Oscillator keeps THD below 1 ppm

Posted on Feb 6, 2014

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. IC1 and associated components form the Wien-bridge and func

Oscillator keeps THD below 1 ppm
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tion as a bandpass filter. IC1`s output goes to the voltage-controlled amplifier (VCA), IC3. IC3 acts as a "smart resistor" whose value the circuit continuously adjusts via IC4. IC2 adds the outputs of IC1 and the VCA and feeds the result into the bridge. These two op-amp inverters eliminate any common-mode signal that might limit performance. 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...

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