[RFI] RFI Digest, Vol 252, Issue 2

[Kenneth Wyatt]

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 at mac.com



> On Jan 16, 2024, at 8:22 AM, rfi-request at contesting.com wrote:
> 
> Date: Tue, 16 Jan 2024 09:32:01 -0500
> From: Pete Smith N4ZR <pete.n4zr at gmail.com <mailto:pete.n4zr at gmail.com>>
> To: N1MMLoggerPlus at groups.io <mailto:N1MMLoggerPlus at groups.io>, RFI List <rfi at contesting.com <mailto:rfi at contesting.com>>
> Subject: [RFI] USB Cable Shield Resistance was: Re: [N1MM+] open
> 	question about RFI & USB
> Message-ID: <19622c6c-9425-42dc-89a4-763251f6945a at gmail.com <mailto:19622c6c-9425-42dc-89a4-763251f6945a at 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