GB553428A - Circuit for generating and modulating carrier waves - Google Patents

Abstract

553,428. Valve generating and modulating circuits. STANDARD TELEPHONES & CABLES, Ltd. May 1, 1942, No. 5891. Convention date, May 3, 1941. [Class 40 (v)] In a combined valve modulator and generator circuit, a piezo-electric crystal functions to stabilize the generator frequency and to suppress the unmodulated carrier. As shown, the crystal 2 has its lower electrode 8 earthed, and its two upper electrodes 7, 9 connected respectively to the plate and to the grid 22 of a valve 20, which generates oscillations at the series-resonant frequency of the crystal. Modulation is applied through a transformer 3 to the inner grid of the valve 20, giving a modulated output through the transformer 6. From this output, the unmodulated carrier is suppressed by the short-circuiting effect of the crystal electrodes 7, 8. One of the sidebands may also be suppressed by a filter 28. The modulated output may be fed through a condenser 26 to a rectifying diode 14. The diode load impedance Z, is high at voice frequencies and as it is included in the circuit of the inner grid, it gives a negative feed-back effect to reduce distortion. An impedance Z 2 between the valve plate and the crystal, and a further network Z 3 between the diode anode 14 and earth, offer high impedance at the carrier and side band frequencies. In a simpler circuit, Fig. 1 (not shown), the diode anode is connected through resistances both to the secondary of the output transformer 6 and to the electrode 9 of the crystal, and the network Z 2 is omitted.