[sdiy] Polyester or Polypropylene for VCO?

[JH.]

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 <--
>
>