Patent GB-673356-A (GB)
Title: Improvements in or relating to receivers for pulsed frequency modulation carrier systems
Inventor: EARP CHARLES WILLIAM
IPC: H03F1/28
Filed: 1950-12-01
Abstract: 673,356. Multiplex pulse signalling. STANDARD TELEPHONES & CABLES, Ltd. Dec. 1, 1950, No. 29466/50. Class 40 (v). Relates to receivers for communication systems of the type in which an electrical wave is periodically sampled and each sample is represented by one or more transmitted wave pulses of phase or frequency modulated waves. In each case means are provided for combining two wave trains produced by selecting each received wave pulse, and/or one or more portions thereof, to produce a wave " packet " having a phase determined by the difference between the phases of the two trains and further means employs two such wave packets to reproduce a sample of the original electrical wave. The principle upon which the systems described operate is described in Specification 673,354, and consists in the transmission of two or more " indices " representing the value of the sample, at least one of which is ambiguous. At the receiver, the indices are used jointly to recover the sample value without ambiguity. Thus in the present receiving systems the frequency of a wave pulse may be used as one unambiguous index and an ambiguous index derived from the phase comparison of the waves of two consecutive pulses. In another case, both indices are ambiguous and are derived by a phase comparison of an early and a late sample respectively of one wave pulse with a late sample of an adjacent wave pulse. When the frequency modulated transmitter oscillator is continuously modulated by the signal, and sample pulses derived subsequently, the indices used at the receiver may be the phase differences between a late sample of one pulse and an early sample of a second pulse and vice versa. A further example comprises a receiver for use with a transmitter which transmits for each sample two or more indices in the form of short pulses of carrier, phase modulated to different degrees. While the invention relates primarily to receiving systems, it is necessary to describe the transmitting arrangements for producing the signals to which the receiver responds. The first embodiments of the invention respond to signals produced by the transmitter of Fig. 1, in which eight microsecond frequency modulated pulses are transmitted at a recurrence period of a hundred microseconds. The 10 kc/s. master oscillator 1 controls the eight microsecond gate pulse generator 3 which provides pulses to open the gate circuit 5 to connect the frequency modulated oscillator 15 to the outgoing conductor 16. The pulses after inversion and differentiation in circuits 6, 7 also provide short positive trailing edge pulses which after a one microsecond delay in circuit 8 are amplitude modulated in circuit 9 by the signal applied to terminals 10, 11. The amplitude modulated pulses charge up a storage circuit 12 which has been discharged one microsecond previously by the undelayed pulses from circuit 7, over conductor 13. The voltage at the output of the storage circuit is represented,by graph B, Fig. 3, in which 28, 31, &c. are levels determined by the amplitude modulated pulses and 27, 30, &c. represent the drops of voltage when the circuit is discharged. This wave, after passing filter 14 which removes the one microsecond gaps, modulates the frequency of oscillator 15 the output of which is gated to conductor 16 by the eight microsecond pulses 26, 29, &c. graph A, occurring at the end of each frequency step. Graph C shows the variations of oscillator phase related to an arbitrary phase 32 occurring during the frequency step preceding the signal level 28, The circuit described may be one channel of a twelve channel time division multiplex system using as synchronizing signal the four microsecond gap occurring once per twelve-channel period. In one receiver for this type of signal the first and unambiguous index used is the frequency of each wave pulse determined by comparison of two samples of the pulse taken four microseconds apart. The ambiguous index is the phase change over a one hundred microsecond period determined by two samples taken from successive wave pulses. The 4 Mc/s. I.F. circuits 38, Fig. 4, feed the received signals to gate circuits 45, 46. These are fed with eight and one microsecond gating pulses from generators 43, 44 respectively controlled by the synchronizing pulses selected in circuit 39 and phased by delay networks 41, 42 respectively. The gate circuit 45 selects the whole of the channel pulses of the channel concerned and circuit 46, a one microsecond sample of each pulse near its trailing edge. Local 2.4 and 2.5 Mc/s. wave sources are provided by feeding the synchronizing pulses to harmonic generator 51 and filter 52, the 100 kc/s. output from which is multiplied by 24 in circuit 53 and by 25 in circuit 54. The 4 Mc/s. wave pulses from circuit 45 are applied with the 2.4 Mc/s. wave to the frequency changing