Wien-bridge oscillator with low harmonic distortion

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) 1kHz of some 0.003%, which tended to The Wien-bridge network can be increase with frequency above this point, connected in a different way in an oscillator as the effectiveness of the common-mode circuit to give a sine wave with very low total harmonic isolation deteriorated. distortion. An I.e.d/photocell However, it is not implicit, in the use of amplitude control is external to the a Wien network as the frequency-control 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 amplifier and the amplitude controlling The Wien-bridge network remains the R negative-feedback signal is taken to the most popular method of construction of other, is the only circuit configuration variable-frequency sine-wave oscillators, which can be employed. In particular, con­ since the basic circuit can be very simple in ov sideration of the phase and transmission form. It is a fairly straightforward matter characteristics of such a network, shown in to design oscillators of this type in which 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· form, using a single operational amplifier z as the gain block, is shown in Fig. 1, and o w_ >Vl the author has shown a practical design of O· z �i: 121 -IVl oscillator, based on this, for a use as a §l O· 6 wz 0:« simple, general-purpose workshop tool. Vl 1 i: 0: Vl O· However, in the form shown in Fig. 1, a �O 4 significant problem exists in that the trans­ 0: mission of a normal Wien network, at the f- O. 3 w Vl operating frequency, is only 113, which O· 2 0 ;! CL means that an inconveniently large propor­ O· 1 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 non-linearities in the transfer characteris­ OSCILLATION FREQUENCY -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 (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 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 3 -41.63° 0.25 5 -57.99° 0.18 Figures in brackets 8 -69.15° 0.12 refer to diagram 3 b 10. -73.14° 0.10 Document Outline WW 1981 May p51 WW 1981 May p52 WW 1981 May p53