Patent GB-607435-A (GB)
Title: Phase and frequency modulation systems
IPC: H03C3/08
Filed: 1946-02-04
Abstract: 607,435. Phase and frequency modulation. BIRD, L. T. Feb. 4, 1946, No. 3500. [Class 40 (v)] Relates to phase-modulation transmitters of the kind in which quadrature phases of a carrier wave are modulated respectively by a sine and a cosine function of the signal voltage, and the resultants are combined; and comprises a system in which the quadrature phases are modulated by voltages which are triangular functions of the signal voltage, and the electrode potentials of the modulated valves are so adjusted that their modulation characteristic approximates to a half sine wave. A carrier source 1 feeds the control grids of three pentode amplifiers 13, 14, 15 by sliders on three shunt circuits of which the first 6 is purely resistive, the second comprises a resistor 4 and inductor 5, and the third a resistor 2 and capacity 3. The grid excitations of the valves 14, 15 have thus 45 degrees lead or lag with reference to the valve 13. The latter is unmodulated, but the valves 14, 15 have modulating- voltages applied to their suppressor grids which are set up by the signal source 16 and are given special and different formations by the multiple diodes 19 ... 26 associated with the secondaries of the signal transformers 17, 18. The outputs of the three valves 13, 14, 15 are combined by two bridge networks which prevent the valves reacting on one another but produce the algebraic sum in the circuit 59 which may feed the aerial through frequency multipliers, amplifiers, &c. Referring to the channel 17, a low positive voltage on the upper side of the secondary renders diode 20 conductive and produces a negative voltage at that end of the resistors 27, 28 which is connected to the suppressor grid of the valve 14. As the input voltage increases, the voltage across resistor 28 rises as indicated by the line FG, Fig. 2. At a voltage determined by the delay bias 36 the outer diode 22 becomes conductive and the voltage across resistor 28 falls as indicated by the line GF. For the other half-cycle diodes 21, 19 operate in a similar manner, and the voltage at the suppressor grid of the valve 14 takes the triangular form shown in Fig. 2. In the other channel, the secondary coil of the transformer 18 has its ends connected to two pairs of oppositely-poled diodes 23 ... 26, all of which are associated with delay bias. When the upper end of the coil is positive, no current passes until the voltage reaches that of the bias source 39, whereupon diode 24 conducts and negative voltage across the load resistor 33 increases as indicated at AB, Fig, 3, until the diode 26 conducts and the voltage across resistor 33 becomes steady as indicated at BC. The voltage across resistor 33 is added to a linearly-rising voltage (see Fig. 4) across the resistor 30 shunted on a further secondary coil, the ratio of which is so chosen that the slope of Fig. 4 is half that of the slope of AB, Fig. 3. The resulting voltage applied to the suppressor grid of the valve 15 assumes the triangular form shown in Fig. 5 which is similar to that of Fig. 2 but in quadrature relationship. Fig. 6 shows that the relation between suppressor grid voltage and carrier output in a pentode modulator approximates to a half sine-wave between the points D, E. If the suppressor voltage follows the curve of Fig. 2 with its limits coinciding with DE, the carrier output of the valve 14 is of the form (1 +cos #) shown in Fig. 7; and similarly the output of the valve 15 is of the form (1+ sin 6) as shown in Fig. 8. If the mean carrier output of either valve is I, the carrier from 14 is 1+I cos # and that from 15 is I+I sin # in quadrature therewith as indicated in Fig. 10. The unmodulated components of these two outputs are neutralized by a carrier of suitable strength and phase supplied from the valve 13; and the modulated components combine to give a wave of constant amplitude I and with a phase angle #. The arrangement described permits a phase shift in either direction of 360 degrees; but by increasing the number of diodes and delay bias sources the permissible angle may be extended. The bias sources are earthed and may be constituted by mains rectifiers. The system may be used for frequency modulation by arranging that the signals from the source 16 are amplified in inverse proportion to their frequency before application to the transformers 17, 18.