[sdiy] Debugging Hammond filter circuit

[Donald Tillman]

On Apr 23, 2010, at 10:55 PM, Aaron Lanterman wrote:

> One of my students (cced here) is working on the circuit from this patent:
> 
> http://users.ece.gatech.edu/~lanterma/patents/hammond_fet_vcf.pdf



Okay class, today's lesson is: Patents don't actually have to work.

I'm gonna take a wild guess and suggest that the circuit you built is oscillating at a high frequency.  It does not look too stable.

Wow, this is a truly awful patent.   There are mistakes in the schematic, the resistor reference designators start from R142 for no reason, the plot has no tick marks or units, there are no voltages or levels specified...  

And it's not a very good circuit topology.  It's depending on the high pass filter feedback at high frequencies where random phase shifts can cause instability.  And the positive feedback thing looks like complete trouble.

  -- Don

--
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com