Patent GB-1229826-A (GB)
IPC: H03J7/26
Filed: 1968-08-05
Abstract: 1,229,826. Radio receiving circuits; sawtooth generators. TELEFUNKEN PATENTVERWERTUNGS G.m.b.H. 5 Aug., 1968 [4 Aug., 1967 (2)], No. 37218/68. Headings H3Q and H3T. A stop on signal receiver can be switched to give manual push-button selection of stations, tuning being performed electronically. In one embodiment tuning is performed by means of varicap diodes 3, Fig. 1, which obtain their control voltage over resistor 4 and switch 10. In the position shown for ganged switches 10, 11 pushbutton control is effective, switch 6 being used to select respective station control voltages from manually presettable potentiometers 5. With switches 10, 11 reversed stop-on-signal operation occurs, there being applied to diodes 3 a saw-tooth voltage derived from the charge on capacitor 12; upon the first station being tuned in a discriminator produces automatic fine tuning. This can be interrupted by means of short-circuit push-button switch 13, associated with regulator 8, capacitor C then being further charged until a second transmission is tuned in. In a second embodiment capacitor 12, Fig. 4, is always charged to the full value and the discharging process is controlled. Capacitor 12 is connected to operating voltage UB via charging resistor 14 and the junction point is connected to the base of transistor 17 which derives an emitter bias equal to the maximum tuning voltage, from Zener diode 18. Negative voltages are applied from the collector of transistor 17 via diode 25 to the base of transistor 26, forming with transistor 27 a multivibrator the pulse duration of which is adjustable by means of preset resistor 20 and variable resistors 21. The collector of transistor 27 is connected via diode 28 to the point 15. Regulator 8 comprises transistor 29 the base of which is connected to earth via diode 31 and via capacitor 30 to point 15. The collector of transistor 29 is coupled via transistor 32 to the emitter of regulating transistor 33, which is connected to earth via setting regulator 34. The collector of transistor 33 is connected to point 15 and its base via resistor 35 to a discriminator (not shown). Setting regulator 34 can be short-circuited by means of finder button 13 which is preferably coupled to switches 36 so that resistors 21 can be switched off upon closure of button 13 and then remain off when it is opened. Operation: upon switching on, and if no signal is present, capacitor 12 charges through resistor 12: with switch 36 the time-constant is so great that multivibrator 19 remains open and charging proceeds until transistor 17 becomes conductive, when a negative pulse is applied to transistor 26 rendering it non-conductive. Transistor 27 then becomes conductive to discharge capacitor 12, until the transistor 27 is again cut off, as determined by the setting of resistor 20 and value of capacitor 22: capacitor 12 then charges again. If during tuning a signal is encountered having sufficient field-strength, as determined by regulator 34, a positive voltage from the discriminator renders transistor 33 conductive and the resultant change in base potential of transistor 29 causes this to become non-conductive, rendering transistor 32 conductive to short-circuit the regulator 34: thus only a small discriminator voltage is necessary at the base of transistor 33 to control it. To move to another station finder button 13 is momentarily closed. If switch 36 is now closed a setting regulator 21 and capacitor 23 are connected to complete the circuit of multivibrator 19, comprising transistors 26, 27; the adjustment of each setting regulator 21 determines the pulse duration of the multivibrator, i.e. the period for which capacitor 12 is discharged following each charging cycle and hence the minimum potential to which the sawtooth will then fall (Fig. 5, not shown). The sweep is thus restricted to a predetermined range of stations. In a modification, switches 36, regulators 21 and capacitor 23 are omitted and switch 37 provided, upon opening which the basic level of the sweep is raised a predetermined amount, since the potential from potentiometer 39 is added to that across capacitor 12. In a further modification the multivibrator 19 is replaced by a thyristor circuit (Fig. 7, not shown), and a number of such potentiometers are provided, together with a selecting switch enabling the sweep to be commenced from a number of levels corresponding to different stations. The arrangements may be used for remote control, and may incorporate waveband switching. Saw-tooth generators.-A saw-tooth generator for use in the invention comprises capacitor 60, Fig. 12, which is charged through resistor 61 and discharged via the collector-emitter path of thyristor 63; a setting regulator 64 is connected between collector and base so that when the capacitor is charged to a predetermined voltage the potential at the base causes conductor, discharging capacitor 60. The thyristor is switched off by means of a negative pulse either as shown, applied to the base via capacitor 66, or derived from the collector current by means of an inductor connected between the collector and point 62 (Fig. 13, not shown).