Patent GB-722205-A (GB)
Title: Improvements in or relating to apparatus for testing the frequency characteristics of electric signal transmission channels
IPC: G01R27/32
Filed: 1952-05-01
Abstract: 722,205. Impedance measurements; cathoderay oscilloscopes. GENERAL ELECTRIC CO., Ltd., and ELLIOTT, E. W. May 1, 1953 [May 1, 1952; April 24, 1953], Nos. 11072/52 and 11381/53. Class 37. Apparatus for testing the frequency characteristics of electric signal transmission channels comprises means for generating a first recurrent train of sine squared voltage pulses and for generating a second voltage waveform having flat topped peak portions and datum amplitude portions recurring at the same frequency as the sine squared pulses and of durations long compared with the half amplitude duration of the sine squared voltage pulses, means for combining the two waveforms in relative phase such that the sine squared pulses occur during the portions of datum amplitude of the second waveform and the peak amplitude of the first waveform is equal to and in the same polarity as that of the second waveform, means for applying the combined waveform to a transmission channel under test, and means for displaying the channel output on a cathode-ray oscilloscope whose display trace is synchronized to the recurrence frequency of the combined waveform so that the amplitude of the sine squared voltage pulses may be compared with that of the flat topped peak portions of the second waveform at a point in the latter at which the amplitude thereof is unaffected by transient distortion. In Fig. 1 (Comp.), a square wave oscillator which may operate at the line recurrence frequency of a television system, e.g. 15,625 c.p.s., drives a generator 2 of negative-going sine pulses, a trigger pulse generator 3 controlling a sine squared pulse generator 4, e.g. of the kind described in Specification 722,204, [Group XL (c)], whose output pulses are of a half amplitude duration equal to half the period corresponding to the normal upper cut-off frequency of the channel and occur a fraction of a microsecond after the corresponding sine pulse, and a blanking pulse generator 5 producing long positive-going rectangular pulses whose leading edges occur 1 microsecond after, and whose peak amplitudes are equal to those of, the sine square pulses. The three outputs are mixed in an amplifier 6 feeding a transmission channel 7 under test, and the output thereof is applied through a variable gain wide-band amplifier 11 to the Y plates of a C.R. tube 10, whose X time-base gives a 3 microsecond sweep controlled by the separated sync. pulses. At each recurrence there appears on the display a sine squared pulse 14 (Fig. 2 (Comp.)) followed by a step formed by the leading edge and the first part if a positive-going pulse, whose flat top 15 and base 12 are adjusted by the Y shift and amplifier gain to coincide at points free from transient distortion with the limits of a transparent scale 13 on the face of the oscillograph tube, which is calibrated vertically in frequency values on which the peak of the emergent sine squared pulse 14 indicates the measured upper cut off of the channel. Fig. 1 (Piov.) shows a modification in which the output of a square wave generator 1 of adjustable recurrence frequency drives a sine squared pulse generator 2, e.g. of the kind described in Specification 722,204, [Group XL (c)], whose output is mixed with that of the square wave generator in an amplifier 20 to give a combined waveform for application to the signal channel 7 in which the negative-going sine squared pulses of half amplitude duration of, e.g., 0.1 microseconds, occur at the mid-points of the positive half-cycles of a square wave, the peak amplitude of the sine squared wave being equal to the peak-to-peak amplitude of the square wave. The output of the signal channel is inverted in a wide-band amplifier 22 which supplies the Y plates of a C.R. tube 10 and also a clipperintegrator 23 (Fig. 2 (Prov.)) which comprises a cathode biased clipping pentode 33 operating at a low screen potential and high anode load with a large capacitor between anode and earth and a resistance-capacitance network 31, 32 in the grid circuit to attenuate the sine squared pulses relatively to the square wave. A square waveform in which the negative-going half-cycles coincide with the occurrence of the sine squared pulses appears across the cathode load at terminal 24 and is applied to the C.R.T. cathode to brighten the trace, and a corresponding integrated symmetrical triangular waveform appears across the anode load, and is inverted by amplifier 39 to develop a push-pull triangular waveform across terminals 38, 40 which is applied to the X plates of the C.R.T. The rectangular trace 45 displayed (Fig. 3 (Prov.)) is adjusted so that its flat top and base coincide at points free from transient distortion with the limits of a calibrated frequency scale 46 on which the peak of the sine squared pulse 48 indicates the signal channel cut-off frequency. The half amplitude duration of the sine squared pulses may be made equal to <SP>1</SP>/ 50  of half the square wave period and the frequency control of the square wave generator 1 may be therefore gauged with the half amplitude duration control of the sine squared generator 2.