Results: Temperature compensated VCO

[Kimmo Koli]

On Thu, 4 Dec 1997, Rene Schmitz wrote:

> The expected phase jitter from the noise in the diffamp is low. I think that
> is due to the 
> cap in the exp. converter (1nF) which I initially ommitted, it shorts the
> ac/noise (?).
> (This cap is in most converters out there anyway. You may have to tweak
> anyway in existing VCOs)
> I had some trouble with oscillations of the amp. thats why I placed a 1nF
> cap on the second opamp.
> 
Well, that 1nF capacitor is there to compensate the opamp feedback loop.
Thetransistor in the loop adds loop gain and it adds also an unwanted pole
which depends on the control voltage. Therefore shunting this transistor
stabilizes the loop.

These things do not show well in simulations unless you break the
feedback loop and simulate just it (AC-sweep, of cource).

Best regards,
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   Kimmo Koli                         Helsinki University of Technology 
   kimmo at ecdl.hut.fi                  Electronic Circuit Design Laboratory
   http://www.ecdl.hut.fi/~kimmo      P.O.Box 3000
   Tel:  +358 9 451 2273              FIN-02015 HUT
   Fax:  +358 9 451 2269              Finland 
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