Ground loop considerations and elimination

Discussion in 'STF Series CCD Cameras' started by funkmeisterfred, Jan 31, 2018.

  1. funkmeisterfred

    funkmeisterfred Standard User

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    Hi all -

    In my endless quest to optimize my system, I'm looking for some information to help me devise a solution to fully eliminate ground loops inherently present over USB connections to laptops/PCs.

    I hope someone can advise:
    • What is the max current draw requirement over the STF-8300M's USB port?
    • Can I expect the STF-8300M to operate over a USB2.0 High-Speed (480mpbs) digital galvanic isolator?
    • Would you expect that doing so would eliminate connectivity issues (such as "TX TIMEOUT") and/or eliminate some read noise?
    The background to my request is that on some occasions, I experience timeout errors during exposure download and also have seen odd noise patterns (less often). I should note that I set up and tear down fully for each of my sessions, image exclusively with a marine deep cycle battery, and image in a huge variety of usually humid weather from -15C to 35C. Though my equipment is the same each time, there may be other small variability in my non-permanent setup.

    Thanks for any information or feedback you can provide.

    Best,
    ~Rick
     
  2. Doug

    Doug Staff Member

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    The STF does not draw power from USB.

    I can’t really comment on the isolator; you’ll just have to try it.
     
  3. funkmeisterfred

    funkmeisterfred Standard User

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    Thanks for the quick response Doug. I may give this a go, and will report back here if I have any notable findings.
     
  4. Colin Haig

    Colin Haig Staff Member

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    Rick, I think we may have discussed this a bit on another forum. It will be interesting to hear what results you get with the isolator.
    A rough schematic of your setup might be of interest as well.
     
  5. funkmeisterfred

    funkmeisterfred Standard User

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    Hi Colin - This is the first I've posted on a forum about it, but I'm not surprised to hear there are other conversations. Ground loops have certainly plagued many users of various gear for some time, but rarely is there a "solution" other than "eliminate anything in between like hubs, etc." I'm not convinced that direct connections are immune to an acceptable level for some of us, either. Depending on how the PC/laptop regulate the USB lines to the ports and taking further consideration to resistive drops over cables, loops could easily still be present. I'm sure this is why we also see suggestions to not to use front-panel ports (which have additional cabling (i.e. resistance) on them) and are advised against long cable runs.

    My laptop may just be particularly bad in this sense with occasional transmission timeouts and undue noise, but loops are the only culprit I can imagine. I have the STF-8300M main power and laptop charger direct connected to my marine battery and a 6ft USB cable to the laptop. I will provide a more detailed schematic if/when I am able to test a solution.

    It seems to me that the only solution is true electrical isolation on the cable. Many low/full speed devices exist for this, but the availability of USB2.0 high speed isolators is very limited and generally expensive. I may be able to land one at a good price and be able to test this out, however. I will certainly post results here if/when I obtain them.
     
  6. Colin Haig

    Colin Haig Staff Member

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    You may find this little article I wrote helpful. It covers some of the issues with disconnects etc. It's over in the public area of DC3 Dreams ACP forums:
    http://forums.dc3.com/showthread.ph...-on-Windows-Preventing-disconnects-and-errors
    If you try changing the Windows Power Management, USB Selective Suspend to OFF as per the article, you may find your disconnects go away.
    If you still suspect ground loop, a few questions further:
    What mount / scope do you have?
    The "classic" aka original Meade LX200 scopes had a current meter built in that was handy, but it offset the ground potential of the entire mount by several tenths of a volt (0.6V if I remember right)

    Are you using a DC to DC convertor for the laptop? e.g. a Best Buy / Rocketfish 12V auto to laptop 19V adapter
    or are you using a +!2V DC to 120V AC inverter? And then using the laptop power off that?

    Dew heaters and controller?

    I run my backyard observatory off 12V solar, and have learned a thing or two about ground loops etc. They are usually not the problem with disconnects and timeouts, unless the situation is particularly egregious.

    Maybe a suggestion that might work:
    Since you're running off a marine battery, am assuming this is a portable type setup.
    One approach that might work is to go from the battery to a West Mountain Radio Rig Runner 4004U or similar model. They are fused and have bypass caps, and serve as a good "common ground" for portable setups. Then you connect everything in to it. e.g. laptop auto convertor, camera, mount, etc. Run individual DC wires from each device to the Rig Runner, preferrably 12AWG for the heavier loads like the camera, laptop, and mount.
    This will provide the lowest resistance path for everything, and will ensure that the laptop isn't "floating" above your "system" ground.
    If you're running a DC to AC inverter, this can lead to big ground issues with floating neutral/floating ground on the AC side, not necessarily at ground potential of the DC side. It depends on the quality and design of the inverter.
    If there is a chassis ground point - like a screw on a VGA connector you might put a voltmeter between it and the system ground, and see what it comes in at. I had one device that used to float at +70V due to improper internal grounding. Got zapped a couple times until I figured out what was going on.
     
  7. funkmeisterfred

    funkmeisterfred Standard User

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    Hi Colin -

    Many thanks for your feedback and interest in this - it is much appreciated!

    I'm not by my laptop at the moment, but I will check the USB power management setting. If it's not set as you indicate, I will certainly evaluate this before putting money into another solution. This may well be a fix or improvement over my biggest concerns.

    I'm using a CGEM mount for this portable setup. No connections here other than the expected ones to the PC (serial) and to the battery.

    Dew heaters are controlled by the Kendrick dual channel controller, powered also by the battery.

    The laptop (Samsung) is fed by a DC-DC converter (12V to 19V) to the battery as well.

    The last power connection is to the focus controller, which is also a straight DC connection to the battery.

    I use no AC in the system - I'm very power conscious and avoid inverters like the plague :) They absolutely cause problems (not to mention their poor efficiency)!

    I've considered a Rig Runner, but was not convinced it would make an electrical difference except for shorter cables and cleaner routing. As it is, I use a home-brew box on the battery utilizing positive (fused) and negative terminal bars to distribute power - all equipment is hooked to these. However, I do not have bypass/filter caps in my setup, and will look into throwing appropriate ones to the box.

    I will also put my voltmeter on there next time I have the system set up to see if there is any appreciable voltage drop. Not sure why I haven't tried this already.

    Again, thank you for the suggestions!

    ~Rick
     
    Last edited: Feb 1, 2018
  8. Doug

    Doug Staff Member

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    Older Kendrick controllers make a LOT of line noise. I think they’ve improved them but worth checking.
     
  9. funkmeisterfred

    funkmeisterfred Standard User

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    Thanks for the thought Doug. May be even more reason to throw appropriate filtering caps in my setup. I'll certainly have a closer look at this.
     
  10. Colin Haig

    Colin Haig Staff Member

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    Also, some of the Kendricks have +12V on the outside of the RCA jack, not the centre pin. This = major hazard for shorts.
    You might try process of elimination as well - eg use the camera with the laptop alone, just to bench test, and then add stuff.
     
  11. funkmeisterfred

    funkmeisterfred Standard User

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    Yikes... what a poor design decision! I'll see how mine is configured (or just replace it with a home brew one).

    I appreciate the heads-up.
     
  12. Colin Haig

    Colin Haig Staff Member

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    Kendrick fixed these issues with newer models, but its worth checking.
     

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