Patent GB-1406901-A (GB)
Title: Radio receiver and a channel scanning and priority channel monitoring system
IPC: H03G3/34
Filed: 1973-01-24
Abstract: 1406901 Tuning radio receivers; pulse circuits; detector circuits and muting circuits MOTOROLA Inc 24 Jan 1973 [24 Feb 1972] 3671/73 Headings H3Q and H3T In a radio receiver and a channel scanning and priority channel monitoring system for receiving signals on a predetermined number of channels, with one channel being designated a priority channel to which the receiver is automatically tuned in the event of a transmission on the priority channel, a pulse responsive switching means 30 is coupled to an oscillator means 20 for controlling the frequency of the oscillator means, clock means 50 provides clock pulses at first and second frequencies to the switching means 30 to change its condition of operation and the scanning of the channels is terminated whenever a signal is received on any one of the channels and detector means 62 detects a received signal on a channel associated with the output frequency of the oscillator means 20. The clock means 50 provides pulses at the first frequency in the absence of a received signal and at the second frequency on receipt of a received signal corresponding to the non- priority channels to sample the priority channel. A delay means 68 is coupled to the clock means for delaying pulses of the first frequency to the switching means 30 for a predetermined time following termination of a received signal and means 72 is coupled to the switching means 30 and is responsive to a condition of operation of the switching means 30 corresponding to the selection of the priority channel for maintaining the switching means in that condition on receipt of a received signal corresponding to the priority channel. With the switch 15 in the scan position shown and no signal received a muting oscillator 74 generates a signal corresponding to a noise signal which actuates a squelch circuit 18 to mute the audio amplifier 17. Clock pulses from scanning clock 52 operate the switching means 30 which selectively connects the oscillator 22, 24, 28 associated with the non-priority channels selected by switches 46-49. Each clock pulse from 52 also triggers monostable 36 which operates the oscillator driver 35 for the end part of each clock pulse from 52 so as to select the priority channel oscillator such as 26 as selected by switch 45. If during scanning of the non-priority channels a signal is received it operates a squelch circuit 62 so as to stop the scanning clock 52 and disable the muting oscillator 74 so that the receiver is tuned to this received non-priority channel. The sampling clock 54 now operates at a lower frequency than the scanning clock 52 so as to operate the monostable 36, the oscillator driver 35 and the priority channel oscillator 26 to monitor the priority channel. If a signal is received on the priority channel while it is being monitored a priority detect gate 72 is energized to maintain the receiver tuned to the priority channel until the priority signal ceases whereon after a delay provided by resistors 205, 207, 209, Fig. 3, and a capacitor 206, in the turn on delay circuit 68, the non-priority channel previously received is returned to provided its signal is still present or if not the scanning operation restarts. The detector means 62 is associated with a variable squelch delay 64 for varying the response time of the squelch in response to the signal being received and a squelch delay control 66 for varying the response time in response to the condition of operation of the switching means 30 to make the response time of the switching means rapid during channel scanning and priority sampling and relatively slow after a signal has been received to maintain the receiver locked to the received signal in event of signal fading or other interruptions. The detector of the squelch circuit 62 may include a diode (114, Fig. 2, not shown) and transistors (116, 118) having an output capacitor (120) chosen to provide a fast detector response time when scanning and priority sampling. The associated variable squelch delay circuit 64 includes three transistor differential amplifiers (122, 124; 126, 128 and 130, 132) which operate at three increasing signal levels together with a capacitor (142) which is chosen to produce a greater delay to slow down the response of the squelch output to provide fade protection. The squelch delay control 66 includes a transistor (160) which enables the fast responding voltage at the capacitor (120) to provide the output of the squelch circuit 62 during the sampling period. When switch 15 is moved to the normal position the squelch circuit 18 is operated by the noise from the discriminator 16. The channel scanning circuit then operates to lock to the channel selected by the priority channel selector switch 45 so that a channel can be manually selected by the switch 45. Lamps 81-84 indicate the oscillator 22-28 which is operating. Reference has been directed by the Comptroller to Specification 1,314,577.