GB585588A - Improvements in or relating to electrical frequency responsive circuits - Google Patents

Abstract

585,588. Frequency modulation ; automatic tuning correction. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. April 27, 1944, No. 7855. Convention date, April 27, 1943. [Class 40 (v)] A frequency-discriminating circuit comprises one or more triode valves with a cathode impedance for providing inverse feed-back and with an anode impedance, one of which is variable with frequency, the circuit being-so arranged that on the application of varying frequency voltages, voltages of oppositelysloping amplitude-frequency characteristics are developed. Fig. 2 shows a F.M. receiver with the I.F. signals applied to one grid of a pair of triode valves coupled by a common cathode resistance 2. The anode of the first triode is connected to a network Z which is series resonant at a frequency F2 and is parallel resonant at a frequency F1 so that the output voltage across the network has the form shown in full lines in Fig. 3 when plotted against frequency of input. The second triode has its anode connected to a resistance 7, its grid being connected to the earthed end of the cathode resistance 2. High voltage across Z is accompanied by low voltage across the resistance 2, and the frequency characteristic of the output across the resistance 7 thus takes the form shown in dotted lines in Fig. 3. The respective output voltages are applied to balanced rectifiers 16, 17 and the resultant voltage across the load resistances 18, 19 may be passed to a signal reproducer or may be smoothed and utilized for automatic tuning. The network Z may comprise a series connection of inductance and capacity resonant at the frequency F2 shunted by an inductance to form a closed circuit of resonant frequency F1 (Fig. 3a, not shown). In a further modification (Fig. 4, not shown), the impedance Z is in the form of a parallel circuit resonant at the frequency F2, and the resistance 7 is replaced by another parallel circuit resonant at the midfrequency Fc. In Fig. 5, a single screen-grid valve 40 is used with its cathode circuit resonant at the frequency F2 and its anode connected to a resistance 60, output to the rectifiers being taken from the anode and cathode respectively through condensers A, B.