Patent GB-1422375-A (GB)
Title: Communications system azide-containing benzo
IPC: H04J1/00
Filed: 1973-05-03
Abstract: 1422375 Frequency multiplex telephone system; phase control systems and detectors LITTON INDUSTRIES Inc 3 May 1973 [4 May 1972 23 June 1972] 21155/73 Headings H4K H4R H3A and H4L In a frequency multiplex telephone system using a common transmission channel, e.g. coaxial cable, each station transmits and receives at frequencies which differ by a constant reference frequency. A signal at the latter frequency is fed over the channel to all the stations from a common source. Each station has a local oscillator, which, when the station is calling, is set to the standby receive frequency of a called station. The frequency band used occupies less than an octave, the standby receive frequencies falling within the lower half of the band. Each station is allocated a 10 kHz channel of which 4 kHz serves as a guard band. General system Fig. 1.-A plurality of stations 12-N are coupled at 16 to a common line 18 over which an electromagnetic wave of reference frequency fr, e.g. 2À5 MHz is propagated from a common source 19. In each station the reference frequency is divided at 28 and applied as a first control signal of 10 kHz to a phase locked loop frequency synthesizer 26. The latter normally generates a standby frequency fs which is peculiar to each station, e.g. 12À01 MHz for station 12, 12À02MHz for a station 13 and so on, within the standby receive band 10-12À5 MHz. Operation.-A calling station, e.g. 14 raises his handset and "dials" the wanted number. A converter 27 applies the dialled signal as a second control signal to synthesizer 26 whose output frequency is then changed to a value fs+fr where fs is now the standby frequency of the called party. An audio signal f AUDIO  from the handset (no mention is made of ringing signals as such) and the incoming reference frequency fr are applied to a balanced modulator 20 whose double sideband output frŒf A  is passed via a single sideband filter 22 to a second balanced modulator 24. The latter also receives the synthesized frequency (fs+fr) so its output comprises the components fs and (fs+2fr). For simplicity the audio frequency f AUDIO  may be ignored since it is merely a constant in each of the frequency expressions. Filter 29 has a bandwidth of less than 2fr and depending on the calling/called mode of the station can select for transmission either the lower or the higher sideband. In this case a signal fs of, e.g. 12À01 MHz is transmitted over the common channel 18. Only one station viz station 12, is tuned to this frequency so that the audio component may be recovered in its balanced modulator 30 and applied via a low bandpass filter 32 to its handset. Signals outgoing from station 12 are transmitted in the same way except that the output of its second balanced modulator viz fsŒfr is filtered such as to provide the upper sideband fs+fr, which latter is now the receiving frequency of the calling station. Modified system Fig. 6. By making use of the fact that signals of widely different frequency may be transmitted in opposite directions through a device without interference, the receiving modulator 30 and the transmitting filter 29 may be dispensed with. The modulator 24 then works independently in both directions. It is connected at its left hand side via a filtering capacitor to filter 22 so as to keep the incoming demodulated audio signal out of the transmitting branch 20, 22 and is also connected to filter 32 via a resistive impedance. An advantage of this arrangement, apart from the saving on components and cables, is that there is now no D.C. component on the input to filter 32. This component arose due to reception by the modulator 30 of identical but out of phase frequencies fs from its own synthesizer 26 via a direct path 120 and an indirect path through filter 29 and coupler 16. It is suggested that the frequency of the called party's transmitter rather than that of the calling party's may be adjusted or that a common source may be used for adjusting the stations' frequencies. Wide band data, e.g. TV signals may be transmitted by allocating more than one frequency channel thereto. Phase locked loop frequency synthesizer, Fig. 3, incorporates a voltage controlled oscillator 36, and a thick film circuit 42 for applying digital information as one input to a sample and hold circuit 46. The latter (Fig. 4) comprises a constant current source 60-70 whose output is applied as a sawtooth signal to an FET 74 under control of 10 kHz reference pulses fr<SP>1</SP> appearing on input 48. The FET also receives enabling pulses at a frequency f DIV  from an input 76 (the dialled digit converter 27, 42). When the synthesizer is stable these pulses f DIV  are of same frequency as fr so that a steady D.C. voltage is provided at the output terminal 100 extending to the controlled oscillator. However when a subscriber dials so as to change the synthesizer's frequency, the effect is to change the dividing ratio of circuit 42 so that the pulse frequency f DIV  changes (becomes lower in the case of dialling). The turn-on times of FET 74 are now no longer synchronous with the input sawtooth so that the potential on capacitor 90 builds up step-by-step with a consequent rise in potential at the source output of FET 92. The latter is connected to a stabilizing network having one branch 96, 98, 102, 104, 106 which is such that, during a rising voltage at point 96, diode 102 breaks down so as to accelerate the charging of capacitor 104. The network's other branch 108, 110, 112 is effective to shunt the capacitor to earth when the voltage at point 96 is falling. The result in both cases is to accelerate the limit for stability in the circuit.