So, after getting back to client work for a minute, I decided to try and nail the clock pulse shaping circuit problem with a more viable solution than throwing in an extra $5 op amp.
The problem with the original pulse shaper circuit was simply that it was designed with a function generator and not a 20 cent opamp in a RC oscillator. The idea was sound (I think) but the values were not.
The real problem is that the square-to-pulse converter has to shape two different clocks — it’s always driving the same IRQ pin, but it can be hooked up to WTPA2′s 4046 based VCO, or the LM358 based on a user switch. The 4046 is HC logic, and has really square edges. The LM358′s edges are not square, and their slew rate seems frequency dependent also. So, you could optimize components for one or the other, but not both. I did some bench tests to figure out what I needed to change to get this right.
Check it. Here’s the rising edge of the output from the VCO…
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