[sdiy] Polyester or Polypropylene for VCO?

[simon.oo.o at xs4all.nl]

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
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