Patent GB-1093675-A (GB)
Title: Improvements in and relating to high frequency apparatus
IPC: H03J3/00
Filed: 1966-06-02
Abstract: 1,093,675. Stop on signal receivers. MATSUSHITA ELECTRIC INDUSTRIAL CO. Ltd. June 2, 1966 [June 2, 1965; June 17, 1965 (3)], No. 24713/66. Heading H3Q. In an automatic tuning arrangement which includes means for varying the frequency comprising a sweep generator incorporating means for varying the charge on a capacitor, means are provided to inhibit change of the sweep voltage for a period following a reduction in or cessation of the received signal, so that the receiver does not tune to a further station if signalfading occurs. As applied to a receiver in which signal and local oscillator circuits are tuned by varactor diodes 6, 7, Fig. 1, respectively, controlled by a sweep generator comprising capacitor 12, this capacitor is charged from source E via resistor R and a switch 10, which is opened by means of control circuit 9, fed from the I.F. amplifier 2, when a station is tuned in. In addition, a " hold " circuit 8 is fed from I.F. amplifier 2 and retains the switch 10 in the open position for a period following reduction in, or cessation of, the I.F. output. " Hold " circuit 8 may comprise a rectifier rectifying part of the I.F. signal, a monostable circuit triggered by the rectified signal, and a differentiating circuit which differentiates the output of the monostable circuit and so provides pulses of opposite polarity which respectively trigger a bistable circuit into one or other of its stable conditions, the bistable circuit operating switch 10. Capacitor 12 may be shunted by frequency-indicating means 13, in parallel with which is a switch 11 closure of which re-initiates the sweep. The resistor R may be replaced by the emitter-collector path of a transistor (16, Fig. 2, not shown), the switch 10 then being used to apply or remove a base bias from the transistor; in this modification, the leakage current of the transistor with its base open-circuited can be made to compensate for the leakage current of capacitor 12, including the current taken by frequencyindicating means 13. In another modification, providing compensation for leakage current, capacitor 12, Fig. 3, is charged through the collector-emitter path of transistor 22, which obtains base bias from diode 23 in the collector circuit of a further transistor 20. Switch 10 is normally closed, transistor 20 conductive and transistor 21 biased non-conductive, so that charging of capacitor 12 proceeds until a sufficient I.F. signal appears in circuit 17. This is rectified by diode 18 and biases transistor 20, non-conductive, whereby transistor 21 becomes conductive and terminates the sweeping action. The base of a further transistor 19 is coupled by capacitor 24 to capacitor 12 and is biased from source 26 over a high resistance; the collector of transistor 19 is returned to one end of resistor 27 across which switch 10 is located. When the " hold " circuit 8 becomes operative and switch 10 opens, transistor 19 forms a feedback path around the circuit comprising transistors 20, 21, 22 which provides feedback tending to maintain the charge on capacitor 12 constant. Diode 25 provides for the discharge of capacitor 24 when the switch 11 is closed to recommence the search. Transistor 19 may be biased via a high resistance, either from a potentiometer across the +B supply (Fig. 5, not shown), or from the base potential of transistor 21 (Fig. 6, not shown).