[sdiy] Re: reducing crosstalk

Derek Holzer derek at umatic.nl
Thu Feb 14 15:49:08 CET 2008

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?



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 # 135:
"Remove ambiguities and convert to specifics"

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