[sdiy] decoupling on modulars power bus

[Harry Bissell Jr]

Sci-fi author Colin Capp wrote in his short story
"The Subways of Tazoo"  (excellent engineering read
btw)

* A snowflake would not stand a chance in Hell unless
you had a ton of refrigeration equipment alongside it.
Now what exactly is the problem... is it that you are
in Hell... or that you are a snowflake ??? *

Point.  The power busses are probably fine. More
likely the VCOs are defective of design.  Most VCOs
assume that the power bus will be perfect, and trust
the raw power rails for some mission-critical
voltages.

These are (in the usual sawtooth core) - 

1) Ramp Reset level (voltage divider from pos. rail)
2) Expo converter reference (resistor to pos. rail)
3) Initial frequency pot (tied to pos. and neg. rails)
4) Any front panel controls (tied to pos. / neg.
rails)

If you inject ANY noise on these rails... you are
hosed.

These points should be connected to on-board precision
voltage references (LM4040 for example).  Any
variation
of supply voltage will not directly affect the
freqeuncy.  This is good engineering practice and is
almost always overlooked.

The other solutions are

1) separate power supplies
2) decoupling the power rails severely... you need an
RC time constant much longer that the interfering
noise... from an LFO that is DAMN low
3) on-board power supply regulators

This just bit me in the @ss last night I might add.
I went with the on-board voltage references, but need
to include the front panel controls as well. It did
help a lot

H^) harry



--- Julian <julian at 22host24.com> wrote:
> I'm having a few problems with interference between
> modules on my modular.  Notably lfos affecting
> oscillators through the power bus.
> 
> At the moment, the bus is just a simple passive
> stripboard affair (with a run of tinned wire along
> the back to increase the gauge of the ground strip)
> - basically a clone of the doefper distribution
> board.
> 
> What are my options for decoupling?
> 
> I have a few such busses in my modular, each serving
> one 19" row of modules.  What's the best way to
> connect these up to the psu?  In some sort of star
> configuration, no doubt, but should i try and
> decouple these from one another at this point also?
> 
> In fact, is decoupling even the correct term?  Any
> suggestions, preferably with component values , )
> appreciated.
> 
> Cheers, Julian
> 
> 
>