Patent GB-797909-A (GB)
Title: Improvements in or relating to the measurement of group delay in electric communication systems
Inventor: RADCLIFFE STUART NORMAN
IPC: H04B3/46
Filed: 1956-02-21
Abstract: 797,909. Phase measurement. STANDARD TELEPHONES & CABLES, Ltd. Feb. 21, 1956, No. 5280/56. Class 37. Apparatus for measuring the group delay distortion of a communication system comprises at the transmitter, a device for periodically sending over the system a test signal comprising at least two waves at respectively first and second frequencies differing by a given indicating frequency, in alternation with a reference signal comprising at least two waves at respectively third and fourth frequencies differing by the same indicating frequency; and at the receiver a device for deriving from each of the test and reference signals a corresponding indicating wave at the given frequency and for determining the phase difference therebetween, which phase difference represents the difference in group delay corresponding to the frequency bands bounded respectively by the first and second, and third and fourht frequencies. Transmitter.-A signal of e.g. 3.5 Mc/s. from oscillator 1 (Fig. 1) is modulated at 2 by a 1 Kc/s. signal from 100: 1 divider 4 energized by oscillator 3, and the fundamental and first order sidebands of 3.499, 3.5 and 3.501 Mc/s. are filtered out and applied to modulator 6 together with alternative signals of 4.05 Mc/s. fixed and 3.503 to 4 Mc/s. variable, switched at 2.5 c/s. by electronic gate 9 controlled by 50 c/s. source 10 divided by 20 at 11. From modulator 6 the channel input 14 is supplied through lowpass filter 12 and amplifier 13 with lower sideband composite signals comprising alternate groups of three reference frequencies of 549, 550 and 551. Kc/s. and three test frequencies of (F - 1), F and (F + 1) Kc/s., where F is variable from 3 to 500 Kc/s. in response to the setting of oscillator 8. Receiver.-The channel output at terminal 15 (Fig. 2) is freed from 1 Kc/s. components by bandpass filter 17 and from switching noise by high-pass filter 18, and is demodulated by square law detector 16 to 1 Kc/s. indicating signals feeding a variable phase shift circuit 20 whose operation is controlled by relay contacts 31 of relay A, from whose output a 1 Kc/s. signal is selected by band-pass filter 19 to excite a harmonic generator 21, whose 97th harmonic is selected. by band-pass filter 22 for phase comparison at 32 with the output of 97 Kc/s. reference oscillator 33. From the output of filter 18, band-pass filter 23 selects the reference 550 Kc/s. component present on odd operations of switch 9, which after detection changes over a bi-stable two condition electric trigger circuit 25 to operate relays A and B (26, 27) at the beginning of each period of transmission of the reference signal to the channel, and to release them at its end. A pulse is also supplied at the beginning and end of each reference signal period to a single-stable trigger circuit 28 (reverting to stability after 100 milliseconds delay) to impulse a second single stable trigger circuit 29 to operate a relay C (30), which is released when the trigger circuit restores after 80 milliseconds. During the period of the reference signal, relay 26 is operated to change over contacts 31 and introduce a fixed phase shift, and relay 27 is operated to prepare a circuit for the output of phase comparator 32 to charge a capacitance storage circuit 37 over normally open contact 34 of relay 30, while during the test signal period relays 26 and 27 are released to introduce a phase shift variable manually by a calibrated control, and to prepare a circuit for charging a capacitance storage circuit 36. Relay 30 operates for 80 millisecond sampling periods during the second half of both the test and reference signal periods to close contact 34 and complete the appropriate storage circuits, after switching transients due to relay contacts 31 and 35 have disappeared. The differential amplifier 38 energized from the test storage circuits supplies meter 39 with the difference between their accumulated voltages, representing the phase difference of the comparator inputs at 97 Kc/s.; the reference storage voltage being applied to a reactance valve frequency control circuit 40 slightly varying the frequency of oscillator 33 to lock its phase to that of the 97 Kc/s. signal representing the reference signal output of the channel; the control voltage being indicated by meter 41. Operation.-Oscillator 8 at the transmitter is set to a frequency e.g. 4 Mc/s. giving a channel test frequency of 500 Kc/s., and at the receiver the variable control of phase shifter 20 is adjusted for zero reading of meter 39, at which the inserted phase shift equals the phase difference between the test and reference indicating waves, equivalent to the difference in group delay between the test and reference transmission frequencies. Repetition of the test for successive frequencies enables the group delay (or rate of phase shift change with frequency) to be determined and plotted for different transmission frequencies; the group delay distortion being determinable by the deviation of the plot from a straight line parallel to the frequency axis. (General.-The oscillators may be crystal controlled, and any harmonic other than the 97th may be selected from the output of generator 21, with adjustment of the frequency of oscillator 33. The effect of mains hum is minimized by making the sampling period an integral multiple of the mains supply period, while the time constants of the storage circuits are large by comparison therewith so that the mains frequency component is always integrated over a whole number of cycles and its net effect is zero. Frequency division may be effected by successive multivibrators (Figs. 3, 6, not shown) switching alternative frequencies by biasing the alternate cathodes of a double amplifier valve having a common load (Fig. 3, not shown), phase shifting by switchable and adjustable capacitance-resistance circuits coupling successive valve amplifiers, and band-pass filtration by valve amplification having negative feedback through parallel band-stop resistancecapacitance T-networks. The variable phase shift control may be calibrated directly in group delay readings. Test and reference signals transmitted over the channel may each comprise two or more frequencies equally and similarly spaced. Specification 749,297 is referred to.