[sdiy] Varactor-based VCF

[ChristianH]

I always thought varactors were only used in high frequency circuits
like tuners. I never looked at a data sheet, but I assume that the
capacitance is in the pF range, with pretty small variation.

Furthermore, the usual target signal is high frequency, which doesn't
interfere with the DC biasing. In our charging/discharching circuits
(and audio is pretty much DC compared to high frequency applications) I
would suspect that the target signal might confuse the biasing.

Chris (who admittedly knows varactors only by hearsay)

P.S.: the only time I used varactors was at school, when I got literally
a bucket full of reject TV tuner boards, and I made a keyboard coding
matrix for my first diy computer from all those varactor diodes, because
I didn't had to buy those "expensive" 4148s. Well, that was in 1977...



On Thu, 7 Dec 2006 02:19:19 -0500 (EST) Aaron Lanterman
<lanterma at ece.gatech.edu> wrote:

> 
> One of my students, Brian, had heard about varactors in his other classes. 
> All semester we've been talking about replacing resistors with OTAs or 
> Vactrols, and Brian thought it would be interesting to try replacing the 
> capacitors. I figured there was a reason varactors weren't popular in this 
> application, but I thought it would be an interesting experiment anyway 
> just to see what would happen.
> 
> Alas, we haven't seem to come up with anything that filters in any 
> remotely variable way.
> 
> Varactors work by backwards-biasing them; the capacitance allegedly 
> changes with the bias. To get interesing C sweeps, you need to swing the 
> bias between something like 5 to 14 or therabout volts. One issue is to 
> get audio range cutoffs, you need a pretty high resistor (Brian's been 
> trying 1 Mohm and 2 Mohm)
> 
> I suggested two ideas for him to try, and he's been banging his head on 
> both. I'm afraid I may have led him astray, and could use y'alls help.
> 
> Brian sent me screenshots of  MultiSim schematics he made of the two 
> ideas. The first is 
> here:
> 
> http://users.ece.gatech.edu:80/~lanterma/Small_Varactor_Circuit.bmp
> 
> My idea was to run +CV through one diode set, and then -CV through 
> another, to get a Q-point of zero volts inbetween them to feed our signal 
> to; and then put the input through the resistor in an R-C structure, 
> hoping that the C's at the CV points are then at "AC" ground. (To get 
> higher capacitance, he's using a couple varacs in parallel; he bought the 
> very last 5 varactors they had at Ack Electronics).
> 
> (The 1 kohm you see at the output is a load MultiSim wanted)
> 
> 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...
> 
> My second (more complex) idea is here:
> 
> http://users.ece.gatech.edu:80/~lanterma/Large_Varactor_Circuit.bmp
> 
> >From Moog ladder type of things I got the idea to add the CV to the input 
> on one "channel" and then subtract the CV from the input on another 
> "channel," and then run each of these through an R-C style structure with 
> the varacs as the C. Then, adding the two "channels" together should 
> cancel the CVs and give you the filtered signal.
> 
> 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.
> 
> Help!
> 
> Thoughts on these designs? Am I seriously misinterpreting how varactors 
> work?
> 
> Any ideas for other varac-based designs? Brian is frustrated and close to 
> ditching the varacs for something else together, but since he's already 
> put so much time into it, and he has the varacs, I'd like to do SOMETHING 
> VCF-ish useful with them.
> 
> Thanks as always!
> 
> - Aaron
> 
> -----------------------------------------------------------------------------
> 
> Dr. Aaron Lanterman, Asst. Prof.
> and Demetrius T. Paris Junior Prof.    Voice:  404-385-2548
> College of Electrical and Comp. Eng.   Fax:    404-894-8363
> Georgia Institute of Technology        E-mail: lanterma at ece.gatech.edu
> Mail Code 0250                         Web:    users.ece.gatech.edu/~lanterma
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