[sdiy] DCO ramp core / capacitor discharge transistors

[Luís Marka]

Hi Tom,
The NPN transistor you mention in the JX8P is just a waveshaper (part of 
the circuit that converts square to saw), the "core" is the CPU + 
associated programmable dividers (IC39..42).

Luis


Tom Wiltshire wrote:
> 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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