Patent GB-1429617-A (GB)
Title: Method and apparatus for measuring the group delay character istics of a transmission path
IPC: H04B3/46
Filed: 1974-06-03
Abstract: 1429617 Estimating group delay HEWLETTPACKARD Ltd 3 June 1974 24572/74 Heading G1U A method of estimating the difference between the group delay at a measuring frequency of a device under test and the group delay at a reference frequency, comprises the following steps:- (a) applying a test signal to the device under test, the test signal comprising alternate bursts of the measuring frequency and the reference frequency both amplitude modulated by an appropriate low frequency, referred to as a "split" frequency, and the end of each reference frequency burst also being modulated by an appropriate identification signal, (b) demodulating the signal produced by the device under test to give signal a, Fig. 3, (c) locating a particular point on the demodulated split frequency in each of three adjacent periods this point being the positive-going zerocrossover labelled A, B, A' in d, Fig. 3, and (d) counting high frequency clockpulses positively from A to B and negatively from B to A',g, Fig. 3, to give a final count which can be shown to be equal to twice the required group delay difference. The principle of the method is shown in Fig. 3. Because of a difference in the group delay times at the two different carrier frequencies (measuring and reference) the split frequency is phase shifted, in each measuring period, from the position - shown dotted in a - it would otherwise occupy. This difference is the difference that has to be determined. From each identification burst, pulses P 1 , b are produced, and from these, pulses P 2 , a fixed time later, are produced. At the next zerocrossover, d of the split frequency after each pulse P 2  a counter starts to count clockpulses thereby to establish windows f. These windows are used to gate zero-crossovers of the split frequency to start, reverse, and stop a counter g, thereby establishing a measure of the required difference. The preferred system operates as follows. Each identification burst is recognised 110, Fig. 5, to generate a pulse P 1 . Each pulse P 1  is followed, after a time T set by a monostable 112, by a pulse P 2  which sets a flip-flop 114 ON. The next crossover pulse d resets the flip-flop 114 which causes a clockpulse 5 to be counted 116, and a de-coder 118 establishes the reference and measuring windows f at counts of 5 to 15 and 55 to 65 respectively. These windows f control a gate 2, Fig. 4, allowing one zero-crossover pulse d to pass to a control unit 3. In the simplest possible case, the three zero crossover pulse d would cause a counter 4 to count up and then down, as described above with reference to Fig. 3. However, to economise on the size of counter required, it is arranged, in Fig. 4, that the first zero-crossover pulse d will cause the counter to start counting upwards, bottom line Fig. 6, and that the direction of counting will be reversed firstly by the next zero-crossover pulse d and secondly whenever the counter either becomes full or empty (if either of these occurs), these states being detected by a unit 7. In this way, the final count, which occurs when the third zerocrossover pulse d occurs, and which is transferred to a store 6, represents the required difference,  and the direction of counting at this instant indicates the sign of the difference. The system can be arranged either to perform repeated single measurements, or to average a sequence of measurements.