thru zero VCOs (was: Re: [sdiy] Temperature Compensated VCO attempt - help?

jhaible at debitel.net jhaible at debitel.net
Mon Feb 3 13:49:56 CET 2003


> Actually, no... I never separate the sign and the absolute value like you
> do.
> I let them all the way into the 4-quadrant multiplier that the Gilbert-cell
> supplies. This comes from the observation that a Gilbert-cell actually has
> three input signals, the standing current, and the two diffrential inputs.
> The standing current of the Gilbert-cell comes from a normal expo-setup
> where
> as the two different input signals each apply it's polarity switching magic
> on
> the current. The beauty of this design decission is that signals requiring
> less
> precission enters the Gilbert-cell on the base inputs (which has the
> annoying
> distorsion) and the really sensitive signal, the expo-current, enters the
> common emitter current path for which the other transistors just sets
> current
> ratios for.


Ah, clever!
I had no idea you were referring to a 4 quadrant multiplier. (I should read
whole threads, not just single mails (;->) - do you have schematics posted?)

> What I didn't do however was to describe how a phase quadrature oscillator
> is designed. 

Well _if_ there are problems with this concept, I expect them to come
from offset voltage drift. In a quadrature VCO, you'd need two such
4 Quadrant multipliers, and what if - due to offset drift - one of them
sees a tiny positive offset, and the the one a negative offset?

And even in the simple (not quadrature) case, what would happen when 
your sum of external voltage and offset voltage changes sign just 
when you're close enough to a comparator threshold? 

JH.

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