[sdiy] Problem with DCO

[Neil Johnson]

Hi Tom,

> I've been experimenting with a DCO design based on the Roland JX8P  
> circuit:
>
> http://www.electricdruid.com/DCOTestDesign.gif
>
> Broadly speaking, it works, in that I can get a ramp wave out of it  
> if I adjust the 47K pot on the left. This pot and op-amp buffer  
> will be replaced by a DAC when I get it working better. I've got  
> reset pulses generated by a PIC being fed into the base of the  
> transistor at 440Hz for the purposes of the test. That part is fine.
>
> The problem is that the output goes from 0V down to -0.5V and then  
> seems to saturate. What's going on? How can the op-amp output be  
> saturated at only -0.5V when it's running on +/-15V rails?  
> Everything I've read suggests that the output from an integrator  
> ought to carry on as far as the op-amp output can take it, so I was  
> expecting a ramp from 0V down to -15V or thereabouts. I'm sure  
> there's something I'm missing here...

As has been said, your ramp is going the wrong way for the reset  
circuit.  If you invert your input control voltage (change the first  
op-amp to be an inverting buffer) then your ramp will increase  
positively and your NPN reset will be fine.

Neil
--
http://www.njohnson.co.uk