GB865572A - Improvements in or relating to automatic distortion correcting arrangements for frequency modulated communication systems - Google Patents

Abstract

865,572. Automatic frequency control systems; frequency modulation. MARCONI'S WIRELESS TELEGRAPH CO. Ltd. Oct. 20, 1958 [March 24, 1958], No. 9401/58. Class 40(5) An automatic distortion correcting arrangement for a frequency modulated communication system comprising in a signal channel an adjustable frequency-sensitive distortion correcting network (a group delay equaliser stage D in Fig. 1), means for injecting a voltage wave of predetermined frequency at a point in said channel (from 50 c.p.s. source M in Fig. 1) which varies the response characteristic of the channel at the frequency of the injected wave and means responsive to intermodulation noise produced by said wave to adjust the network (phase-sensitiverectifier L and D.C motor N driving potentiometer Pin Fig. 1). Fig. I shows an automatic phasedistortioncorrectionnetwork.Theinjectionof a 50 c.p.s. voltage from M into the equaliser D having a straight line characteristic will produce a small change, at the injected frequency, of the slope of the group delay-frequency characteristic of D but if the mean value of this varying slope is such as to provide uniform group delay there will be no D.C. output from rectifier L and motor N will not run. If this is not the case, a varying amount of noise will be fed to one input of rectifier L out of phase with the signal from M thereby causing motor N to run to adjust D. Differentially controlled bias is arranged to be applied to the grids of valves D1, D2, forming part of the equaliser, from potentiometer P, output from source M being also applied in phase opposition to the grids of D1, D2. Two equaliser stages may be used (Fig. 2, not shown) one at which the voltage is injected and one at which control is effected. In a further embodiment (Fig. 3, not shown) the D.C. motor and phase sensitive rectifier are replaced by a two phase motor, while in Fig. 4 (not shown) two adjustable equalisers are used one having a substantially linear characteristic and one a curved (e.g. a parabolic) characteristic. Instead of using two injected voltages at the same frequency but in phase quadrature with one another, two voltages of different frequency may be used. In the embodiment of Fig. 5 amplitude distortion correction is obtained as well as phase distortion correction, by means of an adjustable amplitude corrector D<SP>111</SP> presenting a linear transfer characteristic the slope of which varies about a mean value at the frequency fed in through transformer D15. The transformer input consists partly of a fixed voltage directly from a source M<SP>111</SP> (of a different frequency than M) and partly of a variable correcting voltage derived from potentiometer P<SP>1</SP> driven by a twophase motor N<SP>111</SP>, one phase of which is connected to source M<SP>11</SP> and the other to a noise envelope amplifier K. A complex amplitude correction arrangement may be provided by using one network having a linear and one having a curved characteristic, and a combination of a phase sensitive rectifier and D.C. motor may replace the two-phase motor.