[sdiy] newbie with some questions

[Neil Johnson]

Kris,

> Well, you don't have to shout about it :-)

Oops, sorry, enthusiasm before keyboard!

> Maybe we're not answering the same question:
> This isn't really what we're seeing in say the Paia 9700, which is this

<viewed sketch>

> This is the case of the 9700 as best I understand it.

Which is basically what I thought you'd meant.  Imagine each piece of wire
has a finite, if small, resistance and inductance.  Even 1cm of wire will,
at some frequency, resonate -- on high-speed logic circuits (ECL) even a
1" long trace is considered a transmission line and must be properly
terminated by a resistor to stop it ringing!

> Also, aren't you forgetting a GROUND which plays in this as well?

Yeh, I left the Ground out of my ASCII picture to try and keep it simple.
But yes, the ground wire will also have inductance and resistance too.

In the general case we try and minimise the effect any external circuit
has with the judicious use of supply filters.  But, its always good to
apply sound electrical principles at the start rather than try to fix them
later on.  The Paia route is simple for supply wiring, but needs careful
supply decoupling on each PCB to minimise the external effects.

As an alternative, look at the MOTM modules, which have common
distribution boards that each module's supply connects to.  This would be
the common "star" point in your second diagram.

Then again, the daisy-chain approach has advantages, as long as the
designer allows for its shortcomings (higher external interference).  The
best approach, as used in the Oberheim OBX for example, is to have local
supply voltage regulators on EACH board, then you can daisy-chain at a
higher supply voltage (say +/- 19V) and be sure that the regulator will
filter out a large amount supply noise from other boards in the system.

Hope this helps,
Neil

--
Neil Johnson :: Computer Laboratory :: University of Cambridge ::
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