -, and 28, 28<1> - - -, respectively. The voltage applied to the input side of the amplifier 26 is obtained, therefore, by modulating the carrier frequency fc with the fundamental of low frequency fp and harmonics of frequency nfp where n ranges from 2 to k ; by passing the lower side-band, from fc to (fc - kfp); by passing a small range, from fc to (fc + hfp) of the upper side-band, a sharp cut-off at (fc + hfp) being assumed for the moment; by suppressing the remainder, from (fc + (h +1) fp) to fc + kpf), of the upper side band; and by reintroducing from the generator 28 oscillations in phase with the pure carrier frequency as received. It is shown in the Specification that the input of the amplifier contains a group of frequencies, denoted collectively by #1, representing the correctly modulated carrierwave; a group #3, related to the suppressed frequencies of the upper side-band, this group being suppressed by the combined action of the pushpull pair of valves 26, 27, and of the low-pass filter 32; and a group o2 corresponding to the unsuppressed portion of the upper side-band. The latter group is suppressed as far as possible by narrowing the cut-off of the filters 13, 22 but owing to the lack of sharpness in cut-off it persists to some extent unless additional precautions be taken. This defect is obviated, therefore, by so designing the system that the amplitude distortion in the neighbourhood of the pure carrier frequency (i.e. of the upper cut-off of the band pass filters 13, 22) is equal and opposite for frequencies equidistant from the carrier, while the time-lag introduced into the modulation products is independent of frequency (ie. the phase-lag to the harmonics is proportional to frequency). In Fig. 4 the curve 41 shows the transfer admittance of the band-pass filters 13, 22, for various frequencies shown as abscissal. The lag which is introduced is shown in terms of multi-seconds by curve 46 and in terms of phase-angle by curve 42. As a working-point the frequency fc = 1048.5 is selected as the carrier frequency because (a) the time-lag is approximately independent of frequency through the admitted side band, as is shown by the curve 46, and (b) the amplitude distortion, shown by curve 41, is roughly autosymmetrical about the carrier frequency.