GB591301A - Improvements in or relating to frequency discriminator circuits for angle-modulated oscillations - Google Patents

Abstract

591,301. Frequency and phase modulataon. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. May 3, 1945, No. 11205. Convention date, May 3',' 1944. [Class ,40 (v)] In the reception of frequency - modulated and phase-modulated waves by means of a balanced frequency-discriminator, out - of - balance voltages at the discriminator output due to mis-tuning of the receiver are utilized to redress the balance. As shown in Fig. 1, the incoming .wave is converted to an intermediate frequency at the mixer 2 and is fed through an IF amplifier 8 and band-pass coupling 9 to a parallel pair of valves C, D. The anode circuits A, B of these valves are tuned respectively above and below the true IF,.,as indicated by the frequencyresponse full-line curves of Fig. 3. The voltages across the circuits A, B are fed to rectifiers F, G with load resistances 22, 23 shunted by condensers 39, 40, the cathode end of resistance 23 being earthed. The audiofrequency signal is taken off at the point J across the :resistances 22, 23, and any D.C. component present at J, due to a discrepancy between the actual IF and the true IF, is fed back through a smoothing filter 21, 24, 19 to the grid of the valve D to alter the amplification of this valve so that its. output is represented by the dotted curve B1, Fig. 3. Owing to the coupling between the valves C, D, afforded by the cathode resistor 14, the output of the valve C is altered in the opposite sense, giving the response curve A1, Fig. 3. The crossover-or balance point FC of the two curves is thus shifted to FC<1> in. the direction to redress the balance. In a modified circuit, correcting voltages of opposite sign are derived from the rectifier outputs and are applied to the grids of the valves C, D (Fig. 6, not shown). In a further modification (Fig. 7 not shown), the discriminator is of the phasing type disclosed in Specification 489,094, and each of the two channels to the rectifiers includes an amplifier and a band-pass filter, the gain of the amplifiers being automatically adjusted as in Fig. 1.