[sdiy] Re: stranded or not stranded

[Magnus Danielson]

From: jhaible at debitel.net
Subject: Re: [sdiy] Re: stranded or not stranded
Date: 18 Jun 2003 15:21:59 +0200,Wed, 18 Jun 2003 15:21:58 +0200

> > > One reason for using twisted wires (stranded, of course) might be
> > > stability. But I see an even more obvious reason: to keep noise from
> > > inductive coupling (magnetic fields) as low as possible.
> > 
> > This only works for balanced lines, where you want the interference to be
> > coupled into both lines with equal magnitude.  And even then you want an
> > even number of twists to ensure maximum cancellation.
> 
> In balanced Lines the effect may be *optimised*, but this doesn't mean
> you don't get it in other systems, too.
> 
> If you have an alternating magnetic field, every loop will pick up
> a little of this. The operation is a vector multiplication, so
> by periodically changing the orientation of the area inside your
> loop, you can reduce the induced voltage along your loop.
> With the "even number of twists" you have the best cancellation,
> because for every partial loop you have another one with opposite
> orientation. But even if you have an uneven number of twists,
> like 99, then 49 will find other 49 to cancel, and the remaining
> single twist / partial area will not be compensated, but will
> have much less noise induced because of the much smaller area
> that is enclosed, compared to one big loop of a non-twisted wire.
> 
> I have no idea if this is *necessary* in the application we were 
> talking about. But I presume it has some effect, and the effect
> is positive, and it cannot hurt to have it, especially if it comes
> almost for free.

In addition, the effect has a cut-off at about 100 MHz, so above that the
diffrential-mode signal on a twisted pair does not experience any isolation.
You need to let the sheild take over then.

> > If its just signal and ground, the only benefit is keeping the wires neat
> > (which is a good thing in itself).
> 
> In general, if you have the lines balanced towards GND (one going positive
> while the other goes negative), you have the *additional* advantage of
> a noise reduction from *electrical* filelds. But noise reduction against
> *magnetic* fields comes from the twisting, electrically balanced or not.

That's the low frequency view of things. At higher frequencies, you become
aware of both types of fields in a more balanced way... with eh... about 377
Ohm impedance usually. ;O)

Cheers,
Magnus