[sdiy] Re: reducing crosstalk

[Derek Holzer]

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
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