Problems with the MXR Envelope Filter

[axel]

Hi everyone !
This is my first post on these boards, that I've been watching closely since I began my school project. I'm trying to build a multi-effect pedal that will comprise several analog circuits controlled by a microcontroller (using digital pots). The main features should be MIDI compatibility and storage of presets in memory.

Anyway, I'm currently building an envelope filter using the layout for the MXR EF. I understand this circuit uses a PWM-controlled filter. Apparently there's something wrong with my build, as the pulse duty cycle doesn't seem to vary very much with the envelope of the input signal. I've only been testing with sine generators for the moment, but I'm measuring small variations of only 10% of the duty cycle, when going from a 0 to 1V amplitude on the input. Is that normal ?

Thanks in advance for your answers, I'm sure I will have many more questions as I progress in my work...

axel

[Paul Perry (Frostwave)]

I don't know if that is normal or not, but the sweep of the filter can't (as far as I can see) be greater than the ratio of duty cycle fraction at one end of range to duty cycle at other end of range, which for your figures implies maybe 20%. I would have thought that it would go over a wider range than that.