GB679408A - Improvements in or relating to a device for the measurement of the value of the time autocorrelation function of the voltage developed across an electrical impedance - Google Patents

Abstract

679,408. Measuring bridges; delay pulse voltage measurement. SOC. ANON DE TELECOMMUNICATIONS. June 13, 1950, No. 14788/50. Class 37. In apparatus for determining the time autocorrelation function of the delay pulse voltage appearing across an impedance under test (e.g. a coaxial line for telecommunications transmission) by reflection from impedance irregularities therein when subjected to excitation by pairs of successive short voltage pulses, a pilot pulse generator 13 of recurrence time To triggers a pulse generator 141 directly and a pulse generator 14 through an adjustable phase delay network 30. The generator outputs are combined as a train of pairs of pulses separated by time intervals T which are variable by the network 30, and the generators are adjustable to cancel or reverse the polarity of either pulse of each pair in the output train, which energizes a normally balanced Wheatstone bridge 15 comprising fixed resistance arms 33, 34, a balancing network 17, and a transmission line under test 16, terminated in a network 17<SP>1</SP>. The bridge output is amplified at 18, 19 to deflect a C.R.O. 20 synchronized from the pilot pulse generator, and is further amplified at 21 to energize a quadratic integrator 22 (e.g. an amplifier and thermojunction, or a square law detector and integrating condenser) operating a recording galvanometer 32 whose scanning operation is synchronously controlled by a manual or automatic sweep adjustment 31 of the delay network 30 over a range of values of the pulse pair interval T, so that the integrated and squared bridge output is plotted as a function of T. Since the bridge is normally balanced, the exciting pulse train is eliminated from its output, which comprises only the pulse voltage appearing across the test impedance due to reflections from successive irregularities therein. If the voltage excited by a single pulse is defined by f(t) the required time autocorrelation function is shown to be (A<SP>1</SP>) and the integrated and squared bridge output is shown to be (B) when the exciting pulses have the same polarity, and (Bi) when the second pulse of a pair is reversed in polarity with respect to the first. Thus the required function A<SP>1</SP> is given by A<SP>1</SP>=“[BB<SP>1</SP>]. The required function may be otherwise computed from the measured results. Expressions for the required function and for the measured results when the pulses of a pair are of similar or opposite polarities, may be set forth as functions of the length of the transmission line, the distances of the reflecting irregularities from the line input, and the velocity of propagation along the line. A mathematical analysis is given, and may be extended to the measurement of the autocorrelation function of a transmission line comprising an indefinite number of line sections, equivalent to the section under test, connected in tandem.