unwanted sync

Harry Bissell harrybissell at prodigy.net
Sat Mar 11 18:08:16 CET 2000


Mosk "unwanted sync" events involve iceberg, submarine, etc

see "titanic", "lusitania", etc...

But seriously folks....
The things to watch for are mostly power supply routes. Use the routes to force
current
to flow where you want it. The best layout would have the oscillators
physically separated
at three corners of the board, with common circuits in the fourth... (makes
sense yes??)
The power should start from a single point (the main filter caps right where
power enters the board) and run separately to each oscillator. Use decoupling
caps for each active (IC) device very near the chip (keeps the capacitance near
the current drain, with very little inductance). Inductance will stop the caps
from delivering current in time to meet demand, causing voltage drop...
unstable operation....

IMHO the lack of attention to power supply concerns (esp. placement of
decoupling) is the greatest cause of a good (schematic) design to perform.

The NEXT BIGGEST ERROR is to share a single IC (maybe a quad op-amp) between
separate VCO's. This guarantees that the power supply distribution is WRONG!
and also
puts sensitive circuits within microns of each other, instead of mils (.001").
A sure prescription for interaction !!!

Good circuit boards are art. A proper layout looks good, feels good. Keeping
symmetry
between all three VCO's will assure similar performance. Think about thermal
issues too, the VCO nearest the top of the enclosure is probably the warmest
one...

OTOH... some circuits work because of (intended or unintended) interaction. The
MXR
Bluebox is a prime example.... the 'sneak' path through the power supplies
actually makes the unit perform 10X better. The designer was either very lucky
or very skilled.

IMHO I perfer to keep circuit functions separate... so that changing a gain
resistor doesn't
screw up three other parameters.

Are you going to build and sell these... or are they a kit ??? SMT can be hard
with a
"100 watt American Beauty" (soldering iron).

H^)  Harry

Per Mattsson wrote:

> Hi guys!
>
> I'm drawing loose sketches on a three ASM VCO board. To reduce size I hope
> to make use of as many SMDs as possible.
>
> Are there any specific "demands" on that (or any) VCO board design? A part
> from the lin-log components beeing close/glued together. Will I get any
> unwanted soft sync effects from traces going parallell to long? Or are the
> traces on a PCB the size of a ASM VCO simply to short to have any effects
> of that kind at all? Or does unwanted sync things has more to do with
> porely regultated/filtered power than "badly" placed components .
>
> This will be a real compact design since I aim to build a four voice, three
> oscillator module.  I hope to make room for "global" CV ins as well voice
> specific CVs . I have yet no idea on how to patch this unit together with
> other polyphonic modules. Maybe matrix switch chips or five pole DIN
> suggested on the list before.
>
> If I were to give board a name it samply has to be  'a ASM VCO SMD PCB',
> which I find rather fun...
>
> ---Per Mattsso




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