[sdiy] Wanna help me make up a VCF homework? ;)

[Aaron Lanterman]

On some of my problem sets, I have the students pick a schematic based on 
their birthday, or their student number, or whatever, and then analyze it.

I'm making up a problem now where I'm going to have them look at an 
OTA-based 1-pole filter cell (in the usual four pole setup) and tell me 
what the cutoff is as a function of the current to the pin on the VCA, 
along with analyze the divide-down ladder, etc.

I've gotten three following so far:

1) SSM2040 datasheet design
2) Ray Wilson's 4-pole - Ray's already showed up in HW #2 ;)
3) Polyfusion Lowpass VCF

Any others come to mind?

In particular, all the designs I've found drop a cap straight to ground 
and then put in a buffer. I've yet to find one that actually follows the 
one of the ARP patents, where coming out of the OTA the current goes into 
an integrator formed with a cap in the feedback loop - I showed that in 
class so it would be nice to find an actual example.

I don't want to do the CEM3320 one, since it's not drawn in a 
student-friendly clear way.

Oh, as a side note, I came across a most interesting VCF design for the 
EML synth here:

http://members.tripod.com/urekarm/synth/eml101_exp_osc_vcf.pdf

There's a differential pair, but instead of having a current mirror on top 
to sum up the current the OTA way, it's a standard voltage setup 
(resistors to the top rail), and then that goes into a differential 
integrator with two caps (I remember seeing one of these on Ian's site).
Don't think I've seen that design anywhere else.

Replace the caps with resistors, and you get a VCA - I think of the sort I 
recall on Rene's site...

- Aaron

P.S. I've only made vague references to the class about this list, since I 
don't want to give away my secret weapon yet. When it comes time for them 
to start their own modules designs, I'm going to require them to join the 
list. ;)

-----------------------------------------------------------------------------

Dr. Aaron Lanterman, Asst. Prof.       Voice:  404-385-2548
School 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: GCATT 334B