Patent GB-1410547-A (GB)
Title: Field deflection circuit
IPC: H04N3/18
Filed: 1973-07-24
Abstract: 1410547 Field time-base MULLARD Ltd 24 July 1973 35109/73 Heading H3T A field time-base has its saw-tooth generating driving stage supplied with the output stage supply voltage whereby the flyback time of the generating stage decreases with increase in the supply voltage, the flyback time also increasing with sawtooth amplitude. This ensures that the current in the deflection coils is always exactly reversed and thus improves linearity. In Fig. 2, a capacitor C1 charges through resistor R5 to produce the scan. When the capacitor voltage at the base of a comparator transistor T3 exceeds the reference voltage from R3, T2 switches on, the switching being speeded by feedback via a current mirror T1a T1b. T1a lowers the base voltage of T9 and thus, T8 switches off T4, allowing the capacitor to discharge via a current mirror T5, T6 at a constant rate determined by a resistor R4. Conduction of T1a also lowers the base voltage of transistor T2 of the comparator to 0À6 volt where it is clamped by D1, D2 and T7. When the capacitor has discharged below this voltage the comparator resets the circuit and a new scan commences. As the capacitor is discharged by a constant current circuit the flyback duration will increase as the picture height is increased (as by changing the value of the height control R3 and thus increasing the capacitor voltage at which the flyback is initiated). If the charging voltage at 3 is derived from the supply to the output stages of the time base circuit an increase in the supply will increase the discharge rate of C1 and reduce the flyback time. The supply voltage at terminal 1 determines the picture height and is derived from the vertical scanning circuit so that the aspect ratio remains constant. Synchronizing pulses may be applied to terminal 2. In a simplified circuit (Fig. 2, not shown) the discharge level determining circuit T1b, T7, T8 and T9 is omitted. Fig. 3 (not shown) illustrates an integrated version of the circuit. Emitter follower buffers are provided between R3 and T2 (T13), between capacitor C1 and transistor T3 (T1c) and between capacitor 4 and the output terminal 4 (T16 and T21). Transistors (T27 and T23) supply (at terminal 13) a flyback pulse from the collector of T9. A sawtooth component (from T16) and a pulse component (from T27) are added and applied to a shaping network (R27-R34 and diodes D7 and D8) to provide an overshaped S correcting waveform (at 11 via T28) or used during the first half of the scan. Correction for the second half is effected as a result of the slowing of the charge rate of the capacitor. A delayed flyback pulse is produced (at 13) by comparing the flyback ramp with a reference voltage in a comparator (T17, T19), the comparator being inhibited during the sweep by a transistor (T18) controlled by T3. Fig. 4 shows a co-operating output stage. The input signal is formed by combining the overshaped and undershaped scan voltages applied at terminals 4 and 11 and is amplied at T43 &c. before application to deflection coil L1 which is connected in one diagonal of a bridge arrangement of transistor T61, T67, T72 and T74. During the scan transistors 61 and 74 are on. During the flyback a pre-delay pulse at terminal 41 turns T61 off and in addition a flyback pulse at 12 holds T74 off so that for the first part of the flyback current is maintained by diode D1, D18 and capacitors C15 and C16. When the pre-delay pulse terminates, T68 switches on holding terminal 16 at earth potential and a delayed pulse is applied from terminal 13 via T70 to switch on T72 so that terminal 18 is raised to the supply for a potential and the flyback is completed with reverse supply voltage connected to the coil. As scan and flyback voltages are separately applied to the coils, large scan and flyback capacitors can be used and these will absorb transients due to tube flashover and protect the output stage. Further, a flashover voltage will cause T75 to switch on via D13-D16, lowering the voltage at the base of T75, causing T73 and T74 to switch off and so prevent excessive voltage across T74.