Patent GB-752042-A (GB)
Title: Apparatus for the oscilloscopic study of the transient properties of electrical transmission circuits
IPC: G01R27/32
Filed: 1954-07-19
Abstract: 752,042. Delay pulse voltage measurement; measuring bridges. COMPAGNIE GENERALE D'ELECTRICITE. July 19, 1954 [July 30, 1953], No. 20964/54. Class 37. [Also in Group XL (c)] Relates to the method of testing the transient properties of electrical transmission lines and networks having a finite propagation time as disclosed in Specification 679,408 in which the distortion of a transient signal S(t), e.g. a pulse, due to multiple reflections at impedance discontinuities in the line or network is measured by determining the auto-correlation function F(#) of the signal R(t) reflected back to the sending end of the line where in which T is at least equal to the maximum delay time # of the reflected signal R(t) and # represents a time period whose magnitude is restricted to values between zero and T. According to the present invention recurrent pairs of transient signals S(t) and S(t+#) separated in time by # are injected into the line and the value of # is varied cyclically with a recurrence period T 1  ; the reflected signal {R(t)+R(t+#)} is applied to a square-law detector, the D.C. portion of the resultant detector output due to the terms {R(t)}<SP>2</SP> and {R(t+#)}<SP>2</SP> are eliminated and unwanted components of frequencies equal to higher harmonies of the frequency 1/T are eliminated by a lowpass filter and/or an integrator leaving a signal proportional to the mean value of {R(t). R(t+#)} which is applied to the Y plates of a cathode-ray oscilloscope whose timebase is synchronised with the variation of r, the oscilloscope thus displaying the variation of the auto-correlation function with #. Each pair of transient signals may comprise two D.C. pulses of like or opposite polarity or two step functions of opposite polarity which results in a D.C. pulse of varying duration; in either case the signals are derived from two pulse generators of recurrence frequencies f1 and f2 such that  (f1 - f2) =1/T 1  and the higher of the two frequencies is less than <SP>1</SP>/ 20 . As shown, the outputs from the pulse generators 14 and 15 (recurrence frequencies f1 and f2 respectively) are combined in mixer 16 and applied to the line 3 under test through a bridge network 2. The line 3 is terminated in its characteristic impedance 4 and a balancing impedance 5 is connected to one arm of the bridge so that only signals reflected by the line 3 are applied to the Y plates of the C.R.O. 13 via amplifier 8, square-law detector 9, low-pass filter or integrator 10 and amplifier 11, the oscilloscope timebase 18 being synchronized by an output from mixer 16. The time constant of the filter or integrator 10 should preferably be just less than the periodic time T 1 . Each pulse generator 14 and 15 may comprise a crystalcontrolled oscillator and associated shaping circuits, Fig. 2 (not shown); suitable mixers for producing pulse pairs of like and opposite polarity respectively are illustrated in Figs. 2 and 3 (not shown), and a suitable square-law detection is illustrated in Fig. 6 (not shown). A testing pulse of cyclically varying duration may be produced by applying pulse pairs of opposite polarity to a capacitor-resistor, integrator, Fig. 4 (not shown), or to start and stop a blocking oscillator, Fig. 5 (not shown). Specification 679,408 is referred to.