Patent GB-601013-A (GB)
Title: Frequency comparison apparatus
IPC: H04N5/12
Filed: 1944-07-06
Abstract: 601,013. Automatic frequency control. FARNSWORTH TELEVISION & RADIO CORPORATION. July 6, 1944, No. 12943. Convention date, April 15, 1943. [Class 38 (iv)] [Also in Groups XXXVI and XL (c)] Automatic frequency control of a television timer is achieved by applying negative impulses derived from the timer, and positive impulses of substantially the'same frequency and derived from a standard frequency source, to an electronic trigger device which develops a square wave alternating current, the relative energy content of the positive and negative portions of which is determined by the phase relationship of the two series of impulses. Means including a voltage doubler -circuit develops a control voltage corresponding to the relative energy content which is applied in the corrective sense to synchronize the timer oscillator. As shown in Fig. 1, the frequency of an oscillator 12 used in a television timer is stabilized with the frequency of the A.C. supply mains by comparing, in a pentagrid tube 16, impulses derived from an impulse generator 17 operated at mains frequency with those derived through a frequency divider 13 from the oscillator 12. The impulse generator 17 supplies positive pulses through a condenser 18 to the control grid 19 while the frequency divider 13 supplies negative pulses through condenser 14 to the control grid 15. As explained in the Specification with reference to Fig. 2 (not shown), the pentagrid is so biassed by the resistor network 21 ... 25, 27, 28, 33 ... 35 from the battery 20 that it acts as an electronic trigger device, the anode voltage jumping from a maximum to a minimum value according to whether its control grids are rendered more positive or negative than normally effected by the biassing resistors 27, 28. The anode voltage is applied through a condenser 38 and resistor 42 to a voltage doubler circuit consisting of seriesconnected diodes 39, 41 shunted by seriesconnected similar condensers 44, 45 and resistors 46, 47. Fig. 3 shows how the voltage doubler input voltage jumps to a maximum value 68 as the tube 16 is triggered by a positive pulse 67 over condenser 18, and to a minimum value when the negative impulse 66 over condenser 14 triggers the tube 16. The oscillator 12 is in step with the A.C. mains supply when the pulses 66, 67 arrive on the respective control grids of tube 16, 180 degrees out of phase. In this case, as shown in Fig. 3, the positive and negative portions of the voltage input to the voltage doubler are of equal duration and no voltage appears across the output terminals TT thereof. The relative duration of these positive and negative portions will vary according to whether the oscillator 12 is fast or slow, Figs. 4, 5 (not shown), resulting in a D.C. voltage appearing across the voltage doubler output, which will vary in magnitude and sense according to the amount by which the oscillator is fast or slow. Thus, voltage is smoothed at 48, 49, 51, 52 and appears across a terminal resistor 53 and is applied through a resistor 54 to the A.F.C. circuit 55 to adjust the frequency of the oscillator 12 in the corrective sense.