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Emulating a Lunetta circuit with an FPGA is a flawed idea

Written by Michael Earls
 synthesizers  Lunetta  electronics

Last night, I decided to explore an idea that I have been formulating over the past several months, but haven't actually taken the time to explore. I had a thought that I could emulate a Lunetta circuit (a CMOS 40106 integrated circuit with a potentiometer and a capacitor to create an oscillator) using my FPGA and Verilog.

After all, the 40106 is simply a hex inverter (6 inverter circuits on one chip), so the code was this simple:

assign pin1 = ~pin0;

That simply set the value of pin 1 to be the opposite of pin 2 (inverted signal) at every clock cycle. I plugged in the capacitor and potentiometer according to the regular "Lunetta" arrangement (as seen in the schematic below), but it didn't work.

Lunetta circuit

Replace the U1A IC in the schematic with the FPGA and you'll understand how I set it up.

I think I have a fundamental misunderstanding of the nature of CMOS and FPGA's. While you can emulate the functionality of an inverter chip using an FPGA, I don't think it's possible to emulate the physics of the chip itself. The Lunettas work off of the actual physical reaction and timings of the chip itself, not off of a clock cycle such as the one that the FPGA is running under. Emulating a physical feedback circuit will not render the same results when using an FPGA. I looked at the datasheet for the 40106 and saw that it has ~140ns delay. I'm not sure if I can emulate that kind of timing.

I enjoyed the few hours I spent hooking the circuit up and playing around, though, and it got me into Pulse Width Modulation and other audio generation techniques for FPGAs. I'll be exploring more about audio processing in the future, but I've put away the idea of emulating discreet components for their "out of band" uses.

It's fun to play around with this stuff, but when the theory gets too thick, I lose interest and move onto more playful aspects of the technology. I think it's why I have a cursory understanding of a lot of technology, but only a deep understanding of a limited number of topics (like C#, HTML, and other Web Development).