[sdiy] DCO ramp core / capacitor discharge transistors

[Scott Nordlund]

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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