modulator 49 and the lower 1.6 Mc/s. sideband is selected by filter 55 which introduces a four microsecond delay. The output wave pulse is applied, together with a 4 Mc/s. undelayed wave pulse from circuit 45, to the frequency changing modulator 57 and the lower 2.4 Mc/s. sideband is selected by filter 59. The resulting wave pulse together with the locally produced 2.5 Mc/s. wave, is applied to the frequency changing modulator 61 and the lower 100 kc/s. sideband is selected by filter 62. The resulting 100 kc/s. wave packet is converted into a " comb " of 0.2 microsecond pulses by the generator 63. The one micro. second 4 Mc/s. wave pulses from circuit 46 are applied with the 2.5 Mc/s. wave to the frequency changing modulator 50 and the lower 1.5 Mc/s. sideband is selected by filter 56 which introduces a hundred microsecond delay. The output wave pulse is applied, together with a 4 Mc/s. undelayed wave pulse from circuit 45 to the frequency changing modulator 58 and the lower 2.5 Mc/s. sideband is selected by filter 60. The resulting 2.5 Mc/s. wave packet is converted into a comb of 0.02 microsecond pulse by generator 64 after phase adjustment in circuit 65. The latter is set so that when the transmitter is unmodulated, each pulse from the comb from generator 63 coincides with a pulse approximately at the centre of the comb from generator 64. These combs are fed to a coincidence circuit 66 which produces an output pulse when two input pulses coincide. These output pulses are time modulated representations of the channel wave samples and the wave is recovered in the pulse time demodulator 67. If the frequency deviation which can occur between two wave pulses is small, circuits 42, 44, 46 and filter 48 may be omitted and the dotted line output of filter 47 fed to modulators 50, 58. For each additional channel, all the circuits 41 ... 67 are duplicated and connected between conductors 40 and 69. For the same type of transmission, two ambiguous indices may be derived at the receiver, by the circuit of Fig. 5 which is largely similar to Fig. 4. In this case, the pulse generators 43, 44, Fig. 5, and gate circuits 45, 46 abstract respectively late and early one microsecond samples of each wave pulse, the first index being the phase difference between the early sample of a wave pulse and the late sample of the preceding pulse, while the second index is the phase difference between the late samples of two adjacent pulses. The 10 Mc/s. I.F. pulses are converted to 9 and 9.05 Mc/s. pulses respectively in modulators 49, 50. Filter 55 introduces a hundred microsecond delay and modulator 58 is fed with its 10 Mc/s. source from filter 47 through a hundred microsecond delay circuit 70. The modulator 61 and filter 62, of Fig. 4, are omitted, and the pulse combs fed to the coincidence circuit 66 have recurrence frequencies of 1000 kc/s. and 950 kc/s. The signals for each channel of a multiplex system may be produced by a conventional carrier frequency modulator, continuously modulated by the input wave, and samples of which are transmitted as wave pulses, Fig. 6 (not shown). One receiver for this type of signal derives two ambiguous indices based on the phase difference between an early sample of one wave pulse and a late sample of the adjacent pulse and vice versa. This requires a modification of the receiver of Fig. 5 in which the circuits between the dotted lines are replaced by similar circuits having the appropriately modified connections and delay times, Fig. 7 (not shown). In another receiver, Fig. 8 (not shown), operating at a 3 Mc/s. I.F., the unambiguous index is obtained by demodulating the received pulses conventionally to reproduce the modulating wave and then frequency modulating a local 2 Mc/s. oscillator with a small deviation by the recovered wave. Samples of the resulting wave are treated in a similar way to the first index samples in the receiver of Fig. 7. The second and ambiguous index is of the same character as that of this Figure and the two result in pulse combs which are treated as in the preceding examples of the invention. Similar principles may be applied to reception of the signals in a time division phase modulation system described in Specification 673,354. The transmitting circuit of such a system is shown in Fig. 9 for one channel only of a multiplex system with a sampling period of one hundred microseconds divided into twelve light microsecond channel periods and a four microsecond synchronizing period. Each channel period is divided into two fourmicrosecond index periods, the indices being given by two wave pulses carrying a different degree of phase modulation. The 10 kc/s. master oscillator 101, Fig. 9, provides the carrier for phase modulator 110, to which the input wave is applied, and the modulated output is multiplied in frequency in circuit 111 by a factor of five. The 50 kc/s. output, in one path, is multiplied by ten in circuit 113 and the 500 kc/s. wave fed to circuit 118. In a second path the modulated 50 kc/s. wave is multiplied by nine in c