-- You\'re joking, right? If you start screwing around with the time constant all you\'ll wind up doing is changing the output amplitude and waveshape, which will _never_ resemble a sinewave, no matter what you do.
-- The OP\'s query asked for a circuit with a stable sine wave output with a frequency equal to that of a squarewave input signal. Since he specified a "square wave", I think we can safely assume a 50% duty cycle and, therefore, disregard the even harmonics.
If you can get a good triangle, you can get a very good approximation... easily as good as a standard "function generator", which are typically 1% THD. Although the
8038 uses diode breakpoints for shaping, this is not a very good method for individual use... too many adjustments, even assuming you can figure out how to proceed with the adjustment.My method of choice is an overdriven differential pair. You will end up with 2 adjustments: The amount of overdrive (input level), and the amount of feedback. If you feed the output to an amp and headphones, you can adjust this by ear. (Though a scope helps to get you in the ballpark.) You will be balancing between 2nd and 3rd harmonic levels. 1% THD is easy, and I have gotten as low as 0.25% on occasion, though I don't know how stable that would be. The by-ear method is better than a scope alone for distortion adjustment. An FFT spectrum analyzer is better still.
I have seen a paper where the authors used a matched pair in a temperature-controlled oven and got something like 0.001%. However, if you are shooting for these sorts of levels, then the input triangle itself becomes more of an issue. It's pretty hard not to have some sort of glitch at the peaks.
Best regards,
Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis
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