Double horizontal frequency oscillator

Principle of synchronization between horizontal oscillation and horizontal scanning in TA7609P integrated circuit

  The line oscillator directly generates a 2fH (31,250Hz) pulse, which is outputted as a 50% duty cycle line frequency (15,625Hz) pulse after a 2:1 frequency divider composed of a bistable trigger. This design can improve the accuracy of interlaced scanning, avoid parallelism, and improve vertical clarity. This is because in the equipment of the broadcast TV center, a double line frequency pulse is first generated and then a 15,625Hz line synchronization pulse is generated through a 2:1 frequency division. The 2fH pulse is then divided by 625:1 (actually four 5:1 frequency dividers are connected in series) to form a 50Hz field synchronization pulse.
  This treatment can keep the phase relationship between the line and field synchronization pulses strictly consistent, and can ensure the normal operation of interlaced scanning. If a 2fH pulse is also generated in a TV receiver, and then a 15625 Hz line frequency pulse is generated after a 2:1 frequency divider, then as long as the line AFC circuit ensures that the 2fH pulse and the synchronization pulse are in phase, the field synchronization pulse can make the field scanning circuit interlaced accurately, thereby improving the interlaced scanning accuracy.
  Figure 8-15 is a line oscillation circuit, which consists of Q14~Q24. The capacitor 3C12 connected to the ② pin is a timing capacitor, and the external equivalent resistor R is the charging resistor of the timing capacitor 3C12. 
  Q15? and Q16 form a differential comparator amplifier, Q17 is a constant current source biased by D10, R25 and Q′18? are the collector load of Q16, and its output voltage is output in three ways through PNP transistors? Q″18?, Q19?, and Q20. The collectors of Q″18? and Q18 are connected to the bases of Q22, Q24? and Q21, Q23 respectively. The collectors of Q23 and Q21 are connected to the base of Q15 through resistor R24. The collectors of Q23 and Q24 are connected to the base of Q16 (i.e., pin ②) through resistor R30?. In this way, the output signal of the collector of Q16 is inverted and amplified by Q″18 and Q19, and further inverted and amplified by Q21, Q22, Q23, and Q24 respectively, and added to the base of Q15 and Q16 through R24 and R30 to form a positive feedback loop. The ② foot is connected to the inertial element 3C12, and its charging and discharging process and positive feedback loop are used to generate an oscillation pulse of 31,250 Hz.  Q14 is a clamping tube, and its base is connected to a fixed bias voltage UBO14?. Compared with Q22 and Q24, since the bases of Q21 and Q23 are not connected to series resistors, when Q19 is fully turned on, the base potentials of Q21? and Q23 are clamped at UBO14+UBE14, so that it will not be saturated too deeply, the flipping speed is improved, and the leading and trailing edges of the pulse are steep. 

This picture comes from the Electronics Fans Network: http://www.elecfans.com   For the convenience of analysis, Figure 8-15 is simplified to Figure 8-16. Switches K1 (Q21, Q22) and K2 (Q23, Q24) are two electronic switches controlled by Q16 through Q'18? and Q19. Uces22 and Uces24 are the voltage drops when the electronic switches are turned on, which are about 0.5V. The output waveform of the oscillator is a continuous oscillating positive sawtooth wave formed on the capacitor 3C12. The collector of Q16 outputs a continuous oscillating negative pulse, and its waveform is shown in Figure 8-17.