If you treat the "Pin 1 Problem" as the proper implementation of the
Faraday Cage, there is no problem. I've seen this time and time again! My
succinct comments on the issue is "Keep internal fields inside and external
fields external". I like a quote from one of the Henry Ott siminars: "The
outside of the shielded cable is not shielded". The shield is only an
extension of the properly designed inside of the Faraday Cage philosophy.
PAINT: I will also include what I term "PacRim Spit". Paint looks good,
but is one of the EMC engineers outstanding issues. Masking off contacting
surfaces costs the supplier money, so its not implemented. Besides, it all
looks spiff. And, if the supplier has used powder coat, the number of
Dremmel grinders and wheels and Exacto knives used to remove it to prove a
point is highly frustrating and bad on the lungs. To my "PacRim Spit":
The RoS initiative precipitated two solutions: 1) the western world stuck
with conductive coatings and passivation treatments not based on hexavalent
chrome, and 2) the PacRim went with organic treatments. Those organic
treatments are great for not showing finger prints, but are highly
NON-CONDUCTIVE! They're tough and may as well be powder coat (they can be
punched through for verification with the sharp points of the DMM probes).
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. Also, I'll
point out that 1" of AWG #18 wire exhibits about 18nH of inductance - #22
to 28 is commonly used to connect the shield to several pins of a
connector. At even 200 MHz, this 1" of #18 wire (#22 or 28 is even worse)
presents a series impedance (reactance) of +j22.6 ohms. Since an inductor
(or capacitve reactance) is non-dissipative, it hardly forms a good
termination of the shield to complete the Faraday Cage design philosophy!
Dave - WØLEV
EMC Design & Test, LLC
On Mon, Jun 3, 2019 at 10:21 PM Jim Brown <jim@audiosystemsgroup.com> wrote:
> That has ALWAYS been true -- EU Standards have nothing to do with it,
> other than educating (at least partially) on the subject. And there are
> elements of design that are not well known (or at least well-practiced)
> by the EMC community that have been well known in the pro audio world
> for nearly 25 years! Consider "the Pin One Problem," exposed by the
> late Neil Muncy, ex-W3WJE in a 1994 AES paper. He also exposed a
> manufacturing issue with the shielded twisted pair cable that is still
> ubiquitous as "rack wire" in all sorts of studio systems that he dubbed
> "shield-current -induced noise" (SCIN). Thanks to the defect, shield
> current was converted to a differential voltage on the pair, so that,
> for example, mic wring running through the ceiling of a wood frame
> church would act as a an RX antenna for a nearby broadcast station and
> be detected either as a result of a Pin One Problem or excessive
> bandwidth in the audio chain. Mackie, a popular mfr of otherwise good
> low cost gear believed that his audio chain should pass at least 2 MHz
> to minimize phase shift! These audio mixers and mix desks were unusable
> in downtown Chicago, where TV transmitters lit them up, and anywhere
> close to an AM station.
>
> The very public stink I raised (and documented) forced the company to
> fix both issues. This was late '90s - early '00s. The Pin One issue was
> also present in some very top-line microphones.
>
> I published several AES papers on the RFI aspects of both the Pin One
> Problem and SCIN.
>
> The power-line equivalent of a Pin One Problem is a green wire that
> fails to make contact with the shielding enclosure, and often for the
> same reason. It's present in many (most?) Astron power supplies, where
> the bonding point is insulated from the enclosure by paint. This is a
> VERY common mechanism by which RF is coupled into a victim and out of a
> source. There are applications notes on my website about Pin One and
> SCIN. k9yc.com I suspect Pin One as the mechanism by which backhaul
> signals at HF are coupled out of CATV internet systems.
>
> In bar conversations in the late '90s, Muncy emphatically stated that
> Pin One Problems were the primary cause of RFI. A few years later, I did
> the research and published it in the form of several AES Papers that
> proved him correct. They're on my website -- scroll all the way down.
>
> 73, Jim K9YC
>
> On 6/3/2019 2:35 PM, David Eckhardt wrote:
> > Jim, audio equipment meets its challenge with the immunity
> > requirements that Europe places on it. Analog circuitry is highly
> > susceptible to external fields! Design to withstand the EU standards
> > is no simple task. In addition, even the analog portions of the audio
> > designs of today employ actives devices that, by themselves, could
> > operate happily into the VHF and low UHF region (especially your low
> > noise input stages).
>
>
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>
--
*Dave - WØLEV*
*Just Let Darwin Work*
*Just Think*
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