Ferrite Bead or choke Shields for USB cables??

Discussion in 'General' started by Tim Carruthers, Jun 25, 2016.

  1. Tim Carruthers

    Tim Carruthers Cyanogen Customer

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    Hi there,
    Does anyone know if it is worthwhile to add a ferrite shield at each end of a USB cable for use with astronomy cameras / equipment??
    Cheers,
    Tim
     
  2. Colin Haig

    Colin Haig Staff Member

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    I've done so with some of my USB stuff. Been using TDK snap-on ferrites that go over the USB cables, within an inch or so of the ends. More importantly, the DC power leads for some equipment. I had a real issue with one device making a lot of RFI/EMI and it would show up on the CCD images. Took me a while to hunt it down. Anyway, 2-3 turns of the DC power cable through a ferrite snap on cured it. On the power distribution end, I use West Mountain Radio RigRunners, and have RF bypass caps etc.
     
  3. Joseph Zeglinski

    Joseph Zeglinski Standard User

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    Colin,

    I wonder if the signal noise is coming from the "Earth Grounding" being tied to the DC Ground Return of battery plug type power cords. Other "home appliances" or power supplies chassis, might be leaking into the "earth grounding system", infecting the DC reference, like on the USB or RS-232 cables.

    I did some testing last year and found most astro accessory manufacturer's devices are guilty of this - the one big exception being SBIG and my STL-11000 - which has a separate Earth ground lead (if using the ELPAC supply brick), from the DC Return. I also did a continuity check between the STL body and any scope earth ground point, finding the camera and thus its PCB electronics, is indeed isolated. Kudos to the SBIG electronics designer.

    In my tests, I isolated the side clips of any such "cigarette lighter plug" by wrapping it with a strip of paper or tape, so only the positive center tip touches socket power. I was surprised to see that three of my devices (AP mount controller, RCOS TCC, and Kendrick Premier dew controller), still had completed circuit paths and fully operational displays. The controllers found a DC ground return by going through their RS-232 DB-connector body "outer shields" - which should not have been "conveniently" jumpered to logic/battery ground in the cable or circuit board. I think it might account for fried telescope electronics from distant lightening storms, as the (real) earth ground potential spike oscillates for a few cycles, thus changing the circuit boards "reference" DC ground, making it wider than the nominal 12 volts DC it normally uses.

    This kind of "grounding sharing" of DC Return and Earthing system, may be the path of the electronic AC noise you were seeing, on (some) of your data cables, which also carry, possibly noisy, DC power to their USB-2 and USB-3 devices.

    Joe
     
    Last edited: Jun 25, 2016
  4. Doug

    Doug Staff Member

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    Ferrites are supplied with SBIG cameras. They are intended to reduce RF emissions from the camera, by preventing them traveling down the cable - which is a pretty good antenna. Normally these are placed at the camera end.

    I don't see a benefit to placing one at the other end.
     
  5. Joseph Zeglinski

    Joseph Zeglinski Standard User

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    Doug,
    Is this only for recent SBIG CCD camera models?
    Checked the 10-foot USB cable which came with my (2007 ?) STL-11000, and there is no Ferrite on it.
    When did the STL start coming with the USB attached beads and is it now recommended?

    Your wording implies that the problem is isolating other equipment FROM "camera" generated RF "coming Down, instead of UP" to the camera. I always thought the camera was the RF victim, not the instigator. In either case, the Ferrite should fix things.
    Thanks for the heads up.

    Joe
     
  6. Colin Haig

    Colin Haig Staff Member

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    Hi Joe, thanks for the comments.
    My observatory runs off solar, and is totally 12V DC operated (except for roll off roof motor which will hopefully be converted next week when parts arrive).

    Agree, grounding, bonding, are so important. I have most things bonded to earth through at least 10ga or 6ga copper to a common copper ground rod.
    The DC power all comes back to rigrunner 4005i which then goes to the +12V 220Ah batteries. and the DC ground meets the bonding at the solar charge controller, which is then bonded to the common copper ground rod. The idea is that wherever possible, all grounds go to a single point. This is the result of a couple of lightning strikes and 1 expensive experience related.

    I started the observatory with a Meade LX200 Classic, and sadly, its chassis was at a higher potential than the DC ground. So, if you grounded the camera or pier, you'd have problems, most obviously the DC current meter LED bargraph would not read properly, and worst case, all kinds of electrical madness. Plus, you could not properly shield the RS232 cables, as they used modular plugs. The ports blew on an LX200GPS a few years later during a severe storm - cost my neighbour about $10,000 for new major appliances and furnace.

    I have an STL-11000, and at USB1.1 speeds, haven't needed ferrite on it. I have a couple of faster interface USB2.0 cameras, and they spew RF from faster ADC and USB2.0. One of the video-type cameras would get weird line patterns without ferrite on its USB and power. Eventually tracked the source of the issue to another device on the USB bus. Ferrites on the devices cured it. USB hub power is off of a rig runner as well, so no AC adapters.

    My environment has Amateur Radio gear, which has never caused an astro-toy problem except on my old WMR-968 weather stations. Ferrites on the stupid modular phone cords between the sensor heads and the solar powered 433Mhz transmitters solved that. Although I did upgrade to a Davis, and have had no issues since.

    This setup has generally worked well. I've found some of the AC wall warts supplied by the various vendors are total crap, with poor regulation and lots of RFI. I had one in particular that was giving me several dB of noise on the radio and a cordless phone headset. 3 turns through a ferrite solved it.

    What I like are vendors that build a ferrite into the cable, molded on, or devices with internal filters as well. I wish some of this stuff was better engineered. We're swimming in RFI and don't even know it.
    Was reading an article on the Pin 1 Problem: http://pin1problem.com/ talking about mixing chassis and signal grounds.
    I hope we haven't scared Tim C with all this!
     
  7. Tim Carruthers

    Tim Carruthers Cyanogen Customer

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    Yep...I'm worried!!

    I think I will keep an eye on the RF interference issue and add some ferrite shields on some of the cables!!
    Thanks!
    Tim
     
  8. Colin Haig

    Colin Haig Staff Member

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    Tim, are you experiencing a particular problem? For example, noisy images with repeating patterns?
    The other thing I've found is the quality of USB cables is all over the place - it's worth chopping a few open to see how good the shielding is, and whether the wire gauge is as advertised.
     
  9. Doug

    Doug Staff Member

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    Yes the ferrites are used to ensure that the equipment meets FCC regulations. A lot of computer monitor cables come with ferrites built in.

    The STL may not have needed the ferrites. The decision to use them or not depends on measurements made at a certified EMC compliance lab.
     
  10. Tim Carruthers

    Tim Carruthers Cyanogen Customer

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    Hi All,
    I am actually not having any RF interference issues that I know of, just some camera timeouts, that are related to other problems, it would seem.

    I think I have good quality USB cables....

    Cheers,
    Tim
     
  11. Doug

    Doug Staff Member

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    People don't realize how fussy USB 2.0 is. If there is an impedance mismatch you will get electrical reflections that degrade the signals. Cable quality has to be excellent. Hubs are also an issue.
     
  12. Tim Carruthers

    Tim Carruthers Cyanogen Customer

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    Hi Doug,

    Thanks. So, I have an Icron Ranger hub with cat6 cable through the mount and then expensive - gold plated USB cables running off the Icron to the STX AOX FW RGH + Pyxis rotator+ Focuser. Sounds like some ferrite chokes wouldn't hurt.

    :)

    Regards,
    Tim
     

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