Wien-bridge oscillator with low harmonic distortion - 3

WIRELESS WORLD MAY 1981 53 third harmonic - is that due to the de­ photo-conductive cell and the light-emit­ Measuring tranSient pendence of the resistance of the thermis· ting diode combination shown in Fig. 7, in tors used to control the amplitude of the which the time constant and other dy­ intermodulation oscillation on the instantaneous value of namic characteristics of the control circuit continued from page 47 the signal potential applied to them. This can be optimized by a suitable combina­ limiting filter. characteristic of oscillators with averaging tion of proportional, integral and differen­ The authors are interested in measuring control s�stems has been analysed by tial (p.i.d.) adjustment to the gain of the t.i.m. principally to test the effectiveness Robinson who suggests that the distortion control circuit (A2). Needless to say, the of anti-t.i.m. measures such as input filt­ of such a system, which is shown to be photoresistive element should be chosen to ers, and to design low-t.i.m. monolithic mainly third harmonic, will be have a very low voltage coefficient of resis­ amplifiers. The availability of a simple and tance and an adequate response speed to X3 1 Ao-l'j 1 _ accurate measuring system has already Xl - 8rj avoid the introduction of a further signifi­ . 'Iitji provided useful results, exemplified by the cant time delay into the control loop. R.I.A.A. preamplifier shown in Fig. 12; a where (Ao-l'j)/l'j is the fraction by which Leaving aside the question of the means the low-level loop gain exceeds the gain employed to control the amplitude of the circuit designed around the TDA32 10 ste­ required to initiate oscillation, and T is the output signal (which imposes limitations of reo preamplifier i.c. The filter on the out­ time constant of the control system (ther­ an identical kind on any oscillator system, put is intended to minimize t.i.m. in the mistor or similar). In the case of a Wien­ in terms of the settling time, and the in­ next stage. This circuit, in terms of tradi­ bridge oscillator, l'j == 3. fluence of the control time constant on the tional parameters, represents the current This equation indicates that if the feed­ harmonic distortion at any given fre­ state-of-the-art in i.c. R.I.A.A. preamps in back amplitude is very little above that quency) the improvement in performance which the total harmonic distortion is required to sustain oscillation - which is given by the circuit design shown in Fig. 5 0.02% at 20kHz. The frequency response implicit in the design - the residual over that obtainable from the more con­ is 20Hz to 20kHz ±O.SdB and the dy­ distortion will be dependent on the time ventional arrangement shown in Fig. 1, namic range 100dB. constant of the control mechanism. By the suggests that it would be sensible to regard Further reading use of series and parallel resistors of appro­ the improved circuit as a general replace­ T. Roddam - Calculating transient response, priate values with the thermistor, this can lment for the earlier system in all future Wireless World, Aug. 52, pp. 292-295. be made to control the amplitude of the designs. M. Otala - Transient distortion in transisto­ oscillation at a resistance value which is References. rized audio power amplifiers IEEE Tram. only a little less than its room-temperature I. Linsley Hood, J. L., Hi-Pi News and Record Audio and Electroacoustics, Sept. 70, pp. 234- value. Under these circumstances, the Review, March 1975, pp 63-67. 239. settling time of the amplitude is long - 2. Linsley Hood, J. L., Wireless World, Sept. J. R. Stuart - An approach to audio amplifier perhaps 3-4 seconds at 1kHz, but the 1971,p437. design, Wireless World, August 73, pp. 387-391 - t.h.d. will be very low. The penalty in­ 3. Linsley Hood, J. L., Wireless World, Jan. Sept. 73, pp. 439-446 - Oct. 73, pp 491- 494. curred in this type of adjustment, apart 1973,pp 11-12. www.keith-snook.info M. K. Vander Kooj - Predicting and avoiding from the obvious inconvenience of a relati­ 4. Taylor, E. F., Wireless World, April 1973, p 194. slew-rate limiting, Electronic Engineering, Febr. vely long settling time following any dis­ 5. Robinson, F. N. H., Intl. J. Electronics, 73, pp. 18-19. turbance, is that the two gangs of the po­ No. 2 pp 137-148. (1980) W. M. Leach - Transient i.m. distortion in tentiometer used to control the operating 6. Linsley Hood, J. L., Wireless World, Sept. power amplifiers, Audio, Febr. 75, pp. 34-42. frequency of the oscillator must be 1977, pp 40-42 D B. Sundqvist - Transient intermodulation in reasonably well matched in resistance amplifiers, Wireless World, Febr. 77, pp. 37-39 value across the adjustment range, and M. Otala - Non·linear distortion in audio am­ also, if switched capacitors are used to The impact of new plifiers, Wireless World, Jan. 77, pp. 41-43. E. Leinonen, M. Otala and J. Curl - A method provide step changes in frequency sweep, technology at work for measuring transient intermodulation distor­ the ratios of their values must remain the More than £90,000 is being made available over tion, AES Journal, July 77. same. three years by two research councils for studies E. Leinonen and M. Otala - Correlation audio However, this is merely a statement of into the shop-floor impact of the introduction of distortion measurements AES Journal, Jan. 78, the obvious, that it is a pointless exercise to new technology. pp. 12-19. try to design high-performance equipment Three studies are planned: on telephone ex­ W. G. Jung, M. L. Stephens and C. Todd - using low-performance components. change modernisation; on the adoption of a An overview of SID and TIM Audio, June 79, Nevertheless, within the limitations im­ computer-based freight information system in pp. 58-79 - July 79, pp. 38-47 - Aug. 79 pp. posed by the use of a thermistor as the British Rail; and on the introduction of 42-59. R. Cordell - Comments on "A method for stabilizing element, the performance of a electronic news gathering (ENG) equipment in television. measuring TIM" and "Correlation of audio very simple oscillator, built around a dual The Science and Social Science Research distortion measurements", AES Journal, April operational amplifier (a Texas Instruments Councils are sponsoring the work which will be 79, pp. 295-300. TL072), is very good, as is shown in Fig. carried out by the New Technology Research R. Cordell - Another view of TIM, Audio, 6. The total harmonic distortion from this Group of Southampton University. The Group Feb. 80, pp. 38-49 and March 80, pp. 39-42. arrangment, in which the resistors asso­ has been formed by engineers and social scient­ S. Takahashi, S. Tanaka - A method of mea­ ciated with the thermistor were adjusted to ists committed to interdisciplinary research "on suring intermodulation distortion, AES give a settling time of 5 seconds at 1 kHz, the introduction of new electronic and computer Convention, May 79. and an output voltage of 2 volts r.m.s., is' technologies at the level of the individual S. Takahashi, S. Tanaka - A new method of measuring TIM: a comparison with the conven­ lower than that obtainable from any other workplace". The two main objectives of the work will be tional method, AES Convention, Nov. 79. simple Wien-bridge oscillator (that is to to explore the process of technological change P. J. Baxandall - Audio power amplifier de­ say with the exception of systems with low­ and to develop interdisciplinary research sign, Wireless World, Jan. 78, March 78 and pass output filtering) known to the author. methods for the problems that arise. The team May 78. This distortion is almost exclusively third will be investigating the nature of technological Yuri Miloslavskij - Audio preamplifier with no harmonic - decreasing with frequency - innovation and engineering decision-making in TIM, Wireless World, Aug. 79, pp. 58, 59, 60, which implies that the source of this wave­ the economic and social context of business or­ 86. www.keith-snook.info form distortion is the instantaneous change ganisations; the bearing of organisational A. Morando, Introduzione Analitica alia distor­ in gain of the system, during the excursion structures on the capacity of managers to gener­ sione d'Intermodulazione Dinamica, IAF.I suppl. a "Suono" Aprile 77. of each half sinusoid, due to the limited ate methods and mechanisms for the introduc­ tion and control of new technology; th� de­ P. Nuti, Misure di Intermodulazione Dinamica, thermal inertia of the thermistor. velopment of union strategies towards new Suono, Nov. 77 pp. 115-120. The very high performance obtainable technology; the consequences of technological R. A. Belcher, An experimental investigation of from such a circuit encourages the consid­ change for the nature of work and occupations; test·noise signals for the measurement of non­ eration of alternative methods of ampli­ and the effectiveness of industrial relations linear distortion of sound signal, BBC report tude control such as that employing a procedures in handling new technology issues. RD1974/2. D Document Outline WW 1981 May p51 WW 1981 May p52 WW 1981 May p53