[sdiy] Resistor matching in Bergfotron complex VCO

[ASSI]

On Sonntag 27 April 2008, Aaron Lanterman wrote:
> The Bergfotron webpage emphasizes matching the resistors at the top
> of the OTA - it says "Buy a belt of 100 resistors and find two that
> give the exact same reading on a 4 1/2 digit ohmmeter. Any error here
> will give a non-symmetrical triangle wave." The resistors Greg is
> using currently measure at 4.963K and 4.972K, which are a far cry
> from measuring exactly the same to 4.5 digits. But, before I send
> Greg off to spend an afternoon at a DMM measuring a truckload of
> resistors, are there other sources of nontriangularness we should be
> looking at first?

Simply switch the two resistors and see if you now get an 80% triangle.  
Or combine a 4.7k resistor with a 330 trimmer and try to get symmetry 
by adjusting that.  If not, maybe the matching of the transistors isn't 
all that great.

> Is that circuit really that sensitive to the resistor matching? That
> seems OK for a one-off like the Bergfotron, i.e. a design a person
> makes to build just one or two of - but it seems to me that would
> have severely limited the manufacturability of the original Buchla
> 259 (although perhaps that shouldn't surprise me.)

E96 values are easily obtained in 0.1% tolerance.  While still a factor 
of 5 short of the required matching, these should match fairly well in 
just a few tries on a bridge setup.  Matching pairs of 1% resistors 
takes a while longer I guess, but not overly so.


Achim.
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