| 摘要 |
<p>687,893. Automatic frequency control systems. RADIO CORPORATION OF AMERICA. Dec. 29, 1950 [Dec. 30, 1949], No. 31596/50. Class 38 (iv). [Also in Group XL (b)] A wave generator, particularly the saw-tooth scamiing generator of a television receiver, is synchronized with a pulse train by distorting the waveform so that it exhibits a flattened portion, superposing the pulses on the distorted waveform, and deriving from the composite wave a voltage for controlling the frequency of the wave generator. In Fig. 1, a sawtooth voltage is generated on the condenser 22 by charging it from a source B + through a resistor 21 and discharging it periodically through a valve 26 connected as a blocking oscillator and comprising coupled grid and plate coils 32, 33 and a grid condenser 38. The sawtooth voltage is applied to a valve 12 which feeds scanning- coils 16 through a transformer 14. The sawtooth wave is partially integrated by a resistor 64 and variable condenser 66, and their junction is connected to earth through condensers 71, 73 and low resistance 76. The synchronizing pulses 44 are fed through a cathode follower valve 78 and condenser 73 to the grid of a mixing valve 59 and are there superposed on the wave fed from the condenser 22 through the network comprising resistor 64 and condensers 66, 71. The effect of the network is to round off the base 69, Fig. 3A, of the wave and to flatten the portion 82 that precedes the steep fall 108. The valve 59 is back-biased so that it only passes current when its grid voltage is above the upper limit of the waveform 46. If the generator is running slow the pulses 44 ride on the wave 46 as shown in Fig. 3A, and give rise to plate-current pulses shown in Fig. 3B. These pulses are smoothed by a network 98, 99, 101, and the smoothed positive voltage across the cathode resistor 89 is applied to the grid of the blocking oscillator to adjust the moment of firing and thus to control the frequency of the wave 46. Owing to the flattened crest 82 of the wave 46, a larger proportion of the pulses contribute to the control voltage. Two grid leaks 86, 87 are connected in series between the grid of the valve 59 and the junction of the cathode resistors 88, 89; and the junction of the leaks is connected to the cathode by a condenser 91. A resonant circuit 109, 111 interposed between the condenser 22 and the junction of coils 32, 33 is tuned to the wave frequency.</p> |
