Re: Signal interference in a long cable

From: Gerrit Heitsch <>
Date: Wed, 21 Dec 2016 22:30:49 +0100
Message-ID: <>
On 12/21/2016 10:15 PM, Michał Pleban wrote:
> Hello!
> Gerrit Heitsch wrote:
>> You have short spikes you want to get rid off. How about a capacitor
>> between the signal and GND. Yes, that will soften the signal in general
>> so you have to experiment if there is a cap that will work without
>> softening the rising and falling edges too much. Put the cap at the
>> input of the shift register or the 74LS14 if used.
>> Try 470pF and see what happens.
> The closest I had was 1nF. The signal got visibly better, and I was able
> to read the dist directory - big improvement!
> What is the significance of 470pF? Is it related to the clock speed?

It was just a number that looked OK for the frequency since you don't 
want to distort the real signal too much. You can also create 500pF by 
using two 1nF caps in series.

Also, you could try a small resistor (< 100 ohm) between output and 
input but before the cap. Like this:

  ---|___|----*-----o shift register input

Play around with the values some and see what works best. Then try a 
different cable. :)


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Received on 2016-12-21 22:02:03

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