I can remember a colleague working for weeks on a sinewave generator board for a piece of test equipment.
He came up with a Wein bridge oscillator with several refinements that had wonderfully low distortion.
To be honest, I can’t actually remember which circuit went into the equipment, but his Wein oscillator was a lovely thing.
I have just discovered that Apex Microtechnology of Arizona actually makes a chip, the SWR200 Precision Sine Wave Reference, to do much the same thing, a nice low-distortion sine wave with a well-controlled amplitude – Wein oscillators are a bit touchy about holding both amplitude and low distortion.
From the data sheet:
A1 and A2 are connected as a phase-shift oscillator circuit with the frequency set by the external capacitors C1 and C2. Q4 is included in the feedback loop of A1 as a gain control element.
The oscillator output is fed to the chopper amplifier which develops an absolute value representation of the oscillator output. The chopper output is compared to a precision DC reference in integrator amplifier A3.
This DC error signal is used ot control the gain setting FET Q4.
Accuracy comes from a chopper-based AGC, the temperature characteristic of which compensates the typical non-linearity of the internal DC zener reference, resulting in a nearly linear amplitude-temperature characteristic, says Apex.
I have no idea if it would have fitted my colleague’s bill, but it is neat.
Sine wave output of +7.071V at ±0.5% initial accuracy
Temperature coefficient as low as 20ppm/°C (-55 to +125°C)
Stability: 10ppm/1,000hr typical
0.1% THD @ 3.3kHz
14pin bottom braze hermetic package
Suggested use is as a reference source for LVDT or RVDT-based position sensors and “a programmable AC reference can be constructed using the SWR200 as a reference for a high accuracy multiplying DAC”, said Apex.