Patent GB-1353881-A (GB)
Title: Tuning resonance circuits
IPC: H03J7/26
Filed: 1972-02-17
Abstract: 1353881 Voltage sweep circuits; AFC; tuning circuits MATSUSHITA ELECTRIC INDUSTRIAL CO Ltd 17 Feb 1972 [19 Feb 1971 (5)] 7476/72 Headings H3T H3A and H3Q In an apparatus including a resonance circuit containing a variable capacitance diode 23, a main voltage sweep circuit 1 operates to provide an output voltage without substantial decrease after cessation of an initial sweeping increase of output voltage, an auxiliary voltage sweep circuit 2 operates to provide an output voltage which sweepingly increases after cessation of the initial increase of the output voltage of the main voltage sweep circuit 1, means 25, 26 applies the output voltages of the two voltage sweep circuits to the variable capacitance diode 23 and means controls the sweeping increase of output voltage of the auxiliary voltage sweep circuit so that the increase of output voltage of the auxiliary voltage sweep circuit compensates for any decrease of output voltage of the main voltage sweep circuit. A positive input at 24 ((24), Fig. 2, not shown) causes transistors 6 and 7 to turn on so as to charge a capacitor 5. The voltage sweep output at a alters the tuning of the variable capacitance diode 23 in a tuner 4. When the frequency of the tuner nears a broadcast frequency the voltage at 24 falls and the transistors 6 and 7 turn off so as to stop the charging of the capacitor 5. The fall in voltage at 24 may be obtained from a third voltage sweep circuit (not shown) which is similar to the main sweep circuit 1, and has an oscillator including a variable capacitance diode. The main voltage sweep circuit 1 and the third voltage sweep circuit are controlled such that the output voltage of either one of these circuits increases while the output from the other circuit has stopped sweeping. In each of these voltage sweeping circuits each voltage difference from the start of a sweep to the end of a sweep corresponds to the frequency difference #f between the broadcast channels. The main and third sweep circuits alternately sweep and stop and by counting the starting or stopping of the sweeps of the main or the third sweep circuit in a counter when a desired count is reached which corresponds to a desired channel the signal at 24 is caused to fall so as to stop the charging of capacitor 5 as described above. Also the signal at 18 is caused to fall so that a transistor 17 turns off and allows the capacitor 13 in the auxiliary voltage sweep circuit to start to charge by way of a transistor 14. The rising output at b is added to the slightly falling output a of the main sweep circuit 1 to provide a resultant rising tuning voltage e (at time t 2 - t3, Fig. 2, not shown). When at (t 3 ) the voltage at e corresponds to a certain tuned frequency of the tuner 4 as determined by the IF output, a frequency discriminator produces an input to 16 which reduces the charge rate of the capacitor 13 (b 3 -b 4 ). Then (at t 4 ) when the output b reaches a certain value a Schmitt circuit 21, 22 changes state making 21 off and 22 on so that the output at c is fed to the input of the main sweep circuit 1 via D1 so as to turn on the transistors 6 and 7 which charge the capacitor 5. This increases the output voltage a (a 4 -a 5 ) of the main sweep circuit 1 (at t 4 -t 5 ) which alters the tuning of the variable capacitance diode 23 so as to tune to a higher frequency. This increases-the output from the discriminator applied at 16 (at t 4 -t 5 ) which increases the current through the transistor 15 so as to discharge the capacitor 13. This forms a negative feedback loop consisting of the tuner 4, the IF amplifier, the discriminator and the auxiliary voltage sweep circuit 2 so that the output voltage b of the auxiliary sweep circuit 2 is lowered (b 4 , b 5 ) and the resultant tuning voltage at e becomes constant. The above operation of the auxiliary sweep circuit 2 is repeated to maintain the tuning voltage at e constant so that the tuned frequency is memorized. If the broadcast signal memorized is interrupted the sweep circuit 2 starts to sweep, however the terminal 35 is grounded so that the output from the Schmitt circuit 3 does not affect the main sweep circuit 1. When the broadcast signal is received again the signal at 16 from the discriminator stops the charging of the capacitor 13 and the sweep of the circuit 2 and the main sweep circuit 1 starts to sweep again so that the voltage on the variable capacitance diode 23 is maintained constant as described above. When a broadcast signal is present a picture IF signal or a synchronizing signal is applied to the terminal 35 so as to bias diode 28 off. The capacitor 13 is discharged by a thyristor 19 when the output at b exceeds a certain value. The capacitor 5 is discharged by a thyristor 11 fired by a reset signal at 41. In a modification the Darlington connected transistors 36, 37 may be replaced by a MOS transistor (29, Fig. 3, not shown). In a further modification (Fig. 4, not shown) the output of the auxiliary voltage sweep circuit 2, (2<SP>1</SP>) is made of reverse polarity and is connected to the anode of the variable capacitance diode 23, (23<SP>1</SP>).