Patent GB-457485-A (GB)
Title: Improvements in or relating to radio receiving systems
IPC: H03J7/04
Filed: 1935-05-30
Abstract: 457,485. Valve circuits. KOLSTER-BRANDES, Ltd., and EARP, C. W., Cray Works, Sidcup, Kent. May 30, 1935, No. 15751. [Class 40 (v)] To compensate for frequency drift in incoming signals or for variation in frequency of a local oscillator in a heterodyne radio receiver, signal energy is fed over two parallel paths, one highly selective, the other relatively broadly tuned, to a balanced detector circuit, whereby a change in signal frequency causes a change in relative phase of the two inputs to the detector bridge, thereby producing unbalance which is utilized to effect an automatic compensatory control of a frequency selective portion of the receiver. Fig. 1 shows the application of the invention to a superheterodyne receiver for I.C.W. signals, in which the output from the final stage OP is applied to the primary P of a transformer T having three secondary windings. The output from secondary S2 is applied over a note filter NF sharply tuned to the desired note frequency (e.g. 1000 cycles) to the primary P1 of a transformer T1, the secondary S5 of which is connected to a balanced rectifier circuit BRC. The output from secondary S3 is applied over a phase adjusting network PH to the balanced rectifier BRC in opposed phase to the currents applied thereto from S5, and the circuit constants are chosen so that the rectifier bridge is balanced when the receiver is in exact tune, e.g. when the note frequency applied to P is 1000 cycles. As circuit S2, NF is highly selective while S3, PH is broadly tuned any departure from the required frequency unbalances the network BRC thus varying the impedance of a frequency control triode FCV. The resulting change in anode current of this valve varies the voltage drop in resistance R1 and thereby varies the screen grid potential of the pentode oscillator BO in such sense as to restore the tuning of the receiver by varying the beating oscillator frequency. Alternatively, the suppressor grid potential may be controlled in a similar manner. A time delav circuit RC prevents the control from varying at the keying speed. Fig. 4 shows a superheterodyne receiver for telephony or CW reception, in which the output from the intermediate frequency stage IF is applied to two parallel paths S6, S7 thence over amplifiers SA, NSA and transformer primaries P3, P4 to a balanced detector circuit BRC. The circuit S6 is made sharply selective by a crystal gate CS, circuit S7 being broadly tuned. The output from the selective circuit amplifier SH is applied in push-pull fashion to the circuit BRC while the output of NSA is applied thereto in parallel as in Fig. 1. Departure from exact tune unbalances the bridge BRC and varies the screen potential of the beating oscillator BO through resistance R1. R1, C1 forms the time-delay circuit and the phase relation of currents in circuits P3, P4 is adjustable by condensers a b. The output to the second detector DET2 may be taken either from the selective circuit P3 or the non-selective circuit P4 by means of switch SW; in the latter case the automatic control is made ineffective by closing switch AFC.