GB626103A - Variable frequency oscillators - Google Patents

Abstract

626,103. Frequency modulation ; valve generating circuits. STANDARD TELEPHONES & CABLES, Ltd. April 11, 1947, No. 9709. Convention date, June 14, 1946. [Classes 40 (v) and 40 (vi)] A variable frequency oscillator for frequency modulation comprises two valves 65, 85 in parallel with a common output load 80, a phase reverser 101 with output load 117 connected by resistance capacity networks 82, 81, 69, 70 and 95, 193, 88, 89 respectively to the control grids of the valves and means for push-pull suppressor grid modulation of the valves. The series resistance and condenser 82, 81 and parallel resistance and condenser 69, 70 for each valve, between which each control grid is connected, have the same time constant, but the frequencies f1= 1/2##R69C70 and f2= 1/2##R88C89 for which the net phase-shift on each valve control grid is zero are different so that the control grids respectively lead and lag behind the combined output current of the valves, the output frequency at 121 being an intermediate frequency dependent on the relative amplification of the valves as controlled by the modulation. Amplitude control is effected by connecting the mid-point 77 of the suppressor grid resistances 76, 94 to a diode anode 105 in the valve 101 connected in a voltage divider network 107, 108 between H.T. and earth so that if the output of the valve 101 increases the condenser 109 charge decreases. Alternatively, Fig. 3 (not shown), the condenser 109 voltage is applied to the resistance and condenser networks 88, 89 and 69, 70 and to the point 77 or the latter earthed. Connection also of the cathodes of valves 65, 85 to a voltage divider network C121, R130, R126, Fig. 3 (not shown), provides regulation against supply voltage variations and degenerative feedback. Alternatively, Fig. 4 (not shown), the 180 degrees phase-shift of the valve pair output to the first valve control grid may be effected by a condenser and resistance network, a further condenser and resistance applying voltage to the control grid of the other valve, so that the grid voltages respectively lag and lead on the combined anode current. Both valves have A.G.C. diode anodes connected to a voltage divider network and a condenser to which the network resistances are connected.