GB927801A - Tuning and detection set-up for radiotelegraphy, radiotelephony or radio-broadcast receivers - Google Patents

Abstract

927,801. Valve circuits for frequency-selective amplification; valve pulse-generating circuits. R. L. L. G. GILBERT. Feb. 5, 1960 [Sept. 2, 1959], No. 4117/60. Class 40 (6). The intermediate-frequency section of a radio receiving circuit comprises a valve 3 having a grid 9 to which incoming I.F. signals are applied and a grid 13 to which a rectangular-pulse gating signal is applied, the repetition frequency of the gating pulses being in excess of the highest modulation frequency contained in the I.F. signal, followed by a narrowband-pass filter comprising a crystal 14 tuned to the I.F. and having a pass band considerably narrower than the band width of the received I.F. signals, there being a high-valued resistor 18 connected in series with the crystal and a circuit comprising diodes 20, 201 adapted to short-circuit resistor 18, this latter circuit being controlled by pulses from the generator of the above-mentioned gating pulses so that during the intervals when valve 3 is passing signals resistor 18 is short-circuited and the crystal 14 has a high effective Q while when the valve 3 is blocked, resistor 18 is not shortcircuited and the effective Q of the crystal becomes very low. Tuned transformers 11, 12 and 15, 16 having wide pass-bands are arranged on either side of the crystal 14; the signal derived from tuned secondary 16 is amplified at 5, passed through a further broad-band filter circuit 6 and detected at 7. Incoming modulated signals as shown at a, Fig. 13, are chopped by the valve 3 and then converted to a succession of triangular groups of oscillations at I.F. with a slow build-up due to the high Q of the crystal during the " on " periods of the valve and a rapid decay due to its low Q during the " off " periods, the envelope of these triangular groups corresponding to the original modulation. The triangular " ripple " superimposed on the original modulation may be considerably reduced by connecting three circuits, each as shown in Fig. 11, in parallel, Fig. 1 (not shown), and so synchronizing the respective gating- pulse generators that the phase of each is displaced 120 degrees from that of each of the others. Generating three-phase rectangular pulses. Fig. 12.-The time-constants of the cross-coupling connections of three substantially identical multivibrators I, II, III are so arranged that positive-going pulses with a mark-space ratio of 2 to 1 are produced at the anode of the lefthand valve of each. The commencement of a positive-going pulse at the left-hand anode of multivibrator I is applied through capacitor 28 to terminate a positive-going pulse at the lefthand anode of multivibrator II. Multivibrator II is similarly coupled through capacitor 28<SP>1</SP> to multivibrator III. The three multivibrators thus produce positive pulses separated from each other by one-third of a pulse period. It is stated that the first multivibrator could similarly be synchronized from the third.