[Dorkbot Boston] voltage controlled filter

[Steve Grandpre]

I stumbled across a technique for turning an analog oscilloscope into a spectrum analyzer today.  Your oscilloscope needs to have an input that controls the horizontal location of the trace.  You feed a sawtooth wave into this input, and feed the same sawtooth wave into a voltage-controlled bandpass filter.  Feed your signal into the bandpass filter and plug the output into the scope.  Now the sweep of the trace corresponds to the frequency being passed by the filter, and you have a spectrum analyzer.

I'd love to try this out, but I have no idea where to get a voltage-controlled bandpass filter.  Any thoughts?

-Steve

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[John Luciani]

On Wed, May 5, 2010 at 5:41 PM, Steve Grandpre <steveg...@gmail.com> wrote:

> I'd love to try this out, but I have no idea where to get a
> voltage-controlled bandpass filter. Any thoughts?

An analog synth.

If you want to build one you could probably get a schematic
to a Moog or Arp filter and make the appropriate mods.
A good resource for schematics is
http://www.till.com/articles/index.html

A couple of other ideas --

* ADI and LinearTech have some filter ICs. Not sure if these can be
configured as bandpass
with a voltage control.
* There was a company called CEM that made chips for synths - VCOs, VCFs, VCAs.
They are no longer around but you may be able to get some surplus ICs.

(* jcl *)


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[Tim]

I don't know any directly voltage-tunable off the top of my head, but
you could tie a clock-tunable filter to a voltage-to-frequency
converter. Have a look in Digikey's active filter section
(http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=2556488&k=filter)...these
clock-tunable filters also go under the names of switched-capacitor
filter or "filter building blocks" or simply programmable filter. You
may get lucky and find one that responds directly to voltage, but the
ones I am (distantly) familiar with are set by feeding a variable
frequency clock to set the center frequency.

Tim

On Wed, May 5, 2010 at 5:41 PM, Steve Grandpre <steveg...@gmail.com> wrote:

--
"If you really want something in this life, you have to work for it.
Now, quiet! They're about to announce the lottery numbers..." - Homer
Simpson

[f-go...@media.mit.edu]

> Date: Wed, 5 May 2010 17:41:16 -0400
> From: Steve Grandpre <steveg...@gmail.com>

> I'd love to try this out, but I have no idea where to get a
> voltage-controlled bandpass filter. Any thoughts?

Use a VCO, a mixer, and a fixed-frequency bandpass filter?

[Noah Vawter]

A friend of mine uses your very technique - he's a synthesizer
designer :) one of the possibilities is the SVF (State-Variable
Filter). It's not the simplest thing in the world, but only uses a
few op-amps. try looking for "SVF kit" or "SVF schematic," or you can
write own in a few lines of code on an avr chip, assuming you have the
surrounding adc/dac code working:

// State Variable Filter - s08 filter(s08 in)
//short fc=6; // 128/256 = fc=.5
//short rezdamp=8; // 0 - 63
static int l=0,b=0,h=0;
{
h = input - l - ((b*rezdamp+31)>>6);
b += (fc*h+31)>>6;
l += (fc*b+31)>>6;

filtOut = (b>>1);

[Stephen DeJesus]

The book Active Filter Cookbook by Don Lancaster is pretty much a classic on filter design.  You could definitely get a design for a bandpass VCF out of there.  You will need to pay some attention to the range of frequencies that you are interested in.  Obviously audio synthesizer related designs will be aimed at audio frequencies (<20KHz), but other applications may be in entirely different frequency ranges.

-Steve DeJesus

[Ted Moallem]

This website appears quite useful on first glance:
http://www.musicfromouterspace.com/analogsynth.php?page=ANALOG

Found it through the wikiP entry for "Voltage-controlled filter",
which links directly to
http://www.musicfromouterspace.com/analogsynth/AAA_VCFilterIndex.php?page=ANALOG

But also note http://www.musicfromouterspace.com/analogsynth/synthdiy_links.html

Ted

--
________________________________

Theodore Moallem
moa...@mit.edu
646-872-0283

Sensory Communication Group
Research Laboratory of Electronics at MIT
Graduate Program in Speech & Hearing Bioscience and Technology
Harvard-MIT Division of Health Sciences and Technology


--

[Liam Daly]

beware: the bandpass filter on that multimode unit on there has a pretty wide bandwidth and the voltage control doesn't track very linearly. probably not the best choice in this application. it is very musical sounding though!