Yes, the capacitor termination keeps us high-frequency types happy and
satisfies the 'one end only' termination for the audio TSP designers. I'm
an RF type with some experience with audio from my long ago college days.
My favorite example of the failure of twisted pair (TP) with the digital
designs of today is Ethernet. In the early days when Enet was over coax,
there was little or no major radiation problem. However, once the designers
(digital, with no RF knowledge) got word of TP (far less expensive than
coax), all h#$% broke loose. Our building at HP was originally outfitted
with coaxial cable for Ethernet. There was little interference problem.
However, once TP was installed, many small-signal instrumentation setups
became unusable without a shielded room. Today, modern buildings outfitted
with TP for Ethernet are so noisey that radios, even at FM broadcast
frequencies, are unusable. I have lot's of 'war stories', but I'll spare
those for the time being. In my experience and regarding only RF up to low
UHF, the RF noise becomes common mode over roughly 10-wavelengths of TP
length. It radiates like an intentional antenna. The present-day
implementation of differential drive does not yield true differential
signals as the 'ground' plane serves as the reference for both the 0 and
180 phase signals. It's not true differential drive . Yes, I'm an old
fossil at 72 years old (wise) who cut my teeth on vacuum tubes (remember
those glass 'things' filled with nothing? We might label them 'thermionic
FETs' today.).
Dave - WØLEV
On Tue, Jun 4, 2019 at 3:23 AM Jim Brown <jim@audiosystemsgroup.com> wrote:
> I led the writing group for On 6/3/2019 3:54 PM, David Eckhardt wrote:
> > One additional item I should mention: The low-frequency designers
> > tend to stick with "grounding" (returning) the shield at only one end
> > (to prevent "ground" loops). The high-frequency designers know this
> > will enable the shield to become a wonderful antenna and they better
> > "ground" (return!!! !!! !!!) both ends of the shield at the
> > penetration of the 'cage' - the shield should be viewed as an
> > extension of the internal gutts of the Faraday Cage design
> > philosophy. If both ends of the shield are "grounded" and there is
> > still a radiation / susceptibility problem, the shield is too thin
> > and/or not of properly covering the shielded conductors.
>
> I led the writing group for AES48, AES54-1, AES54-2, and AES54-3, which
> specifies good engineering practice for balanced audio
> interconnections. Everyone in the writing group was fully aware of the
> issues you raise. There ARE good reasons for lifting the shield
> connection at one end, and by correctly analyzing the balanced audio
> interface as a classic Wheatstone bridge, Bill Whitlock proved that it
> should never be lifted at the sending end. His work subsequently caused
> IEC Standards with respect to the balance of such circuits to be revised.
>
> The Standard (AES4-1) specifies that, and recommends a capacitor with
> good properties at HF through UHF. The most commonly used connector for
> shielded twisted pair audio cables is the XL3, of which Neutrik and
> Switchcraft are the principal manufacturers. I came up with the idea of
> a cylindrical capacitor terminating the shield to the connector shell,
> with a ferrite bead around the designated shield contact, Pin 1. The
> bead serves two purposes. First, it lowers the Q of the parallel
> resonance between the capacitor and the inductance of the connection via
> Pin One to the shielding enclosure. Second, it blocks the flow of
> VHF/UHF shield current into equipment that is non-compliant with AES48
> ("the Pin One Problem" Standard, which specifies terminating Pin One to
> the shielding enclosure). We recommended capacitance in the range of
> 10-50 nF.
>
> Neutrik, an active participant in AES Standards activity, developed and
> manufactured a practical implementation based on our concept. At the
> time of publication of these Standards, existing products experienced
> serious RFI because they were not AES48 compliant, and I consulted with
> two major mic mfrs on the solutions to their problems.
>
> In part as a result of this work (we all did lots of other things),
> Muncy, Whitlock, and I were subsequently elected Fellows of the AES,
> along with another important contributor, Ray Rayburn.
>
> 73, Jim K9YC
>
>
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>
--
*Dave - WØLEV*
*Just Let Darwin Work*
*Just Think*
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