Jelly Bean Function Generator
This function generator was originally designed to mount on top of the PPMScope Oscilloscope as a second board. I later trimmed the sides of the PCB to reduce it's size. It generates sine waves, square waves, and triangle waves. You can adjust the frequency, amplitude, and offset. It only utilizes a comparator, a few op amps, transistors, and passive components. I call it the "Jelly Bean Function Generator" because it is doesn't utilize a function generator IC. The design was inspired by a college project, but it operates fairly well for a simple low cost function generator.
Files For Download
The central component of the schematic is the oscillator. The oscillator uses a comparator and an op amp configured as an integrator to generate a square wave (comparator output) and a triangle wave (op amp output). The advantage to this configuration is that it is easily to tune the frequency using only a single RC time constant. R30 and R32 and C1 thru C4 are adjusted to set the frequency.
A buffer takes the triangle wave output and sends it through a passive wave shaping circuit. The wave shaping can be adjusted using configuration pots R41 (to zero the offset) and R25 (to adjust the amplitude of the triangle wave) to reduce the distortion of the output sine wave. After the sine wave is adjusted to the lowest possible total harmonic distortion, R38 and R44 are adjusted to calibrate the amplitude of the triangle and square wave to match the amplitude of the sine wave.
The last section of the circuit is the output stage which includes the function selection switch, amplitude adjustment, offset adjustment, and the output amplifier.
The schematic files do not include the power supply (since the function generator was powered by the same power supply as the oscilloscope). The schematic was drawn in TinyCAD and PDF versions are linked below:
The PCB Layout was drawn in FreePCB and PDF versions are linked below:
I'd be glad to help with the construction of the Function Generator hardware or the understanding of the schematic. If you just need someone to e-mail every now and then, I don't mind making friends. I am not nearly as enthusiastic about doing your homework or helping on some unrelated project unless I find it interesting. That said, you can ask, but I don't give any promises about my expertise in anything except matters directly relating to the software and hardware schematics delivered here.