GB571314A - Frequency modulation detector and converter - Google Patents

Abstract

571,314. Circuits for sound reproduction. PHILCO RADIO & TELEVISION CORPORATION. Nov. 23, 1943, No. 19574. Convention date, Nov. 24, 1942. [Class 40 (iv)] [Also in Group XL] A frequency discriminator for detecting or converting a frequency-modulated carrier wave comprises a filter network of either the bridged- T or the parallel-T type. In Fig. 1, the tuned circuit of a reaction oscillator 7 includes a phonograph pick-up 9 of the variable-inductance type described in Specification 558,680. The frequency-modulated output is fed to a bridged-T network 1 comprising series condensers C/2 bridged by a resistance 4R and a shunt inductance L of resistance R. This network has a transmission characteristic as shown in Fig. 2 and the mean oscillation frequency is such that the network operates midway along the slope A of the curve, giving an amplitude-modulated output which is rectified in the grid-cathode circuit 4, 5 of the valve 6. Other electrodes of this valve are connected to a tuned circuit 13 to produce radio-frequency oscillations which are amplitude-modulated by the audio voltages on the grid 4. These oscillations are radiated and are picked up by a nearby broadcast receiver to reproduce the record traversed by the pick-up 9. The zero transmission frequency of the network may be adjusted by a movable comminuted iron core in the inductance L (Fig. 3, not shown). Two such networks and two rectifiers associated in balanced relation may be used with their zero transmission frequencies symmetrically displaced on either side of the operating frequency (Figs. 5 and 6, not shown). Fig. 4 shows a modified network comprising two T-sections in parallel, one section comprising two series resistance 2R and shunt condenser C, the other section comprising two series condensers C/2 and a shunt resistance R. The zero transmission frequency is given by WCR=1. The Specification, as open to inspection under Sect. 91 refers also to an arrangement in which the network 1, Fig. 1, is employed for amplitude modulation by feeding a carrier to it, and by using an inductive phonograph pick-up as the coil L. This subject-matter does not appear in the Specification as accepted.