EDN -- 11.10.94 Oscillator keeps THD below 1 ppm
Jeff Smith, Analog Devices, Wilmington, MA -- EDN, November 10, 1994
Design Ideas: November 10, 1994
The Wien-bridge sine-wave oscillator uses a light bulb to stabilize its amplitude. The circuit in Fig 1 doesn't have a light bulb; it sports several enhancements that lower its distortion and generate a test signal pure enough for testing modern op amps and high-resolution A/D converters.
Click to enlarge
The circuit's AGC loop begins with diodes D1 and D2. These diodes half-wave rectify the outputs of IC1 and IC2. These outputs are 180° out of phase; so IC4 sees a full-wave rectified signal through R1 that is proportional to the output signal's amplitude.
Integrator IC4 compares the average value of the rectified current to a constant current through R2. Any imbalance in these currents causes IC4 to output a correction signal, changing the gain of VCA IC3. The VCA's gain adjusts the oscillation's amplitude until IC4's input currents are equal. R3 and C3 further filter the correction signal to remove harmonic components that would manifest themselves as distortion at the circuit's output. D3 minimizes damage to C4 and C7 in the event of reverse polarization.
The ac performance of C1 and C2 is critical to this design. I recommend polystyrene or polypropylene film types; and make sure you connect the outside plate as Fig 1 indicates. Mylar capacitors can degrade the circuit's performance by 6 dB. C5 and C6 are peculiar to IC1 and IC2. They eliminate distortion arising from VBE nonlinearities in the op amps' output stages.
The large ratio of output signal to distortion and noise floor makes verifying the performance of this circuit with standard test equipment difficult. Therefore, I used the tunable, buffered-output, twin-T filter in Fig 2 to reduce the fundamental (1 kHz) in the output by 70 dB. Spectral analysis of the filter's output permits calculation of THD.
When properly tuned, the filter reduces the second and third harmonics by about 10 and 5 dB, respectively. Harmonic-distortion calculations must take this reduction into account. Harmonic-distortion calculations must factor in the gain of IC1 (Fig 2). Be sure to use the same high-performance capacitors used for C1 and C2 in Fig 1 for C1, C2, and C3 in Fig 2.
To tune the filter:
Adjust R9 so that R7 shorts to R8.
Monitor TP3 on an oscilloscope and adjust R27 for a visually undistorted sine wave.
Using the filter, adjust R10 for a minimum second-harmonic distortion at TP3.
Note: You may need to make small adjustments to R9 for successful power up. (DI #1617)
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