[sdiy] Varactor-based VCF

[René Schmitz]

Hi Aaron and all,

Aaron Lanterman wrote:

> http://users.ece.gatech.edu:80/~lanterma/Small_Varactor_Circuit.bmp

> What's very odd is that if you measure point C to ground, and point A to 
> ground, and point B to ground, and then measure C to A, the various 
> voltages make sense. But... if you measure C to B and then add the 
> voltage from B to A... that's a few volts lower, it seems, than 
> measuring straight from C to A. I'm at a loss to explain why... I 
> wondered if there was too much current and we were losing voltage in the 
> wires, but measuring voltage drops at various connection points along 
> the wires didn't yield anything interesting. Confusing...

On a very high impedance node like this, don't forget your DVMs internal 
impedance... Try measuring to the output instead of B.

> My second (more complex) idea is here:
> 
> http://users.ece.gatech.edu:80/~lanterma/Large_Varactor_Circuit.bmp
> Neither filter seems to do anything to the signal that could be 
> considered voltage controlled. I'm not sure if there's a small tweak 
> missing, or if there's a more fundamental flaw in my thinking. I'm 
> thinking my thinking is fundamentally flawed.

It seems this circuit does forward bias the diodes.

> Thoughts on these designs? Am I seriously misinterpreting how varactors 
> work?

Just one thing, varactors come in various varieties, I suppose the ones 
you got there have a pretty small capacitance and maybe also just a 
small Cmax/Cmin ratio like 3:1. (They are used in FM/VHF tuners)
There are some varactors for AM like the now obsolete BB212, which have 
a 10:1 ratio and capacitance of 500p at -0.5V bias. With higher ratio 
you would get more sweep, and also your varactor capacitance moves away 
from any stray capacitance. Speaking of sweep range, the capacitance 
moves roughly like 1/x^2 I think, so the useful range would be fairly 
small and nonlinear.

Cheers,
  René


-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159