[sdiy] decoupling modules from power lines?

[Sowa Roman]

Hi and welcome.

Let me describe it on my quad VCO board. There are four VCOs on one PCB,
using one quad OTA as VCO core. Every opamp used is decoupled with 100nF
and 10uF on both supply rails, OTA is also decoupled this way. Supply and
ground traces on PCB are the thickest possible and go thru the middle of
PCB.
>From those 3 rails there are smaller traces supplying each opamp. Better
would
be to route each GND trace and maybe even power rails too, from one point
like power connector, to each VCO, but I was too lazy for that.
Whole board is supplied via small chokes, about 4-5 windings on small
ferrite
circle (bead is it called?).
Result - it's locking only 2 of 4 VCOs (don't know why just the 2) when they
are
0.2Hz apart at 4kHz. Not too bad I guess.

My suggestion is: 100nF paralel with 4.7-10uF on every opamp as a rule,
good tracing layout, and if it doesn't help, small choke, less than 10uH on
supply
rails before caps, never on GND.
Larger chokes, and 1mH is rather huge for me, have also considerable DC
resistance,
take that into account. And if they are not screened, thay can collect
interferences
serving as an antenna and make more trouble than there was without them

Roman

""-----Original Message-----
""From: Christian.Bergmiller at daimlerchrysler.com 
""[mailto:Christian.Bergmiller at daimlerchrysler.com]
""Sent: Thursday, January 10, 2002 9:19 AM
""To: synth-diy at dropmix.xs4all.nl
""Subject: [sdiy] decoupling modules from power lines?
""
""
""Hi,
""
""here is just another one joining the list...and the first 
""message i'm sending 
""is a question (what a shame) !
""
""I'm currently building a (small) modular synth with ASM-1 
""VCO's (added 
""tri/sine), polyfusion VCF (OTA design), 
""selfmade midi to vc (at89c2051) etc...
""Yesterday i discovered the VCO's had the tendency to lock 
""(they even modulated 
""each other - without any connection
""between them but the same power source) i fixed that by 
""adding a tantal 
""capacitor from the V- to ground on both VCO modules
""(from V+ to ground seems to have no effect so i left them out so far)
""
""Now i'm thinking about equipping every module with a coil 
""followed by a 
""capacitor to gound for the V+ and V- power lines.
""(tro prevent further problems so far undiscovered)
""I would try 1mH and 2,2uF, but would like to hear your 
""suggestions bevor i 
""order a greator amount of these values from
""my favorite supplier...
""
""Hope to hear something helpful :-)
""
""Christian
""
""(and please don't mind the bad english...)
""
""
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