[RFI] USB qrm - what to do?

[Jim Brown]

Hi Barry,

It's always best to use high quality cables with a good shield when they 
are available, but as noted, they only solve problems when the connector 
on the equipment they're plugged into is properly bonded. Shielding 
prevents DIFFERENTIAL radiation from the cable (the signal inside the 
cable). The toroid works on COMMON mode radiation (the signal outside 
the cable).

73, Jim K9YC

On Mon,3/13/2017 1:45 PM, Barry Merrill wrote:
> K9YC states:
>
> Shielded cable rarely solves RFI problems, partly because
> to do anything useful cable shielding must continuous with equipment
> shielding, and lots of
> equipment to which you would like to connect it is unshielded!
>
> My question:  does it matter whether the USB cable is shielded when run thru
> a toroid?
>
> Barry, W5GN
>
> -----Original Message-----
> From: RFI [mailto:rfi-bounces at contesting.com] On Behalf Of Matthew King -
> KK4CPS
> Sent: Monday, March 13, 2017 9:28 AM
> To: Pete Smith N4ZR <n4zr at comcast.net>
> Cc: RFI List <rfi at contesting.com>
> Subject: Re: [RFI] USB qrm - what to do?
>
> Pete - I had similar problems until I purchased shielded USB cables with the
> small ferrites already molded in. The ones I have are silver and look
> braided under a clear insulation. They are noticeably thicker than a regular
> cable.
>
> Those these don't LOOK the same as mine, I'd trust the Tripp-Lite brand to
> be properly shielded, I do believe.
>
> *https://www.amazon.com/Tripp-Lite-Hi-Speed-Ferrite-U023-006/dp/B003MQ29B2
> <https://www.amazon.com/Tripp-Lite-Hi-Speed-Ferrite-U023-006/dp/B003MQ29B2>*
>
> This is more what mine look like - I don't know that it's necessary to be
> able to "see" the shielding or not! *:-)*
>
>
> *http://www.ebay.com/itm/1-5M-USB2-0-A-Male-to-B-Male-M-M-Extension-Shield-C
> able-Blue-for-Printer-Scanner-/321535280583?hash=item4adcff05c7:g:n2oAAOSw~A
> VYuMEa
> <http://www.ebay.com/itm/1-5M-USB2-0-A-Male-to-B-Male-M-M-Extension-Shield-C
> able-Blue-for-Printer-Scanner-/321535280583?hash=item4adcff05c7:g:n2oAAOSw~A
> VYuMEa>*
>
> Anyhow, maybe that'll help ya out.
>
> Be good!
>
> 73
>
> Matt
> AK4MK
>
> On Mon, Mar 13, 2017 at 7:31 AM, Pete Smith N4ZR <n4zr at comcast.net> wrote:
>
>> My shack computer has 6 different USB devices attached.  When I
>> disconnect all but the keyboard, it cuts the background noise level on
>> 160M by a whopping 25 dB, from S9+18 down to S8 (as monitored by HDSDR
>> and my QS1R, calibrated by an Elecraft XG3).
>>
>> What is to be done?  It doesn't seemvery feasible to wind each usb
>> cable on a small type 31 toroid and get enough attenuation .
>> Available snap-ons all seem to roll off dramatically below 25 MHz. Can
>> a common mode filter be built using discrete components that will have
> decent attenuation at 160 M?
>> Or, maybe, is such a filter available commercially?  I have Googled
>> and reviewed K9YC's tutorial but found nothing directly apposite.
>>
>> Thanks for any ideas!
>>
>>
>> --
>>
>> 73, Pete N4ZR
>> Check out the Reverse Beacon Network
>> at <http://reversebeacon.net>, now
>> spotting RTTY activity worldwide.
>> For spots, please use your favorite
>> "retail" DX cluster.
>>
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>>
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