WIRELESS WORLD MAY 1981 51

Wien-bridge oscillator with low harmonic distortion

New way of using Wien network to give 0.001 % t.h.d. by J. L. Linsley Hood, Robins (Electronics)

The Wien-bridge network can be 1kHz of some 0.003%, which tended to connected in a different way in an increase with frequency above this point, R oscillator circuit to give a sine wave as the effectiveness of the common-mode with very low total harmonic isolation deteriorated. distortion. An I.e.d/photocell However, it is not implicit, in the use of a Wien network as the frequency-control amplitude control is external to the Output circuit. method, that the configuration shown in Fig. 1, in which the output of the network is taken to the non-inverting input of the R The Wien-bridge network remains the amplifier and the amplitude controlling most popular method of construction of negative-feedback signal is taken to the variable-frequency sine-wave oscillators, other, is the only circuit configuration since the basic circuit can be very simple in which can be employed. In particular, con­ ov form. It is a fairly straightforward matter sideration of the phase and transmission to design oscillators of this type in which characteristics of such a network, shown in Table 1 and Fig. 2 for equal values of C the harmonic distortion is only of the order Fig. 1. Basic Wien-bridge oscillator circuit of 0.01-0.02%, and which allow frequency control by means of a simple 2-gang poten­ Fig. 2. Gain and phase characteristics of tiometer. Wien network 1-0 131 The basic circuit for an oscillator of this O· 9 form, using a single operational amplifier z 0 o 8 w_ as the gain block, is shown in Fig. 1, and >Vl _Vl 7 the author has shown a practical design of z O· �-IVli: 121 oscillator, based on this, for a use as a wz §lO· 6 0: Vl « " 0: simple, general-purpose workshop tool. 1 f- i:Vl O· ...... However, in the form shown in Fig. 1, a �O 4 ...... significant problem exists in that the trans­ 0: ..... 111 f- 3 mission of a normal Wien network, at the O...... z�-- w ,/7 " '/}:;II)' Vl operating frequency, is only 113, which O·2 0 ;! / '-... � CL means that an inconveniently large propor­ O·1  I tion of the output signal voltage appears at 0 the inputs of the amplifier, and will lead to 01 02 04 0·6 0 2 4� OSCILLATION FREQUENCY non-linearities in the transfer characteris­ - 9Cf' tics of the amplifier due to 'common mode' defects. An oscillator design, which em­ ployed an input device operated in a cas­ Fig. 3. Rearrangement of Wien network code configuration with a junction f.e. t. to R between signal sources gives small in­ minimize this type of defect, was shown by phase signal at point X the author in 1977,6 and allowed a t.h.d. at -..... '---...... ov (al Ibl Fig. 4. Use of arrangement of Fig. 3 in TABLE 1. Phase and transmission charac­ oscillator circuit teristics of simple Wien network.

FIFa phase transmission 0.1 73.14° 0.10 R 0.2 57.99° 0.18 0.3 45.32° 0.23 0.4 34.99° 0.27 0.5 26.57" 0.30 Outr>ut 0.6 19.57° 0.31 I-Eyl 0.7 13.65° 0.32 0.8 8.53° 0.33 R 0.9 4.03° 0.33 1.0 0° 0.33 1.2 -6.97" 0.33 1.5 -15.52° 0.32 2 -26.57" 0.30 ----4�------�------_OV 3 -41.63° 0.25 Figures in brackets 5 -57.99° 0.18 refer to diagram 3 b 8 -69.15° 0.12 10. -73.14° 0.10 52 WIRELESS WORLD MAY 1981 and R, implies that if, instead of the net­ be significantly lower, because of the very operating frequency of the oscillator. Fast work of Fig. 3(a) being connected between small input-signal amplitude and the ab­ response-speed, high h.f. gain op.-amps. a signal source Ein and the OV line, it was sence of any internal transfer errors be­ can therefore be used without problems. connected between two signal sources + E x tween the inverting and non-inverting in­ and -Ey, where these are sinusoidal and puts, than is the case for an identical For these reasons, it can be expected identical in frequency and the negative amplifying element in a series-feedback that the residual harmonic distortion of sign implies phase opposition, as shown in configuration.3,4 this oscillator design will be exceedingly Fig. 3(b), then a small, in-phase signal • The time-delay errors in the second am­ small, and measurements on two proto­ would exist at the point 'X', at the fre­ plifying stage (A2) no longer contribute to types have indeed shown this to be the quency of maximum transmission, (fo), if loss of stablility in the system, but only to a case. So far as can be determined, the +Ex was slightly greater than -2Ey• very small compensatory shift in the residual distortion - almost exclusively This could then be used as a positive­ feedback signal in a circuit such as that shown in Fig. 4, to sustain oscillation at R the frequency fo. Indeed, such a circuit will work quite well, and will sustain a Thermistor Rs constant output magnitude of oscillation if a thermistor is employed, as shown, to make the gain of the second, inverting, Output amplifier stage dependent on the ampli­ tude of the input signal. However, there is, in practice, a small snag with such an R arrangement, and that is that the inverted negative-feedback signal applied to the in­ put of Al will suffer an additional phase ____�� ------��------��OV error due to the internal time lag within Figures in brackets Rp. Rs and Rin chosen to SUIt individual thermistor A2, and this will cause unwanted h.f. insta­ refer to diagnom 3b employed bility if '3rd generation' high speed op.­ amps. such as the CA 3140, or the 174 1 S, Fig. 5. Final form of new configuration in low-distortion oscillator are used in the realisation of this circuit. It is, fortunately, an easy matter to re­ solve this difficulty if the circuit is recast in 0·01 the form shown in Fig. 5, in which the ·009 , (Includes negative-feedback signal, equivalent to ,50 Hz "hum" I ·005 " -Ey in Fig. 3(b), is derived from the am­ plifier AI> and the positive-feedback signal "- , is obtained from the output of the second "- � ..... inverting amplifier A2• :z o ...... This configuration offers several signifi­ 0001 .... �o ... cant advantages. l­ ... �--- V)

