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Re: 10 feet / 3 meters USB extension cord

Posted: Thu Jul 19, 2018 9:13 pm
by Remon
snn47 wrote:
Thu Jul 19, 2018 8:57 pm
I will not recommend phone charging cables for SDR's since they have barely to no shielding.
I didn't propose to use such a cable, I was just trying to explain that cable diameter is not ameasure of resistance of the power lines.
It was just a general advice against in case someone find this topic and buy a unshielded cable. Because most of us try to move away the SDR from any noise source.

EMC of USB cables and extension cords

Posted: Sat Jul 21, 2018 7:47 am
by snn47
While having shielding to improve electromagnetic Compatibility (EMC) is always a good advise it is imho, if at all, hard to achieve and therefore only make believe.

Unlike coax cable, you will not be able to find the necessary shielding information for USB cable or connector because the USB as system system was designed to be cheap and dirt cheap, EMC was therefore never a consideration until USB got faster and interference increased, similar to AC wiring used by Power Line Communication (PLC) or RF-leaking Cable Television (CATV) distribution networks. Even though CATV was designed to provide adequate shielding leakeages not exceeding 20 dBpW in reality those values were meassured to exceed this max. allowable leackage by a factor of 1000000.

Unlike USB CATV use has a defined impedance and shielding characteriscs. The faster USB gets the more problematic EMC and shielding will get, especially if you are only willing to spent just a few $, € for a complete cable. While twisted pair for a data line was intended to a cheap way for slow signal connections, but never at the speeds we use today. USB has neither a defined impedance nor shielding requirements. from start to end like coax cable and connectors were designed for. Even coax connectors perfomance will degrade the more often they are used, the most sensitive ones are rated for less than 5 connections.

Then there is not only interference from the data line,but also the power supply line noise to consider. Power supply line noise depends in addition to shielding of cable and connectors on the quality of the switching circuits regulating the supply voltage,

Wurth provides an overview on USB EMC and this example of power supply noise. The reference is *10 dBm and the vertical axis in 10 dB steps!
power supply noise.JPG
power supply noise.JPG (48.36 KiB) Viewed 3263 times ... f_view.pdf

Wurth also odders a WE-USBH Connector with Integrated EMI & ESD function (WE-USBH EPLE connector) which is around 6 € a piece in quantities of 100.
USB Jack Kombi.jpg
USB Jack Kombi.jpg (19.01 KiB) Viewed 3263 times

DS can be found here ... 369809.pdf

Unfortunately USB shielding does not com cheap, unless you are happy with a so called placebo effect to keep your mind at peace.

Re: 10 feet / 3 meters USB extension cord

Posted: Sat Jul 21, 2018 2:09 pm
by Username
:D How about the idea with buy an big box with this Adpter and resell them?

USB Filter box already offered by Würth as design kit

Posted: Sat Jul 21, 2018 4:50 pm
by snn47
Würth already offers this design bag that provides you with the PCB layout and design shown in ANP024c_EN_The_USB_interface_from_EMC_point_of_view.pdf. I only wonder if replcing the plastic with a metall shield box box would not reduce RFI further.
The Complete USB 2.0 EMI & ESD Solution

You have trouble with noise on your USB 2.0 interface and don’t know how to solve it?
We have the solution for you.

In most EMI labs you can find our "USB 2.0 EMC Adapter" (p/n 829 999 STICK). This stick can be connected to your device during test to improve its EMC susceptibility. In general you will pass all the EMI and ESD test now.
USB-Dongle_geschlossen_res09.jpg (6.43 KiB) Viewed 3248 times
Now you have the choice:

You can either redesign your board and integrate the filter layout like shown above. We have prepared a special design bag (p/n 829 999 BAG) containing the necessary parts. ... USB2_0.php

There might be people interested to buy a already assembled box, but I wonder if they are willing to pay as much as production would cost.