Patent GB-595602-A (GB)
Title: Improvements in or relating to phase modulation detectors
IPC: H03H9/54
Filed: 1944-08-11
Abstract: 595,602. Valve circuits for radio reception ; phase modulation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Aug. 11, 1944, No. 15302. Convention date, April 3, 1943. [Class 40 (v)] A phase-modulation detector comprises a piezo-electric crystal tuned to the mean carrier frequency for removing the modulation from a portion of the modulated input, two unmodulated energy portions of like polarity so derived being combined with respective modulated portions of opposite polarity of said input energy after the relative phases of the modulated and unmodulated portions have been displaced by an angle in dependence on the modulation, the resultant of the two combinations being separately rectified and then differentially added. As shown in Fig. 1, the P.M. signal energy at the intermediate frequency is applied to the detector through a transformer 1, the primary 2 of which is tuned to the mean I.F. A portion of the modulated energy is applied to a piezo-electric crystal P, which removes the modulation side bands, the filtered energy being fed to the anodes of opposed rectifiers 3, 4 in like phase by way of resistors 8, 10 respectively. This unmodulated carrier energy which is fed to the anodes without phase shift, is represented by Ep in Fig. 2. Unfiltered P.M. signal energy is applied to the rectifier anodes in phase opposition through condensers 5, 6 and is phase-shifted 90 degrees by virtue of the capacity feed. These voltages are represented by E5, E6 in Fig. 2. For the condition when the instantaneous I.F. of the signal energy is equal to the mean I.F. to which the transformer primary 2 and crystal P are tuned, vectors E5 and E6 are in quadrature with Ep, Fig. 2a. The resultant voltages E<1>0 and E0 applied to the rectifiers are also equal and the rectifier outputs will be equal so that no effective voltage will appear across the rectifier load resistors 14, 15 in series. As the phase of the signal varies the vectors E5, E6 will swing about the Fig. 2a position as shown in Figs. 2b, 2c, giving a voltage output across the load resistors 14, 15 corresponding to the signal. 'In the case of relatively slow frequency variations, due, e.g. to local oscillator drift, the crystal filter acts as a very narrow-band frequency modulation discriminator or phase-shifter. The vector Ep in this case thus shifts in phase, Fig. 2d, relative to the in-time position, Fig. 2a, and produces corresponding A.F.C. potentials across the rectifier load resistors 14, 15, which are fed to the A.F.C. circuit through a filter 17 which removes the rapid signal modulation component. The capacity of the crystal P is neutralized by a condenser 9. A circuit 12, 13 normally tuned to the mean I.F. and connected between the mid-point of the secondary of transformer 1 and lead 11, may, by de-tuning, be used to increase the selectivity of the crystal P, the phase-shift of the filtered carrier thereby introduced being balanced out by compensation condensers 8<1>, 10<1> shunting the resistors 8, 10 before application to the rectifier anodes. Crystal filtered energy may be taken off at 18 and used for carrier exhaltation as described in U.S.A. Specification 2,063,588. The resistor and condenser combinations 8, 8<1> and 10, 10<1> may be interchanged respectively with the condensers 5, 6. In a modification, Fig. 3 (not shown), the filtered P.M. signal energy. is fed from the crystal P to the rectifier anodes through condensers. The unfiltered energy is in this case taken from opposite ends of the primary 2 of transformer 1 which being tuned to the mean I.F. gives the desired phase relationship at the rectifier anodes between the filtered and unfiltered energy. In another modification, Fig. 4, the filtered carrier energy is derived from the primary 2, the crystal P and its neutralizing condenser 9 being connected between opposite ends of the primary 2 and the mid-point of the secondary. The unfiltered P.M. energy is shifted 90 degrees by virtue of the magnetic coupling M between the transformer windings. Pure carrier energy is again taken from line 18. Undesirable second harmonics in this carrier may be removed by arranging the rectifiers 3, 4 to conduct simultaneously and rearranging their load resistors as in Fig. 4a (not shown) for differential combination of the rectified outputs. In another arrangement, Fig. 5, where both the unfiltered and filtered energy are derived from the secondary of transformer 1, the filtered energy is shifted 90 degrees in phase before application to the rectifier anodes by detuning the circuit 12, 13, this detuning serving at the same time to increase the selectivity of the crystal P. The'rectifiers of this arrangement may also be arranged to conduct simultaneously, Fig. 5a (not shown), to suppress second harmonics in the pure carrier taken from line 18.