• The input signal to Al is extremely a .0005 small, since it is only required to be EOUl/2M, where M is the open-loop gain of AI - typically 100dB for a good modern op.amp. i.c. - and, as pointed out by the 2 author in an earlier article , with semi­ 00001 100 lk conductor amplifiers the non-linearity of 1Ok FREQUENCY such devices is essentially an input charac­ (Hz) teristic, dependent on the magnitude of Fig. 6. Measured total harmonic distortion of improved oscillator of Fig. 5 the input signal. • The second-stage amplifier is operated as a shunt-feedback element, and the non­ Fig. 7. New oscillator with external optoelectronic amplitude-control circuit. Silonex linearities of such a stage can be shown to (formerly National Semiconductors) cell, Type NSL395, is obtainable from Cheston Electronics Ltd., Vanguard House, 56 Oughton Street, Ormskirk, Lancs. Tel: 069572456

1/ Photoconductive cell

10k NSL395

10}! 47k Si

3k3 56k Si

Si I1 3k3 3k3

3k3

\�------�vr------�/ \�------�vr------�/ \�------,v�----�/ Feedback control Oscillator Full wave rectifier circuit Output Note: PC and LED are in optical contact 1 1 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 of oscillation on the instantaneous value 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 -- avoid the introduction of a further signifi­ Xl l'j provided useful results, exemplified by the - 8rj . 'Iitji 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 circuit designed around the TDA32 10 ste­ the low-level loop gain exceeds the gain employed to control the amplitude of the reo preamplifier i.c. The filter on the out­ required to initiate oscillation, and T is the output signal (which imposes limitations of put is intended to minimize t.i.m. in the time constant of the control system (ther­ an identical kind on any oscillator system, next stage. This circuit, in terms of tradi­ mistor or similar). In the case of a Wien­ in terms of the settling time, and the in­ tional parameters, represents the current bridge oscillator, l'j== 3. fluence of the control time constant on the state-of-the-art in i.c. R.I.A.A. preamps in This equation indicates that if the feed­ harmonic distortion at any given fre­ which the total harmonic distortion is back amplitude is very little above that quency) the improvement in performance 0.02% at 20kHz. The frequency response required to sustain oscillation - which is given by the circuit design shown in Fig. 5 is 20Hz to 20kHz ±O.SdB and the dy­ implicit in the design - the residual over that obtainable from the more con­ namic range 100dB. distortion will be dependent on the time ventional arrangement shown in Fig. 1, 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 Sept. 73, pp. 439-446 - Oct. 73, pp 491- t.h.d. will be very low. The penalty in­ 3. Linsley Hood, J. L., Wireless World, Jan. - 494. curred in this type of adjustment, apart 1973,pp 11-12. www.keith-snook.info 4. Taylor, E. F., Wireless World, April 1973,p M. K. Vander Kooj - Predicting and avoiding from the obvious inconvenience of a relati­ 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. B. Sundqvist - Transient intermodulation in frequency of the oscillator must be 1977,pp 40-42 D 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 plifiers, Wireless World, Jan. 77,pp. 41-43. also, if switched capacitors are used to The impact of new 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. 42-59. posed by the use of a thermistor as the British Rail; and on the introduction of electronic news gathering (ENG) equipment in R. Cordell - Comments on "A method for stabilizing element, the performance of a 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 Convention, May 79. give a settling time of 5 seconds at 1 kHz, the introduction of new electronic and computer S. Takahashi, S. Tanaka - A new method of and an output voltage of 2 volts r.m.s., is' technologies at the level of the individual workplace". measuring TIM: a comparison with the conven­ lower than that obtainable from any other 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 sione d'Intermodulazione Dinamica, IAF.I in gain of the system, during the excursion structures on the capacity of managers to gener­ ate methods and mechanisms for the introduc­ suppl. a "Suono" Aprile 77. of each half sinusoid, due to the limited 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