Patent GB-538035-A (GB)
Title: Measurement of phase change and envelope delay of a transmission line
IPC: H04B3/48
Filed: 1940-03-01
Abstract: 538,035. Phase-difference measurements. STANDARD TELEPHONES & CABLES, Ltd. March 1, 1940, No. 3869. Convention date, March 3, 1939. [Class 37] Phase change and envelope delay in a transmission circuit are measured by transmitting an alternating current over the circuit from the near end to the far end and back, and comparing the phase of the returned current with that of the current as originally transmitted. In the arrangement shown in Fig. 1 a source of variable frequency current such as an oscillator 10 supplies a current which is transmitted to the transmission line 100 by means of a highbrid coil 20 with a balancing network 21. A second hybrid coil 120 with its balancing network 121 at the far end of the line removes a part of the transmitted test current from the line and through its conjugate pair of terminals returns some of the current to the line for return to the near end, where it is removed by the near end hybrid coil, a pair of whose terminals are connected to the phase measuring equipment 30. The returned portion of the test current may be amplified by an amplifier 130. The returned test current is taken through an impedance 31 to a two-way switch 32 by which it can be connected to an impedance 33 or a reversing switch 34 connected to one primary winding of a transformer 40. Test current obtained directly from the source 10 is fed through an artificial line 50, impedance 51, and two-way switch 52 either to an impedance 53 or to the second primary winding 42 of the transformer 40. The secondary winding 43 of the transformer 40 is connected through a second transformer 60, artificial line 70, impedance 71, and amplifier-rectifier 80, to a meter 90. With the switches 32, 34 in the right-hand position and the switch 52 in the left-hand position the output current of the line 100 is indicated on the meter 90. By throwing the switches 32, 52 to the opposite position the output of the artificial line 50 is measured. The attenuation is then adjusted until these two valves are equal. By throwing the switches 32, 52 into the right-hand position and the reversing switch 34 into the left-hand position, the vector sum of the two equal currents is indicated on the meter 90. By reversing the switch 34 their vector difference is indicated. With the switch 34 in the position giving the larger reading the calibrating artificial line 70 may be adjusted to produce sufficient attenuation to give the same reading as was obtained with the switch 34 in the other position and the artificial line 70 cut out. The phase angle between the two currents can then be determined by the amount of artificial line inserted. The far end of the line 100 may alternatively be terminated by an open circuit or a short circuit, the return current being reflected with a phase displacement of 180 degrees or in phase respectively. A phase compensator 15 may be employed at the near end of the line to neutralize the phase change of the apparatus. In a modification, an harmonic generator is arranged at the far end of the line so as to raise the returned current to an harmonic frequency, for example double that of the source 10. The returned current is demodulated at the near end to produce a current of the original frequency before being transmitted to the phase measuring equipment 30. Filters may be employed instead of hybrid coils for separating the currents of different frequency. Fig. 5 shows an arrangement for determining envelope delay. An oscillator 10 of adjustable frequency is connected through a potentiometer 11 to a modulator 14 in which it is modulated by a current of a standard low frequency, such as 25 cycles per second, obtained from a source 12 through a potentiometer 13. In the output of the modulator 14 the low-frequency component of 25 cycles is suppressed and the output current after passing through the hybrid coil 20 passes over the line 100 through the hybrid coil 120, back to the line through the conjugate terminals of the hybrid coil 120, back over the line and through the hybrid coil 20 to the amplifier-demodulator 18, the output current from which is of a frequency of 25 cycles. A dynamometer detector 210 has two windings, one receiving the 25 cycle current directly from the source 10 through a phase shifter 220 and the other receiving 25 cycle current from the amplifier-demodulator 18. A zero delay transducer, such as an artificial resistance line, is first substituted for the transmission line to be measured. The oscillator 12 is then adjusted to a frequency of 25 cycles and the setting of the phase shifter 220 is determined to produce a zero reading on the dynamometer. The line 100 is then substituted for the zero delay transducer and the phase shifter 220 is again adjusted to produce zero reading on the dynamometer 210. The envelope delay is then calculated from the two readings of the phase shifter.