GB641915A - Improvements in or relating to electric frequency discriminators - Google Patents

Abstract

641,915. Frequency and phase modulation. STANDARD TELEPHONES & CABLES, Ltd. Feb. 24, 1948, No. 5405. [Class 40 (v)] Relates to discriminator networks designed to give linear response over a wide deviation band, such as are required in multiplex systems employing a wide band of signal frequencies, and consists in employing resonant transmission lines as network elements, and in feeding the F.M. wave to the lines through valves delivering constant current. In a known arrangement shown in Fig. 1, the F.M. wave is fed to input terminals 17, 18 of unilateral devices 19 such as pentode valves of high internal impedance, the output circuit of which comprises parallel combinations of inductance, capacity and resistance I, 2, 3, 4, 5, 6. Each of these combinations feeds a rectifier 7, 8, a load resistor 9, 10, and a byepass condenser 11, 12. The Specification indicates the necessary relationships between the elements of the network in order that the equation connecting the output at 13, 14 with the frequency deviation may be a linear one, design formulµ being given which result in elimination of the quadratic term, leaving only negligibly small terms involving cubic and higher powers. According to the invention, the resonant circuits of Fig. 1 are replaced by lengths of coaxial line 43, 55 as shown in Fig. 3, in which these elements together with resistors 42, 56 are arranged in the anode circuits of pentode valves 31, 32 fed in parallel with the received F.M. wave. If the lines 43, 55 are of length #/8 and one is shortcircuited and the other open-circuited at their outer ends, and if the resistors 42, 56 are given a suitable value higher than the characteristic impedance of the lines, both the quadratic and the cubic terms of the equation representing the frequency characteristic can be eliminated. The presence of capacities across the resistances 42, 56 may be compensated by making the lines 43, 55 of different lengths. In Fig. 3 the voltages produced across the frequencydiscriminating networks are rectified by diodes 44 connected to load resistors 46 through lowpass filters 45. The load resistors are connected across grids and cathodes of amplifiers 47, 51 and the detected signal is taken across the respective anodes at terminals 29, 30. Germanium or other rectifiers may replace the diodes 44. The discriminator may be employed in a F.M. transmitter by feeding the modulated wave to the discriminator through a frequencychanging stage, and passing the discriminator output as a negative feed-back to the input of the modulator (Fig. 4, not shown). Specification 641,916 is referred to.