Patent GB-1020957-A (GB)
Title: An arrangement for compensating amplitude and phase
IPC: H04L25/03
Filed: 1963-09-13
Abstract: 1,020,957. Compensating line distortion. STANDARD TELEPHONES & CABLES Ltd. Sept. 13, 1963 [Sept. 17, 1962], No. 36176/63. Heading H4R. [Also in Division H1] A network for compensating distortion in the amplitude and phase characteristics of wideband V.H.F. and U.H.F. signals comprises a series of directional couplers of the kind shown in Fig. 4 which are used to derive a reference signal and a series of leading and lagging signals which are subsequently combined in an adding circuit. The network is similar in principle to that described in the article " Experimental Transversal Equalizer for TD-2 Radio Relay System " in the Bell System Technical Journal, Vol. 36 (1957), pp. 1429-1450 (November). It comprises a delay line 15 (which may be a coaxial cable or a Lecher line) terminated in a matched load 12 and coupled by directional couplers 500-504 spaced at intervals of n #/4 (# is the wavelength of the central frequency) to similar lines 60-64 each (with the exception of line 60) containing an adjustable linestretcher 701-704 and an adjustable attenuator 71-74. A signal e r  is derived by adding together in network 45 the reference signal e 0  and the leading and lagging signals respectively, e 1 <SP>1</SP>, e 2 <SP>1</SP> and e<SP>1</SP>- 1 , e<SP>1</SP>- 2 . According to the invention, each directional coupler consists of two like lengths of transmission line 51, 52, Fig. 4, coupled at intervals of n #/4 by non-inductive resistors 531, 532 . . . having a resistance greater than the characteristic impedances of the lines. It is stated in the Specification that a good approximation to total directivity may be obtained with such an arrangement and a detailed discussion of the choice of values for the resistors is given. In particular, where there are three resistors R 3  = R 1  and R 2  ¸ R 1 [1 - 2Z 0 /R 1 <SP>2</SP>)]/2. Where there are five bridging resistors R 1  = R 5 , and R 2 =R 4 , R 3  being the smallest value and the values increasing outwardly from the central resistor (values are given). The directivity extends over a wide band. The characteristic impedances of the coupled cables need not necessarily be equal. Pairs of couplers for deriving leading and lagging signals are ganged as indicated in Fig. 8 and can be adjusted in position along the delay line 15 by increments of #/4. The line-stretchers 701- 704 are similarly ganged. The Specification is largely devoted to a theoretical discussion of the equalizer of Fig. 8 not susceptible to satisfactory abridgment. Both amplitude and phase compensation may be obtained without introduction of further distortion. As illustrated, in order to obtain an amplitude/frequency curve 82, Fig. 9, to compensate the characteristic 81, the curve 85a, Fig. 10, is first obtained by adjustment of the first pair 501, 502 of directional couplers. Similar subscripts in Figs. 8 and 10 indicate approximately equal frequencies. The curve 85b is obtained by adjustment of couplers 503, 504 and the curve 85c results from the addition of curves 85a, 85b and the reference signal characteristic. The final compensating characteristic 85e is obtained by passing the signal through an amplifier 47 of characteristic 85d.