Replying to Pete’s excellent question on cable shield resistance. I performed a
couple alternative shielding tests of several types of coaxial cables and this
might shed some light on USB-type I/O cables as well. Part of the experiment
was applying a calibrated 1-amp of current from connector to connector and
measuring the voltage drop, thus calculating the DC resistance. There was quite
a variation depending on cable quality. To answer Pete’s question, 10 to 15 mΩ
would be an acceptable value in my opinion. Most DVMs are not sensitive enough
for measurements this low, thus the described technique to assess DC shield
performance.
https://www.edn.com/quickly-assess-relative-coax-cable-shielding-quality/
<https://www.edn.com/quickly-assess-relative-coax-cable-shielding-quality/>
Now, the other issue (besides the fact manufacturers may not even include a
shield), especially for I/O cables, is the method of termination of the shield
to the connector ground shell. In many cases, the shield is terminated with a
thin wire to the ground shell (often referred to as a ‘pigtail’). Ideally,
shields should terminate to the connector ground shell directly and in what we
EMC engineers call a ‘360-degree’ connection. This is commonly done for
military and aerospace connectors. Pigtail shield terminations are a major
cause of RFI and even susceptibility to external RF transmitters or ESD. I have
a short video demonstration as to why pigtailed connections are ‘bad news’ for
EMC in general and this also applies to ham radio applications.
https://www.youtube.com/watch?v=XoLBOuLH8t0&t=1s
<https://www.youtube.com/watch?v=XoLBOuLH8t0&t=1s>
There’s usually a reason some cables cost more than others!
______________________
Kenneth Wyatt
wyattphoto@mac.com
> On Jan 16, 2024, at 8:22 AM, rfi-request@contesting.com wrote:
>
> Date: Tue, 16 Jan 2024 09:32:01 -0500
> From: Pete Smith N4ZR <pete.n4zr@gmail.com <mailto:pete.n4zr@gmail.com>>
> To: N1MMLoggerPlus@groups.io <mailto:N1MMLoggerPlus@groups.io>, RFI List
> <rfi@contesting.com <mailto:rfi@contesting.com>>
> Subject: [RFI] USB Cable Shield Resistance was: Re: [N1MM+] open
> question about RFI & USB
> Message-ID: <19622c6c-9425-42dc-89a4-763251f6945a@gmail.com
> <19622c6c-9425-42dc-89a4-763251f6945a@gmail.com">mailto:19622c6c-9425-42dc-89a4-763251f6945a@gmail.com>>
> Content-Type: text/plain; charset=UTF-8; format=flowed
>
> Interestingly, I just tested the shielding on a USB A to USB C cable I
> have, which looked like it should be fine, with a transparent jacket
> over woven shielding and a big ferrite bead - about 1 meter long, and
> measured 2.7 ohms - not good.? What should be consider to be the upper
> permissible shield resistance?
>
> Moving this issue to the RFI reflector now...
>
> 73, Pete N4ZR
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