Patent GB-683727-A (GB)
Abstract: 683,727. Radio signalling. PHILIPS ELECTRICAL INDUSTRIES, Ltd. Oct. 19, 1950 [Oct. 22, 1949], No. 25491/50. Class 40 (v). A radio system for enabling mobile stations, such as vessels on a busy system of waterways, to communicate with the subscribers of a normal telephone system, comprises a series of fixed two-way radio stations, each arranged for traffic on one of a plurality of carrier-frequency pairs, and each adapted when not in an engaged condition to transmit an idle signal; each mobile station being adapted, by variation of its tuning, to search for an idle fixed station until it receives a carrier modulated by the idle signal, whereupon the fixed station assumes a busy condition by suppressing the idle signal. The fixed stations have their audio input and output circuits connected by land line to a central station or exchange. To enable the mobile stations to be called, a calling carrier may be emitted by the fixed stations and its modulation controlled from a central station or exchange to give a characteristic signal peculiar to the wanted mobile station. The fixed stations are so situated that the area of action of a station using a particular carrier-frequency pair does not overlap the area of another fixed station using the same pair. Fixed station, Fig. 1. Each fixed station comprises a radio transmitter A and a radio receiver B associated with the same or with separate aerials 7, 8. The transmitter comprises a crystal-controlled source Z modulated, preferably in frequency, by a microphone 2 and feeding frequency multipliers 3, 4 and a power amplifier 6. When the fixed station is disengaged, an idle signal is emitted preferably by pulsing the stages 4, 6 at a low frequency (400-500 c/s.) as represented at 44. The receiving aerial 8 is connected to a heterodyne stage 9 which feeds IF to a limiter 10 and discriminator-rectifier 11, the detected signal passing through an audio amplifier 12 to a telephone 13. When no signal is being received noise currents are fed through a highpass filter 15, 16 to an amplifier 17 which feeds two oppositely-poled rectifiers 18, 19 to create negative voltages at the left-hand terminal, and positive voltages at the right-hand terminal, of a resistor 20. The negative voltage is fed to the control grid of the limiter 10 and the positive voltage to the control grid of a valve 21 connected in the grid circuit of the audio amplifier 12. The anode of the valve 21 is connected to its positive supply terminal through resistors 24, 26, the junction being connected to a point on the cathode resistor 25 of the audio amplifier 12. When the receiver is idle, noise currents drive the grid of the valve 21 positive which has the effect of cutting off the anode current of the valve 12, so that the telephone 13 is silent, and the anode relay 14 is de-energized. When a carrier wave is received on the aerial 8, the noise currents are relatively small, and the audio valve 12 passes current which energizes the anode relay 14 and closes contacts 59, 60. Contact 59 short-circuits the pulse source 44 to switch off the idle signal, and contact 60 energizes an indicator 62. The microphone 2, telephone 13 and indicator 62 are situated at a normal telephone exchange C and are connected by line to the fixed station. Mobile station, Fig. 2. The transmitter D at each mobile station comprises an oscillatormodulator 28, frequency multiplier 29 and output amplifier 30 feeding an aerial 31. The transmitter frequency is determined by crystals KZ<SP>1</SP>, KZ<SP>2</SP>, KZ<SP>3</SP>, connected in circuit in turn by relays energized by a stepping-switch 41. The receiver E comprises a heterodyne and IF stage 33, detector 34 and audio amplifier 35. The local oscillator frequency is determined by crystals KO<SP>1</SP>, KO<SP>2</SP>, K03 connected in turn by relays energized by the same stepping-switch 41. The mobile station is thus tuned in turn for communication on the wavelength pairs associated with the respective fixed stations. When the mobile station desires to make a call, the instrument 37 is lifted from its cradle 50 to throw a series of switches upward. Switch 51 connects the telephone receiver to the amplifier 35, and switch 52 connects the microphone to the modulator 28. Switch 53 connects battery 54 to a pulse source 43 which energizes the stepping magnet 42 of the switch 41. When an idle fixed station is found, the idle signal passes through a tuned filter 38 and is rectified at 39 to energize a relay 40. This releases a relay 56 whose contact 57 arrests the stepping- apparatus 43, 42, 41, and energizes a relay 58 to switch in the previously-inoperative output stage 30 of the transmitter D. The last contact 70 of the cradle switch de-energizes a relay 67 to disconnect the call receiver 65 and call indicator 36 from the audio amplifier 35. Reception of the carrier on the aerial 8 at the fixed station, Fig. 1, activates the audio valve 12, cuts off the idle signal source 44, and gives a signal 62 at the exchange, so that the mobile station is now in communication with the exchange and can be connected to any desired subscriber. At the end of the conversation, the instrument is replaced, and the downward movement of the switch 53 releases the relay 58 to disconnect the transmitter output stage 30, and energizes a relay 64 to connect a crystal KO to the local oscillator which thus tunes the receiver to an idling frequency which is the same for all the mobile stations. The contact 70 of the cradle switch energizes the relay 67 to connect to the audio amplifier 35 a selective call receiver 65 and call indicator 36. For the purpose of receiving a call, each mobile station is allotted a combination of low audio frequencies peculiar to that station, and a special carrier frequency is used for transmitting and receiving these selective calling combinations. This calling carrier is sent out from each fixed station under the control of a central station, which may be the telephone exchange, and modulated by the selective call combination allotted to the desired mobile station. The calling carrier is received at the mobile stations, and at the particular station required it operates the call indicator 36 through the selective device 65. The called subscriber lifts the instrument 37 and the stopping switch operates to find an idle fixed station. He thus obtains communication with the telephone exchange, and is connected to the calling subscriber.