DIY Audio Function Generator Part 1

Posted on Feb 5, 2014

A simple low end Audio Function Generator. I want to be able to use the Sine Wave output to test ADC resolution and as a base band signal for RF projects and having very Low Total-Harmonic-Distortion (THD) would be nice. So I`ve decided to build my own Function Generator. Most DIY Function Generators I found online start out with a Square Wave Oscillator

DIY Audio Function Generator Part 1
Click here to download the full size of the above Circuit.

feed into an integrator to get a triangle wave, then you feed the triangle wave into a wave shaper and you get a rough Sine Wave output. This is similar to a project I built up last year. This method works, but the wave shaped Sine wave isn`t going to have the low THD I was looking for, so I am taking the opposite approach and starting with a Sine Wave Generator based on the #327 Lamp Wien-bridge Circuit to get the low THD. I`ll then feed that into a Comparator to produce the Square wave; I am then using the Square Wave to turn on/off a ramp generator circuit. I would rather have a ramp output vs a triangle wave. The ramp circuit will let me play around with different single & dual slope ADC concepts later on. I`ve added a +3. 3V Square wave output to be feed into a frequency counter, so I don`t need to bother to add a display to the function generator. Mine as well make use of the Frequency counter I have for a display, also having a +3. 3V square wave output would be nice for any micro projects later on. I`ll have a Freq. Adjust Pot, Freq. Range switch, Ramp Current Source Adjustment Pot, and an Output Amplitude Adjustment potentiometer. Below is a rough layout of the proposed design: After sketching out the concept drawings my next step was to test the Wien-Bridge Oscillator circuit I was basing this whole design around. I prototyped up the circuit with the Frequency Range Select switch and 10K pot and was pleasantly...

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