Patent GB-1436847-A (GB)
Title: System for measuring group delay distortions and attenuation distortions on part of a transmission channel which forms a loop
IPC: H04B3/462
Filed: 1973-04-13
Abstract: 1436847 Measuring group delay and attenuation WANDEL U GOLTERMANN 13 April 1973 [29 April 1972] 18031/73 Heading G1U In a system measuring group delay and attenuation with respect to frequency by transmission of an amplitude modulated switched frequency carrier and measuring the change in phase and amplitude of the received modulation, a transmission channel comprises a number of part channels interconnected to form a loop by like test sets each having a receiver and transmitter and in order to measure any part channel, each test set optionally transmits an internally generated switched carrier frequency signal amplitude modulated by the demodulated envelope of the signal received from the preceding part channel or an internally generated modulation signal on a switched carrier whose frequencies and level are controlled by those of the signal received from the preceding part channel. The test set receiver input is amplitude demodulated to provide the envelope signal at terminal 4. Low pass filter 5 removes the auxiliary frequency signals and sample and hold 6 closes during reception of the comparison frequency to provide an output 7 representing the carrier signal level. Any carrier signal residue in the demodulated signal is removed by band-pass filter 8 and the envelope signal is treated to phase discrimination in a phase-lock loop comprising phase differences 9, filter 10 and voltage-controlled oscillator 11. The phase change between the measuring frequency and comparison frequency cycles is indicated on meter 13. Two further D. C. signals are provided by the receiver representing the magnitude of the measuring and comparison frequencies as received. These are derived by frequency discriminator 16 after AGC using variable gain amplifier 3 controlled by comparison of the amplifier output with a reference voltage 19. In the test set transmitters, three modes of operation are possible, these being selected by switches 25, 27, 30, 31, 33, 37. The output signal to the line under test is provided by mixer 24 supplied with carrier signals via switch 25 and modulating signals via switch 32. With the selecting switches in the left hand position the carrier comprises alternate signals from oscillators 28, 29 whose frequencies are set by variable resistors. The modulating signal is supplied from mixer 32 whose output frequency is set by oscillator G via switch 33 and whose output level is set by potentiometer 38. The change over switch 26 for the carrier signals is operated by relay 36 operated via a divider 35 from the modulating frequency generator G. In the central positions of the switches the carrier signal is constant at a frequency set by potentiometer 39 and the modulating signal is provided by the demodulated envelope of the receiver output 4, connected to terminal 4'. The modulation level is set by potentiometer 40. In the right hand position, the carrier is again switched alternatively between the oscillators 28, 29 but now their frequencies are set from the received signal frequencies via terminals 21', 23' corrected to terminals 21, 23 in the receiver. In a modified system Fig. 2 (not shown) the frequency measurement in the receiver and frequency control in the transmitter during the right hand position is achieved using phase-lock loops. In a further modification Fig. 3 (not shown), the AGC output in the receiver is fed directly to the mixes 24 in the transmitter during the right hand position and the output of the oscillator 11 in the receiver phase measurement loop is likewise fed direct to the modulation signal mixes 32.