book review: physics and contemporary needs

6 Comparison with analogue meters

Two points of particular importance are discussed here,namely accuracy and flexibility.

In analogue meters, many factors contribute to the error inthe result. Among these, the stability of the low-frequencyoscillator is of prime importance. Crystals are now used toimprove stability at the expense of flexibility, in that themodulating frequency can only take preset discrete values.In this processor-based meter, the modulating frequency is keyselectable, taking any value, and the crystal stability ismaintained.

The accuracy of the carrier, or measuring frequency, isequally important. In analogue meters, this is chosen withrotary knobs and switches, or an oscilloscope may be used,leaving a lot of uncertainty regarding the validity of themeasurement. In this processor-based meter, the measuringfrequency is key selected with an accuracy of within 0.1%.

The display techniques in analogue meters leave much tobe desired. Although two perfectly matched channels andequal signals are required to obtain a correct reading, theindicator has also to be linearly proportional to the reading,regardless of its value. In Reference 11, one is led to thinkthat the error increases with the value of group delay to bemeasured, limiting the accuracy of the meter in the rangewhere it is most needed. In the processor-based meter, digitaltechniques sort out errors due to mismatches, harmonics andtemperature effects. With the very reliable display techniqueused, the error is independent of the value of group delay,resulting in lower percentage errors with higher readings.

Some care is required when measuring systems whichcontain nonlinearities. This technique is based upon phasedifference, which only has meaning for two sinusoids of thesame frequency. Most systems, such as telephone networks,endeavour to keep nonlinearity to a minimum, and treat itsproducts as noise. The presence of noise is indicated by spotmeasurements having a variation outside the limits given byalmost noise free measurements.

As far as expandability is concerned, a purely analoguemeter, given swept-frequency measuring facilities, can be, at

best, connnected to an xy-plotter. This processor-based meterinterfaces to a visual-display unit or a printer, allowing easytuning of filters under test. The results may also be fed to acomputer for further processing.

7 Conclusion

Group-delay measurement, at audio and lower frequencies, isanother domain where digital processing shows superiority overclassical techniques. Replacing the squarers, lowpass filters andzero-crossing detectors by a processor board performing thesefunctions in real time, will be a further step towards the all-digital group-delay meter which can be integrated as a singlecircuit.

8 References

1 JONES, I.O., and ADCOCK, R.C.: 'Group delay in the audio datanetwork', Post Off. Electr. Eng. J., 1971, 64, pp. 9-15

2 K.D.D.: 'Are group delay time and/or phase delay time usefulparameters for defining low distortion transmission?', IEEE Trans.,1973,COM-21,pp. 1446-1448

3 CHOHAN, V.C.: 'On the usefulness of group delay and/or phasedelay parameters for low distortion transmission', ibid., 1974,COM-22,pp. 1147-1148

4 MAR, H.: 'Comments on "Are group delay time and/or phase delaytime useful parameters for defining low distortion transmission?"',ibid., 1974, COM-22, p. 1148

5 FETTWEIS, A.: 'Significance of group delay in communicationengineering', ,4£"t/, 1977, 31, (9), pp. 342-348

6 NYQUIST, H., and BRAND, S.: 'Measurement of phase distortion',Bell Syst. Tech. /., 1930, 9, pp. 522-549

7 MATSINGER, J.H.: 'An automated group delay measurement',Rev. Sci. Instrum., 1972,43, pp. 1568-1574

8 MATUKA, L.: 'Measurement of amplitude and phase characteristicsof transmission paths', Budavox Telecommun. Rev. (Hungary),1972, Pts. 3-4, pp. 23-27

9 MONTEFUSCO, N.: 'Test set for group delay and attenuationdistortions', Siemens Rev., 1977, 44, (5), pp. 221-224

10 ALLEN, B.N.S.: 'Development of group delay measuring sets',Post Off. Electr. Eng. J., 1973, 66, pp. 47-52

11 SARAGA, W., and MILLER, A.K.H.: 'A novel high precision groupdelay meter'. Conference on electric test and measuring instrumen-tation, Testmex 79, London, June 1979, pp. 70-73

Book reviewPhysics and contemporary needsRiazuddin (Ed.)Plenum, 1980, 587pp., $65.00ISBN: 0-306-40474-5

The Pakistan Atomic Energy Commission (and the TriesteInternational Centre for Theoretical Physics) clearly feel thatthere is a contemporary need for them to hold regular physicssummer schools in Pakistan. The wide range of topics mustprovide an interesting and varied education for thosescientists with 19 days to spare in the foothills of Himalayas.This collection of lecture notes is probably less successful afterthe event when it appears as a fairly expensive text-book.

Not surprisingly, more than half of the lectures on energyand natural resources are on nuclear topics. Technology in thisinstance covers only nuclear reactor materials with amorphousand photovoltaic materials. Computer topics include trendsin programming and information retrieval, together with acouple of specific problems (hydrodynamic studies requiringcomputation). The frontiers of knowledge are represented by

electron impact excitation spectroscopy, tests of generalrelativity and particle physics.

Marchetti on energy systems gives a fascinatingly readabletreatise on extrapolating economic (and other) trends,mixed with a few ingredients such as biochemistry. But mostof the other major chapters read as fairly formal reviewsby experts concentrating on their own specialities. Thismeans that, for example, 'the status and prospects of nuclearpower' does not seem to include thermal reactors other thanPWR, that the 'present state and future prospects' of fusiondo not appear to encompass the laser (nor any ions heavierthan D— T), and that there is a trend to replace Fortran andBasic with Pascal (as the introductory programming language)which is 'worldwide'.

Those familiar with the three volumes from previoussummer schools should not be disappointed, but other readersneed to be prepared for a mixed bag of topics.

D.T.SWIFT-HOOK

IEE PROC, Vol. 128, Pt. A, No. 5, JUL Y1981 361