[sdiy] Polyester or Polypropylene for VCO?

[JH.]

> 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.

Because it's not the same.
If that offset current is multiplied with your expo CV, it's not a constant
value anymore.

JH.



>
> 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 <--
> > >
> > >
> >
> >
>
>