1429618 Measuring group delay and amplitude distortion HEWLETT PACKARD Ltd 3 June 1974 24573/74 Heading G1U In a system for testing a channel AB, Fig. 1, extending from a remote location A via central office C to a remote location B, by means of a test signal consisting of alternate periods of a reference and a measuring carrier frequency, a control signal is sent from the central station to the remote station A causing it to apply the test signal to the channel under test and the remote station B upon receiving the distorted test signal recognises that it has received a distorted test signal (as opposed to a control signal) and thereupon transmits to the central station a signal from which, at the central station, measures of the distortion introduced by the channel AB can be extracted irrespective of any further distortion from the remote station B back to the central station. To test the channel B'A', switches S1,..S4, are changed over, the same control signal is transmitted by the station but this time to station B, and the remote station recognising that it has received a control signal (as opposed to a distorted test signal) responds accordingly similarly to the above test for channel AB. The preferred embodiment measures relative amplitude distortion and relative group delay time, this latter measurement being made by the method described and claimed in Specification 1429617. The test signal comprises alternate 120ms periods of reference and measuring carriers, each amplitude modulated by a split frequency of 41 2/3c/s, the last cycle of which, during each reference period, is itself amplitude modulated by an identity frequency of 168 2/3c/s to allow the end of each reference period to be detected. The receiving and transmitting equipment at each station is essentially the same. In essence it comprises a receiver Fig. 2 coupled by interfacing circuitry Fig. 4 to a transmitter, Fig. 3, and, in more detail, it operates as follows:- Central station A two-position mode switch 45 having banks 45A and 45B, Fig. 4, in the interface at the central station, not shown in Fig. 1, is set to its "normal" mode with its wipers on contacts a, and a switch 61 in the transmitter Fig. 4 is set to its remote control position as shown. Front panel controls 31 in the transmitter are then operative to set the reference and measuring frequency values and a control signal is transmitted, this signal differing from a test signal in that, as a consequence of the setting of switch 61, the split frequency modulation is omitted from each measuring period. Remote station A The control signal is received 1, Fig. 2, full-wave rectified 2, low pass filtered to reject both the reference and measuring carriers but leaving the modulation superimposed upon a D.C. proportional to the amplitudes of the received carriers, amplified 4, bandpass filtered 11 to extract the modulation, and applied to a circuit, shown in a dotted box, which recognises the absence of modulation in the measuring periods and maintains a switch 66, with banks 66A, 66B, in the interface, Fig. 4, in its normal position on contact c. As a consequence signals from buffers 42, 43, which are arranged to store counts of the reference and measuring carrier frequencies, and passed, via switches 66 and 45, which is set in its remote/remote control mode on contact b or c, as shown, to the transmitter Fig. 3, which now generates a test signal (switch 61 being in its 'remod' position), the reference and measuring carrier frequencies being those set by the front panel controls in the central station transmitter. Remote station B The test signal is received, as described above, and since modulation is detected in the measuring periods a trigger 65, Fig. 2 is set to put switch 66 Fig. 4 on its b contacts, as shown. The output signal from amplifier 4, which consists of the split frequency modulation superimposed, in the respective periods, on D.C. levels proportional to the amplitudes of the received carriers, contains the required distortion information in two ways: firstly the phase difference between the modulation during a reference and a carrier period approximate to the relative group delay time between the two carrier frequencies, and secondly the ratio of the two D.C. levels in reference and carrier periods is the relative amplitude distortion between the two carrier frequencies and it is this data that has to be sent to the central station. This is accomplished by adding in the interface a fixed D.C. level to the output from amplifier 4 so that the D.C. level during each reference period is brought up to a reference level, passing the resulting signal through an anti-log amplifier 47, Fig. 4, and then causing the transmitter to modulate the resulting signal continuously on the reference carrier, so that any further distortion between station B and the central station will not affect the data being transmitted. Central station The continuous reference carrier (which still carries the identification modulation of 166 2/3c/s) at the end of each (now notional) reference period, is received 1, Fig. 2, the phase difference between the modulation in the reference and measuring periods is derived as described in Specification 24732/74, and the ratio (in dbs) of the D.C. levels in the reference and measuring periods is derived by a dual-ramp type A/D converter 5.
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