On 9/24/2011 10:13 PM, Tom Thompson wrote:
> This does not imply that the currents in the wires are equal and
> opposite and if they are not, then a common mode current is present.
Exactly. There is an excellent AES paper from 1994, published in the
June 1995 Journal of the AES (Audio Engineering Society) by Bill
Whitlock analyzing a balanced interface as a Wheatstone Bridge. The
function of a balanced interface is the rejection of noise and
crosstalk, and Bill shows that the performance of a balanced circuit is
defined by the balance of the IMPEDANCES in the circuit, including those
on each end of the line. Whitlock's paper can be downloaded (for a small
fee) from the AES website, and a less formal version of the paper is
available free from the Jensen Transformers website (Jensen is
Whitlock's company).
It is important to realize that while the usual application is audio,
the analysis is entirely valid at all frequencies. The only thing that
changes is the complexity of the impedances (including strays) both in
the cable and at both ends. For example, it's fairly easy to balance
impedances at mid-audio frequencies, but a much bigger kettle of fish to
do so from low audio frequencies to high RF frequencies, but that's what
is required if you want the balanced interface to reject interference
over that range. One thing Whitlock showed us, with supporting measured
data on real cables, is that the capacitance between the two conductors
and the shield is typically unbalanced on the order of 5-10% for GOOD
cables. A colleague at Belden on the AES Standards Committee noted that
some reasons are the differing dielectric constants of different color
dyes, the tolerances on the dies that place the insulation on the
conductors, and the centering of the conductors within the shield as the
cable is manufactured.
Following the publication of this work, IEC Standards relevant to
defining and measuring the balance of a circuit were revised to reflect
the realities that Whitlock's analysis exposed, using a test method that
he devised.
73, Jim Brown K9YC
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