[sdiy] Re: reducing crosstalk

[Tom Corbitt]

Well, the simplest improvement with your current setup would be to add
decoupling caps. Add 0.1uF caps between +12 and the -12 and gnd on
each TL074. You should always always always use decoupling caps with
op amp (or any other chip for that matter) You should see a big
improvement from this.

For the future, I would suggest separate gnd and power runs to each,
not the power bus topology it looks like you're using now, with a zero
ohm resistor connecting the "channel" gnd to common ground (and if you
want to get serious throw a series inductor into the power
connections)  Each channel needs to be an island, with only the zero
ohm resistor and the power connecting it to the rest of the circuit. A
large ground pour wouldn't hurt either, and you could do fancy things
like guard rings on the inputs and separate power supplies for each
channel, but all of the is but this is pretty hard to do with
protoboard.

>From a pcb design standpoint, the common channels would make for an
easy first project to to learn from and there are companies out there
that that churn out small runs of boards for cheap ($10-$15 per board
with a 5 board min) This might seem expensive up front, but measured
against the hassle of protoboard changes/fixes, it's cheap in terms of
sanity and time saved. If you went with pcb mounted jacks, it would
also save a lot of wiring hassle.

Tom

On Thu, Feb 14, 2008 at 9:54 AM, Derek Holzer <derek at umatic.nl> wrote:
> One point of clarification: the 8 pole DIN plugs on the front of the
>  board go to the phototransistors, which I did not photograph. There are
>  two phototransistors per DIN cable.
>
>  d.
>
>
>
>  Derek Holzer wrote:
>  > Hi Peter,
>  >
>  > I was away performing and teaching the last few weeks. Now I'm back
>  > working on this project again. I've put up a ZIP file with photos and a
>  > schemo of the module in question. Maybe you can spot some of the trouble
>  > areas? The PS is a standard DOEPFER one, I shouldn't need to regulate it
>  > on the module again, should I?
>  >
>  > http://www.soundtransit.nl/tonewheels/tonewheels.module.zip
>  >
>  > best!
>  > d.
>  >
>  > Peter Keller wrote:
>  >> On Fri, Jan 25, 2008 at 09:01:26PM +0100, Derek Holzer wrote:
>  >>> Where should I start with this? Is the crosstalk coming from a bad
>  >>> grounding? The ICs? The connection wires? I'd be happy to upload a
>  >>> photo of the circuit and a drawing of the schemo if that would help.
>  >>> In general, where would such crosstalk come from? Do I need to etch a
>  >>> board with a good ground plain? Since I want to use the output as
>  >>> either audio or CV, high-pass caps for DC removal isn't an option. Or
>  >>> is it?
>  >>
>  >> I'm kinda interested in this problem. Can you post a picture of it?
>  >>
>  >> I've run into serious crosstalk a few times, and usually it is a
>  >> result of:
>  >>
>  >> A) Since you have an unregulated supply it might variably droop too much,
>  >> which would induce signals into other parts of the circuit.
>  >>
>  >> B) If you run lines parallel to each other for any moderate (like 5
>  >> inches
>  >> or more) length of distance with high currents (10-15mA) in them, they
>  >> will easily capacitively and magnetically induct between themselves. Also
>  >> be aware about parallel analog/digital lines--especially clock lines. For
>  >> your photodiodes, I'd have a plus/gnd pair of wires for each one and
>  >> twist the signal wires to quell the magnetic interference. You can use
>  >> shielded 2-pair conductors for this, and connect the shield on the
>  >> receiver/board side to ground.
>  >>
>  >> C) You should use bypass capacitors for all of your IC Vdd pins, have
>  >> them
>  >> be 0.01uF and keep the leads as short as you can.
>  >>
>  >> D) Check this page out which seems to do a good job explaining grounding.
>  >> http://ai.kaist.ac.kr/~suh/DIY/ground
>  >>
>  >> I'm sure there are more ways crosstalk can show up, but these are ways
>  >> I've
>  >> had personal experience with.
>  >>
>  >> Later,
>  >> -pete
>  >> _______________________________________________
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>  >>
>  >
>
>  --
>  derek holzer ::: http://www.umatic.nl ::: http://blog.myspace.com/macumbista
>  ---Oblique Strategy # 57:
>  "Do the words need changing?"
>
>
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