[sdiy] Varactor-based VCF

Thu Dec 7 15:44:36 CET 2006

Hi Aaron, Brian, list,

Simulation of the first circuit suggests to me that it ought to work: using
a BB545 model (one of only two varactors my sim package has), I do get a
6dB/oct response, with cut-offs in the region of 5kHz to 10kHz, which seems
to be a pretty ballpark ratio for a varactor. I would suggest however
increasing those 1k resistors at least 10 fold: with such low values you are
needlessly running the risk of limiting the range of the op amp output
swing.

Simulation of the second, more complex circuit, was not so successful:
altering the CV impacts only on the overall gain, with little perceptible
effect on the cut-off frequency, which is really high, 50kHz+. Thinking that
the freq responses of the op amps are coming into play, I replaced all of
them with high gain (200,000x) VCVSs (i.e. ideal op amps), and sure enough
it does then exhibit changing cut-offs, from around 135kHz to about 500kHz,
but still the change in passband gain is there too. (I was stepping the CV
from 5 to 15V.) It thus seems that this configuration has some 'non obvious'
(at least to me) facet to it which is made worse by the frequency
limitations of the op amps (maybe the imposition of the signal on the
bias?). (I also didn't check that the simulation was finding a good DC op
point before running the AC analysis...)

My conclusion is that with enough ingenuity you could probably use them in a
VCF somehow, but the limited range of variation in capacitance, hence
cut-off frequency, doesn't seem that useful.

Hope that helps, and if you'd like to see simulations traces etc., give me a
shout!

Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk

> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl 
> [mailto:owner-synth-diy at dropmix.xs4all.nl] On Behalf Of Aaron 
> Lanterman
> Sent: 07 December 2006 07:19
> To: Synth DIY; btippins at gatech.edu
> Subject: [sdiy] Varactor-based VCF
> 
> 
> 
> 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
> Atlanta, GA 30332                      Office: Centergy 5212
> 
> 
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> 
> 
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