[sdiy] dare I say - decoupling?

[Tim Daugard]

I know, I'm late to the party, but I was on a vacation, errrr,
there was a hurricane, yah that's it, errrrr, no wait, there was
two hurricanes, yah thats the ticket.

From: "Roy J. Tellason" <rtellason at blazenet.net>

> I have boxes of 'em,  ceramics,  electrolytics,   whatever,
and board
> space costs but not *that* much -- at worst just tack solder
the darn things
>
> No reason I can see not to use a whole mess of decoupling caps
all over the
> place.  Lots and lots of them...

I used to be of this philosophy, then the problems started.

1) Used electrolytics can leak current. Even new ones have some
current leakage.
2) Lots and lots of capacitors put a heavy capacitive load on the
power supply. This is okay until the power supply can't provide
sufficient current to provide the intial charge on all the caps.
This is where resistors in the power lines of non-voltage
critical circuits help.
3) As discussed before, all these caps combine with the various
power supply impediances to form resonances. This wasn't critical
before, but with large bandwith opamps and some transistor
circuits, it can cause oscillations.

I use individual grounds from the modules to the power supply -
okay I cheat a little - no more than 6 modules on any power
cable. I have had to trouble shoot problems by disconnecting a
power plug from individual modules on a cable until the problem
went away.

I have reached the point of calulating the frequency response
need from a module and insuring that the input caps or filters
are adequite for the needs. This includes ensuring that an RCs in
the power path have an RC time constant twice the lowest
frequency.

--Semi OT

I have designs for a module that will have an power supply RC
time constant so that the power supply cap discharges
signficantly with a large audio signal through the path, and will
only recover slowly when the signal level declines. I haven't
built this yet. I can patch it with an envelope follower, env
gen, and a VCA. The effect simulates power supply sag in tube
amps.

Tim Daugard
AG4GZ 30.4078N 86.6227W Alt: 12 feet above MSL
http://home.sprintmail.com/~daugard/synth.htm

with spell check still broken