Electronics > Beginners

Setting up Tune Voltage Circuit

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metrologist:
Hi All,

I am controlling the tune voltage of an oscillator with a 16-bit PWM DAC out from Arduino (from the Lars DIY GPSDO project).

My oscillator needs a smaller tuning range than 0-5V, probably 1-3V or less would be fine. The current tune voltage is about 2.1V. I figured I can wire the DAC out to a 10k trimpot (simulated R1-R4) and use the center tap to the tune pin to increase effective DAC resolution.

Is there an equally easy way to improve noise and temperature stability? I was wondering if a 2.5V voltage reference could be easily used. I probably won't consider anything too complex, including op amps, etc.

David Hess:
Improving the noise will mean adding an active low pass filter and removing any ground loops.  Improving the temperature stability will require a stable reference, stable gain, and stable offset.

Potentiometers do not make good precision voltage dividers; a pair of precision resistors should be used instead.

A great improvement could be made by doing the PWM switching closer to the oscillator with the 2.5 volt reference but it would require an analog switch or logic gate.

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