Patent GB-764895-A (GB)
Title: Apparatus for studying the transient properties of transmission lines
IPC: H04B3/46
Filed: 1954-10-04
Abstract: 764,895. Impedance/frequency measurements; cathode-ray oscillographs. COMPAGNIE GENERALE D'ELECTRICITE. Oct. 4, 1954 [Oct. 5. 1953; Feb 10, 1954], No. 28509/54. Class 37. [Also in Group XL (c)] Apparatus for examining the alterations of the profile of a pulse transmitted over a line comprises a pulse generator connected to the far end of the line and synchronized, over an auxiliary line and a main phase shifter, from a sinusoidal voltage generator which also synchronizes a sawtooth generator to provide the timebase voltage of an oscillograph at the near end, which is deflected to display the voltage output of a limiting amplifier energized by the line output signal; the limiting amplifier being periodically blocked by control pulses derived from a pulse generator synchronized with the sinusoidal voltage generator over an auxiliary phase shifter circuit, so that what is displayed by the oscillograph is the parasitic distortion superimposed by the line on the excitation pulse, the latter being suppressed. Fig. 1 shows apparatus for testing the transient impedance properties of a coaxial transmission line 101, to the end B of which is applied a train of periodic pulses of frequency nF 0  (n being an integer) from pulse generator 103 synchronized over auxiliary line 111 and adjustable phase shifter 110 from master generator 102 of a sinusoidal wave form of frequency F 0 , which also synchronizes timebase generator 108 horizontally deflecting C.R. oscillograph 106. The pulse train appearing at end A of the coaxial line, which is distorted by reason of the attenuation and phase shift due to the line and also by reflections of spurious signals from impedance irregularities thereof, is applied to a non-linear limiting amplifier 107 (Fig. 3) transmitting signals of low amplitude undistorted while restricting the amplitudes of greater signals, wherein the input is applied over limiting resistance 204 to the grid of limiting pentode amplifier 201 having a low variable value of screen voltage and capacitance coupled to a limiting cathode follower whose grid is biased from a variable point of its cathode resistance and which has a diode 218 shunting grid resistance 219 to reduce momentary positive grid excursions; the amplified output from a tap 213 on the cathode resistor being connected over linear wide-range amplifier 104 to vertically deflect cathode ray oscillograph 106. The suppressor grid of pentode 201 is pulsed negatively over terminal C from pulse generator 112 synchronized at frequency nF 0  from generator 102 over adjustable phase shifter 113, to periodically block the amplifier. In operation, phase shifter 110 is adjusted to display the output pulse train waveforms in a suitable position on the oscillograph screen, and phase shifter 113 is adjusted so that amplifier 107 is blocked in time coincidence with the reception at end A of a high-amplitude pulse corresponding to the exciting pulse at B. In conjunction with the limiting action of amplifier 107 this ensures that the waveforms displayed on the oscillograph represent only the parasitic distortion due to each exciting pulse; from inspections of which at different frequencies conclusions may be drawn as to the impedance/frequency characteristics, and the impedance irregularities of the coaxial line. The limiting amplifier may be unblocked (Fig. 2, not shown) and in a modification thereof (Fig. 4) the input is applied to the control grid of pentode 301 over parallel LC circuits 303, 305 and 304, 306 whose junction is returned to earth over condenser 316 in shunt with resistance 317, and to the control grid of triode 302 over capacitance 307 and adjustable potentiometer 314. The cathode potential of triode 302 is adjustable by potentiometer 311 and its anode is connected to the control grid of pentode 301 biased by cathode resistance 309, whose anode is supplied with a low D.C. voltage over resistance 312 and inductance 313 and is capacitance coupled to the subsequent amplifier 104, while the screen is also fed with a low voltage over resistance 318. In operation, the arrival of a high positive signal at the input initiates a positive grid current flowing in high resistances 305, 306 to limit the signal which charges the parasitic grid-earth capacitance 310 to discharge slowly over resistances 305, 306. The discharge is hastened by the application of the input signal direct to the control grid of triode 302; whose anode voltage falls to rapidly reduce the potential on the control grid of pentode 301, and the threshold of limitation is adjustable by potentiometer 311. Resistors 305, 306 are by-passed for high-frequency signals, and resistors 317, 309 are by-passed for all alternating signals. Specification 752,042 is referred to.