[sdiy] twisted pairs?

[Simon Brouwer]

David G. Dixon schreef:
> I'm not sure about the theory, but wrapping signal wires with ground wire
> in
> one of my circuits virtually eliminated the noise that the signal wires
> were
> broadcasting to the rest of my circuit.  However, I was careful only to
> ground one end of the ground wire; the other end was unconnected.  I don't
> know if this matters or not.

On a high impedance signal wire, e.g. one from a wiper of a voltage
divider pot to a PCB, some loose shielding may have already a big effect.
The capacitance between the signal wire and the very nearby ground wire
will be much bigger than the capacitance between it and an interfering
signal wire a bit further away. You get a capacitive voltage divider that
effectively attenuates the interference.

I would, however, not expect that much of an effect of loosely shielding a
low impedance (output) signal wire. The capacitance between the signal and
the ground wire, at audio frequencies, will be negligible relative to the
low impedance, and exposed area of the signal wire will still capacitively
couple to other signal wires. To prevent a signal "escaping", the signal
wire needs to be largely covered by shielding. The smaller the size of the
holes in the shield, compared to the distance of the shield to the signal
wire, the better.

Not grounding one end of the shielding is good practice, if the circuits
at both ends are already both grounded in another way, because it prevents
ground loops. Ground loops inductively pick up magnetic fields, such as
the stray field from a power supply transformer. This will result in
(sometimes quite large) currents through the ground conductor, which cause
differences of electric potential between different points of the ground
(as no ground conductor has zero impedance really).
If one part of the circuit generates a signal referenced to one point on
the ground, then a receiving circuit that references the signal to a
different point on the ground will see it with this unwanted component
added to it.

-- 
Vriendelijke groet,

Simon Brouwer
-*- nl.openoffice.org -*- http://www.opentaal.org -*-