[sdiy] DCO ramp core / capacitor discharge transistors

[nicolas]

One other possible consideration is the switching voltage.  If the cap is charging up to say 10V before discharge you might see different behaviour compared to a reset at say 1V.  The lower voltage also leads to a smaller cap being required (for the same frequency and charge current), which overall means a lot less energy through the transistor - hence probably less critical what you use.

Cheers,
Nicolas


--- On Wed, 17/12/08, Scott Nordlund <gsn10 at hotmail.com> wrote:

> From: Scott Nordlund <gsn10 at hotmail.com>
> Subject: RE: [sdiy] DCO ramp core / capacitor discharge transistors
> To: synth-diy at dropmix.xs4all.nl
> Received: Wednesday, 17 December, 2008, 11:54 AM
> A VCO depends on a rapid discharge time so that it responds
> properly to
> the control voltage.  If it took 0.1 ms to discharge the
> integrator's
> capacitor, the high frequencies would be off as the
> discharge time
> would significantly contribute to the total period of the
> output
> waveform.
> 
> A DCO (specifically a voltage-controlled ramp with a
> digital clock)
> only needs a quick discharge to make the proper waveform-
> the frequency
> is not affected, so the component isn't as critical.
> 
> Of course I don't know if this was a design issue in
> either case, but if it matters, I'd guess this would be
> why.
> 
> > To: synth-diy at dropmix.xs4all.nl
> > From: tom at electricdruid.net
> > Date: Tue, 16 Dec 2008 22:00:47 +0000
> > Subject: [sdiy] DCO ramp core / capacitor discharge
> transistors
> > 
> > Hi All,
> > 
> > I'm having one of my periodic "Should I build
> a DCO?" moments...
> > 
> > Consequently, I've been looking at the
> Electronotes/ASM ramp core,  
> > and various schematics for DCOs, including the JX8P.
> > 
> > I'd have thought that since the ramp core of both
> of these circuits  
> > is identical, you'd see similar components, but
> you don't. The ASM  
> > VCO uses a FET chosen for low on resistance (2N4856
> seems favourite)  
> > whereas the JX8P uses a normal NPN, the 2SC945. The
> JX8P also uses  
> > half the capacitor value (1n versus 2n2) so probably
> doesn't need  
> > such a low on resistance.
> > 
> > Can anyone tell me why we need to use a FET for the
> VCO, if the DCO  
> > can get by with a NPN?
> > 
> > Thanks,
> > Tom
> > 
> > 
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