[sdiy] dare I say - decoupling?

[harrybissell]

inline


> For my new synth I want to use a clean ground + dirty ground setup. What are
> some guidelines as to which devices use which ground? Especially in a
> discreet VCO...

Anything driving a low impedance (like charging a cap fast - envelope
generator?)
or sporting an LED might benefit from the dirty power connection.

Usually if these have the charging or discharging on the same PCB, they can be
handled locally without fvcking the power up too badly.  An example... the reset

transistor of a sawtooth VCO dumps the current from one side of the cap through
the
transistor... and to the other side.  It will not go anywhere else... unless you
make a big LOOP in that circuit. Do that and the reset will take longer, and it
will radiate
to other circuits.

> Also, since I am planning to have no less than 5 VCOs what should I do to
> prevent soft-syncing (ground coupling?).
> Maybe it would be helpful to me and others if an overall system grounding
> strategy was outlined.

Keep on card inductances low. Keep power and ground traces close together,
avoid all loops in the power traces, use lots of decoupling caps.

> I noticed some people use resistors on the power rail inputs for each
> module. What exactly is gained from this?

MORE decoupling.  The resistors form a LPF that stops high frequencies from
coupling into the module. they ALSO form a voltage drop. So its a trade-off. You

sacrifice stable voltage in the name of making it more isolated.

Use a small value resistor for minimum voltage drop, you need a really BIG
cap. Some folks just use ferrite beads... very close to zero ohms at DC but
maybe tens to hundreds of ohms at high frequencies.

You can use inductors, but you have to be yery careful with them... a series LC
could resonate at some frequencies and make noise worse. Usually you need
some small series resistance to prevent or reduce this.

Onboard regulators go WAY past this. Essentially, a regulated power supply
per module.  Run your rails at maybe +/-20V then drop to 15V.  Now a glitch
on one module has to fight backwards across the regulator, and then get forward
through another.

I like to use local zener diode regulators on the voltages for the front panel
controls
on each module. Maybe I go +/-10V from a +/-15V supply. This stops variations
in supply voltage from modulating synth parameters.

Even better, always use onboard voltage references for critical circuits. The
biggest
mistake mmost people make is setting some critical parameter (like VCO expo
reference or sawtooth reset level) from the main power supply. This is asking
for
trouble !!!

Best idea... don't use LED indicators. They look cool but cause a lot of current

variation as they switch on/off

H^) harry