[sdiy] VCO voltage reference

[René Schmitz]

Antti Huovilainen wrote:

> So, given that 1) TL431 can provide any voltage with three resistors and 
> 2) it costs peanuts, why don't all VCO schematics/boards use them? (or 
> similar references)

It wasn't always like that. And some designs can be traced back to the 
60s. So call that "tradition". (Besides in early days a voltage 
reference was a U shaped piece of glass filled with some nasty 
chemicals. (weston cell) Too fragile for PCB mounting, and terribly 
sensitive to mounting position :-))

I for one also (mostly) didn't, because I use a PSU which uses just one 
reference that is used for deriving all supply voltages. There is one 
regulation amp per VCO, all using the same reference, so if they drift, 
they all drift alike... (And if some dirt was modulating the VCO it 
would have to come from within the VCO.)

Also as Magnus was pointing out, a VCO will be fairly tolerant to slow 
changes in the power supply voltage.

You can get away without fancy references if you like. Some influences 
actually cancel. For example the threshold of the comparator and the 
expo current. (Only the voltages from the pots are actually 
contributing, due to the nonlinear behaviour of the expo circuit.)

Btw, I also use dustbins for each VCO. Its all centralized, and easy to 
backpatch to existing designs without references. The VCOs are perfectly 
isolated from each other and other circuitry.

There are different philosophies about this. If I say yes, then just 
because you can do that, not because I want to imply that it was the 
only thing thats right. There are always a lot of different ways doing 
the same thing. Nowadays I would include references, and there is one 
design which does, but is not going to be published.

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159