[sdiy] decoupling on modulars power bus

[James Patchell]

As an example of what Harry is talking about...take a look at a VCO I did:

http://www.oldcrows.net/~patchell/synthmodulesII/200-1007.pdf

I have three of these on a simple power backplane that connects power up to 
all of the modules:

http://www.oldcrows.net/~patchell/synthmodulesII/back.jpg

You can...if you look carefully...see a PC board that connects all of the 
modules together...this is the power distribution.

I use the LM4041-ADJ for the voltage reference on the VCO's...plus a lot of 
bypassing (Harry can testify to this :-)....

I get no soft sync...no cross talk between modules...and the VCO's are rock 
solid stable...


At 01:13 PM 5/24/2005 -0700, Harry Bissell Jr wrote:
>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
> >
> >
> >

         -Jim
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