The design started on the breadboard as usual. I ruined one chip by putting a 5V regulator backwards.
With the testing on the breadboard I gained a better understanding of how this chip works, the most interesting thing was the PDM output rather than PWM like the VCLFO10 in the previous design. I hadn't come across using PDM before.
In this scope image it's possible to see the PDM and the resulting wave (after amplification to 10V).
The output stage just has a filter (passive filter from the data sheet) to tame that PDM signal. Much easier to implement that the PWM filter for the VCLFO10. I like my CVs 0-10V so I've amplified the output of the chip and that meant another op-amp was free to have an inverted signal too, so that's the outputs.
As for inputs, there's a pot to select one of the 8 available waveforms, and two more pots to change the offset and depth of the output. There's a sync input and tap tempo button to manually set the frequency.
Here's a simulation.
Unlike that simulation the actual circuit's LED is driven by the PDM signal. It behaved better than when driven by the filtered output.
Issues on v1:
This was a very straight forward design. In general everything I've checked on the v1 boards works, there are just a few component that weren't quite the correct values.
- Filter from the data sheet that I used with the output buffer/amp's R4 created a voltage divider that makes the output 0-5V instead of 0-10V.
- Using (R8+R5=430k and R7=30k) cranks it up to 0-10V as desired.
- R10 could be two 1k resistors instead of a 2k resistor to reduce component variety.
- There are two 5V regulator footprints, of course only one is needed at a time, I wanted to try some SMD options.
- There are alternate capacitor footprints for through hole and surface mount. I wasn't sure I'd have the right values for the filter so I wanted options.
- I've used a BC547 instead of the data sheet's 2N3904 because that's what I have on hand, many others would likely work fine.
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.