Patent GB-801629-A (GB)
Title: Apparatus for measuring and correcting phase delay distortion in a facsimile communication system
IPC: H04L21/00
Filed: 1955-11-18
Abstract: 801,629. Phase measurement. CREED & CO., Ltd. Nov. 18, 1955 [Nov. 19, 1954], No. 33062/55. Class 37. [Also in Group XXXIX] Apparatus measuring phase delay distortion in a facsimile transmission system comprises a first fixed frequency source modulating a variable frequency carrier oscillator which is cyclically swept over the transmission waveband into upper and lower side-bands, which together with the carrier are applied to the transmission channel, whose output is demodulated at a receiver to a signal at the modulation frequency for comparison with a comparison source of similar frequency in a device comprising a first pulse generator driven by the demodulated signal to pulse at a predetermined amplitude of each cycle of such signal, and a second pulse generator driven by the comparison signal to similarly pulse at a predetermined amplitude of each cycle thereof, with a visual indicator (which may be a C.R. oscilloscope with a frequency sweep time-base) for comparing the times of appearance of the generated pulses and displaying an indication of the difference in time between the occurrence of corresponding amplitude points of the demodulated and comparison signals. In Fig. 14, a 200 c.p,s. tuning fork frequency standard energizes a circuit 4 generating one pulse per cycle to drive a frequency divided square wave generator 5 comprising three successive double triode multivibrator circuits which produces a balanced 25 c.p.s. square wave output integrated by RC circuits 7, 8 to a triangular waveform exciting parallel resonant circuit 9, 10 generating a 25 c.p.s. sinusoidal frequency to energize push-pull triode amplifier 12 transformer coupled to voltage divider 17, whose output is applied to the secondary centre-point of input transformer 20 feeding balanced triode modulator 18 transformer coupled to a line transmission circuit over attenuator 48. A variable frequency oscillator 23 is linearly swept by motor 24 over the test frequency range, and the sweep signal is combined with the output of a fixed frequency oscillator 33 at the secondary of the input transformer 30 of balanced triode modulator 32 to produce a frequency modulated sweep carrier signal, separated from its components by a low-pass filter comprising series inductances 38, shunt capacitances 39, and shunt series resonant capacitances 42 and inductances 43, and transformer coupled to the input of modulation transformer 20. The emergent signal at the receiving end of the line comprising a carrier frequency and upper and lower side-bands is transformer coupled to balanced attenuator circuit 52 (Fig. 16) and push-pull triode amplifier 54 transformer coupled to bridge rectifier demodulator 61 energizing signal level meter 64. A signal derived from one anode of amplifier 54 is coupled over capacitance 101 and series resistance 102 to a four-stage limiter (Fig. 15) comprising resistance coupled cascaded double triodes 103 whose grids are biased positively to appropriate levels from potential divider resistance chain 106, 107, 108 and 115-119 energized from the positive high voltage supply; the resistance 119 being shunted by temperature variable resistance 116 to maintain the final stage bias constant independently of ambient temperature. Oppositely poled rectifiers 109, 111 connected to each grid conduct when the appropriate input signals reach the respective positive or negative clipping levels, and the limited square wave signal from the final stage is amplified by double triode 122 connected as a single cathode follower biased from resistance voltage divider 124, 125, 126. The output from cathode resistance 128 is differentiated by series capacitances 127, and positive pulses flow to ground through series rectifiers 131 and resistance 132 against a standing direct voltage level developed across capacitance by-passed resistance 126. Negative pulses flow through rectifiers 135 to moving coil meter 133 to give a unidirectional indication of frequency. The drop across resistance 132 is filtered from A.C. by resistance 136 and capacitance 137 and applied to the X-deflection plates of a cathoderay oscilloscope to provide a linear frequency base. A portion of the rectified output of transformer 59 (Fig. 16) derived across resistance 63 may be used for oscillographic display signal of level in preliminary adjustment, while the output across load resistances 62, 63 in series is coupled through carrier frequency attenuating capacitance 67 and resistance 66, and coupled to the grid of a cathode follower comprising one section of double triode 73 feeding a variable twin T filter network comprising resistances 76, 77, 78, tuning variable resistance 79 and capacitances 80, 81, 82, which excludes the 25 cycle modulation signal but admits all other frequencies to pentode amplifier 85 resistance coupled to the grid of the cathode follower triode, thus cancelling all but the 25 cycle signal from its output, corresponding in repetition rate to the modulation signal and in phase shift to the phase change over the line of the carrier frequency. The latter is amplified by the second section of double triode 73 and resistance coupled over RF choke 90, series capacitance 152, and resistance 151 to the input (Fig. 17) of a six-stage limiter circuit comprising three cascaded double triodes 153 whose individual stages operate similarly to those of Fig. 15. The square wave output is phase split by the final limiter double triode 172, amplified by double triode 184 and coupled to the cathodes 201 of double diode 202 (Fig. 18) across cathode resistances 203 in combination with a positive bias from voltage divider 204, 205. Negative pulses at 50 p.p.s. appear at the common anode circuit (positive pulses being blocked) synchronously with positive and negative excursions of the 25 p.p.s. square wave, and are applied to the grid of the first section of double triode 207 connected for operation as a symmetrical bi-stable multivibrator ; the grid of the second section being supplied with another train of 50 p.p.s. negativegoing square wave pulses from a pulse shaper and two-stage frequency divider fed from an independent local 200 c.p.s. tuning fork oscillator. The multivibrator cathodes are variably biased by common variable resistance 221, and the signal appearing between the respective cathodes comprises bipolar square wave pulses whose average value represents in magnitude and sense on the relative timings of the pulse trains applied to the grid; the signal being zero when the pulses arrive alternatively at equal intervals. It is averaged by back-to-back electrolytic capacitances 225 and applied to centre zero meter 226 as a coarse measure of delay time or line phase-shift; currents outside the meter range being measurable by the application of a positive or negative bias current adjustable by ganged switches 231, 232. For more precise measurements the anode of the second section of double triode 207 is coupled through capacitance 234 and resistance 235 forming a long time constant circuit to the grid of the first section of double triode 237; transients being removed by shunt capacitance 241. The second section of the tube 237 operates at a constant level of current set by bias resistances 246, 247 and its cathode voltage is compared with the first section cathode voltage by meter 249 as a sensitive measure of line phase-shift or delay. The voltage on the first section cathode is connected over conductor 300 and resistance 301 (Fig. 16) for connection to the Y-plates of an oscilloscope for display of phase-shift against a frequency time-base while a signal proportional to the input level is derived from resistance 63 through capacitance 302 and applied to the low potential input of the oscilloscope to regulate the display to a convenient amplitude. Phase distortion in the transmission system is equalized by plural active network sections (Fig. 12, not shown) in each of which an input signal is phase split and applied over a balanced transformer to a shunting voltage divider asymmetrically tapped to earth at a ratio of 1: 3 or 1 : 5 ; opposed ends thereof being connected to the output respectively over a single variable resistance and a parallel network of inductance capacitance and variable resistance in parallel; each circuit being adjustable for phase equalization at a specific frequency. A modified equalizer circuit (Fig. 20, not shown) comprises a pentode amplifier resistance coupled to a network in which the amplifier output is connected over a single variable resistance and over a parallel network of variable resistance, capacitance, and inductance to opposite ends of a voltage divider asymmetrically tapped to earth at a ratio of 3 or 5 to 1; the signal across the voltage divider being transformer coupled to the output. The single variable resistance may be replaced by a network comprising a variable resistance in series with an inductance shunted by a capacitance (Fig. 21, not shown).