SV: Re: [sdiy] Polyester or Polypropylene for VCO?

[karl dalen]

Why fuzz with two capps, more or less the same effect
are achived by applying a small DC voltage at the linear 
input in the expo.

Reg
KD

--- "JH." <jhaible at debitel.net> skrev:

> The original cap is replaced by a series connection of two capacitors.
> One being much bigger than the other, the total value is not much changed
> compared to the original circuit, so the VCO operates in a similar region as
> before.
> The tap between the caps is used to feed a dc current to offset the (linear)
> CCO core, bypassing the expo converter. So you can detune the VCO as
> if it were a linear (V/Hz) VCO. As the offset current is fed into the bigger
> cap only
> (i.e. the lower impedance part of a capacitive divider!), the offset is
> scaled down
> accordingly, allowing for more practical current values.
> Of course you cannot divide DC currents with a capacitive divider - thus the
> tap needs an extra discharge path to make the offset current dc-free
> over-all.
> 
> The page contains a link to my former web site on synthf**l.
> This isn't valid anymore. Looks like from ancient times, hard to imagine
> now.
> The link should be
> http://home.debitel.net/user/jhaible/hj.html
> Or, more easy to memorize: http://www.jhaible.de/
> 
> JH.
> 
> 
> 
> ----- Original Message ----- 
> From: <simon.oo.o at xs4all.nl>
> To: <synth-diy at dropmix.xs4all.nl>
> Sent: Monday, August 01, 2005 10:34 AM
> Subject: Re: [sdiy] Polyester or Polypropylene for VCO?
> 
> 
> > Hi Harry,
> >
> > harrybissell zei:
> > > Simon Brouwer wrote:
> > >
> > >> In this circuit you have a capacitor of 150nF in series with one of
> > >> 1000pF.
> > >> The bigger capacitor hardly contributes to the combined capacitance, so
> > >> the
> > >> same is true for its tempco. So in this case it probably doesn't matter
> > >> much.
> > >
> > > Nope. These capacitors are NOT in series. There are two capacitors here.
> > > One
> > > is reset (?) by Q10, the other couples a sync pulse into the VCO.
> >
> > OK, strictly they are not in series, but as far as I understand, for the
> > purpose of determining the stability of VCO frequency they are.
> >
> > Referring to the component numbering used in
> > http://home.swbell.net/roy_tate/info/lindetun.html ,
> >
> > Q7 resets C3 and Q7a resets C3a at the same time.
> >
> > In theory, linear detuning could be implemented by a very small offset
> > current directly into C3. However, you would then need an absurdly high
> > value of Roffs.
> > Instead, a larger current is dropped into the bigger capacitor, resulting
> > in the same effect, while a practical resistor value can be used.
> > Because of the detuning current, C3a receives additional charge and for
> > this reason discharging by Q7a is needed. Except for this, Q7a has no
> > practical effect, so that the capacitors can be considered to be in
> > series.
> >
> > In any case, that's how I understand this circuit.
> >
> >
> > Vriendelijke groet,
> >
> > Simon Brouwer
> > --> nl.openoffice.org <--
> >
> >
> 
>