Sunday, September 1, 2013

Unstable opamps, VCO and low pass filter

As I mentioned earlier there was some oscillation problems with the VCO sawtooth output. The problem was easily solved by biasing the opamp high. This makes the opamp faster and gives it a higher phase margin, thus making it more stable when driving capacitive loads. Since this solved the problem, I removed the opamp buffer from the output. The low pass filter had the same kind of issue, and it was solved in the same way.
 The filter also suffered from some clipping, which I solved by replacing R1 with a potentiometer and trying out the best value, which was 3.45k\(\Omega\). When P1 was set to the lowest resistance, there where some oscillations in the output. This was solved by putting 1k\(\Omega\) in series with each potentiometer terminal. I'll see if I can get my function generator to sweep out a bode plot of the filter, to see some details on it's characteristics.
Updated active filter schematics

 To get some higher tones out of the VCO, replaced C1 with a smaller capacitor. I needed a few tries to find a good match. A small capacitor gave a higher frequency, but the wave forms got scewed, probably due to unmatched resistors in the input stage. A smaller capacitor also raised the lower end frequency too high. This shows that I probably need to find a better VCO design, to get a higher bandwidth.

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