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ground loop

Discussion in 'Electronic Basics' started by [email protected], Aug 8, 2007.

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  1. Guest

    I'm trying to understand a system with rf circuit.

    I found that there is a concept called "groundloop"

    I searched it at the Wikipedia, but I don't think I now know it.

    How can I explain the concept?

    What is floating potential?

    below is the original statement I'm trying to understand

    "
    Ground loops can add excess noise and therefore efforts were made
    to avoid these throughout the whole system. As a consequence,
    many signals are transmitted differentially, the cable shields are
    typically only connected to ground at one end, many of the power
    supplies use floating potential, and isolated inputs with optocouplers
    are used for most digital signals. These precautions were only
    taken in the parts where the DC level is important, and not in the
    pure radio frequency (RF) circuitry.

    "
     
  2. Floating potential just means that the voltage has no direct
    connection to earth. A transformer secondary is a common
    source of floating potential. Of course, the static charge
    on a balloon that holds it to the ceiling is another.

    A ground loop is just a conductive loop that is made up, in
    part, by two separated connections to Earth. This is what
    you get if you ground both ends of a wire. The Earth
    completes a loop that connects the two ends through another
    path than through the wire. Any changing magnetic field
    that passes through the area enclosed by this loop will
    generate circulating current around the loop. The
    resistance, inductance and propagation delay of that current
    will produce voltage differences between the ends of the
    wire. So it you are using that wire as a zero volt
    reference for another signal at both ends of the wire, the
    measure of that signal with respect to this reference will
    be different at the two ends of the wire. The loop will
    have coupled energy from the magnetic field that passed
    through the loop into the signal, corrupting it to some extent.

    If that reference wire were earthed at only one end, there
    would be no loop, but at the other end, that "ground"
    reference may not have exactly the same voltage as the local
    Earth potential. So an isolated receiver that subtracts the
    reference voltage of the remote ground from the signal, and
    re references the difference to the local ground potential
    will break the loop and eliminate the corruption.
     
  3. Circa Wed, 08 Aug 2007 03:43:40 -0000 recorded as
    Differential transmission is another way of describing a balanced line,
    such as a twisted pair cable, where neither polarity of the signal is
    referenced to ground. Google "balanced line."
    This is now describing an unbalanced line, such as a coaxial cable, where
    the shield is referenced to ground at only one end of the line, in order to
    reduce the possibility of current finding a path through the shield. This
    statement could also refer to a technique of grounding chassis in a "star"
    pattern, meaning that there is a central point where connection to ground
    is made, and all chassis are isolated from each other except at the point
    where ground cables from those chassis are tied to the central point.
    Describes not grounding either end of a power supply. This way, current
    that is produced by the power supply does not return through a ground path,
    thus does not add to current through ground connections. The return path
    is separate from ground.
    An opto-coupler will isolate return paths for small signals, again keeping
    the paths for ground current to a minimum.
    I'm not sure what this means. It seems specific to the application. In my
    experience, these methods of isolation are used to minimize the effect of
    60Hz radiation on RF circuitry.
     
  4. Rich Grise

    Rich Grise Guest

    John, this is awesome! Even _I_ get it now! ;-)

    So, when's your book coming out? ;-)

    Cheers!
    Rich
     
  5. neon

    neon

    1,325
    0
    Oct 21, 2006
    lets start with floating potentials. Look at the sky during a storm now that i what a call floating potential. it is there just floating millions of volts just sitting there. you got now potential difference?. now if the lighting hit your house which is tied to mother earth but when that happens the house will experience a voltage dirfference because of the massive current flowing through it. electrical substation have the same problem. soluton tie a heavy copper cable to earth ground to pass the current without elevating the edifice to 1000 of volts. ground loops are caused by payin attention to where current flow. example you have a 40w amp and you tie the gnd to the power supply then you tie the same gnd to at the end to the preamp. so now whatever current the power amp use it elevate the gnd it becomes a signal for the preamp. that is a loop . best thing to do is tie each comonent path to the source the power supply. this is the basic at hi freq more things gets invoved. and yes a cable shiled should never carry any current for any reasons and should be tied at one end only the lowest gnd potential usualy the very input that you are trying to shield.
     
  6. Rich Grise wrote:
    (snip)
    Sorry, I haven't yet read nearly enough questions to force
    me to face all the stuff I still don't know.

    But there is nothing like trying to stuff more than you know
    about something into a single paragraph, to make you think
    really hard. That is what I get out of being here. I'm
    just getting close to understanding some other people's
    books, at this point.
     
  7. Guest

    Ground loop: You need to draw a picture and think about it, like this:
    Your (ancient) hi-fi amp is plugged into the wall socket. It has an
    earth in the mains plug.

    Your (ancient) tuner is also plugged into the wall socket. It also has
    an earth in the mains plug.

    You connect them together using a screened co-axial cable because...
    it is screened and you don't want any mains hum... but you get... lots
    of mains hum!?

    When you listen to the tuner with headphones (only) it is perfect.

    When you plug a microphone (only) into the amp it is perfect.

    You only get the hum when they are joined together by the coax.

    The reason is that both the (ancient) amp and the (ancient) tuner have
    an continuous earth between their coax screens, to their wall sockets
    so when they are plugged together a loop is created, an earth or
    ground loop via the coax, though the amp, through the mains loop,
    though the tuner back to the coax screen.

    Here is the hard-to-understand-bit: both the amp and the tuner have
    huge, bulky, mains transformers in them. Each transformer is radiating
    magnetic fields at mains frequency. These changing fields are cutting
    the ground loop. The ground loop now has changing currents induced in
    it.

    Any conductor carrying a current *must* have a voltage across it
    driving that current - otherwise there would be no current :)

    Therefore the coax screen *must* have a changing voltage (at mains
    frequency) between each end.

    So even with no signal in the coax's center conductor, there is a
    signal (mains hum) in the screen. And it gets amplified and comes out
    the speakers.

    The problem ceases by breaking the "gound loop" e.g. disconnecting the
    earth in the plug of the tuner.

    That's were the name "ground loop" came from but it applies to any
    loop grounded or not. But evidently it is a common error particularly
    in screens and earthing plans.

    *******************************************
    The twisted pair: suppose you plug a mike into the amp but you *still*
    get mains hum?!

    How can that be? The coax screen of the mike should prevent mains
    radiation getting to the centre conductor! The answer is that is
    *does* prevent radiation getting to it and *that* in itself, is the
    problem. Like this:

    Imagine the coax screen at the amp's imput as being zero (or put it
    another way, that you are standing on it so as far as you are
    concerned, you and it are always the same voltage i.e. zero) then
    mains radiation (and any other nearby source) will be inducing
    currents in the coax screen. These currents flow back and forth
    between you, and via the mike to the signal wire and thence to the
    amp's input - hence the mains pickup.

    The screen *is* the problem.

    An answer could be to use two plain wires instead of the coax - now
    this is the clever bit, any radiation is now picked up on *both* wires
    the same and currents flow back and forth in each IN THE SAME
    DIRECTION and the result is that they *cancel* each other out (you
    might have to draw a picture here).

    Evidently if both wires are laying straight then one wire might pick
    up a little more than the other if it is closer to the source of
    radiation so you avoid that by twisting them together to even out the
    likely pickup from all directions (as well as keeping them close).

    This twisted pair thing is usually done inside a screened cable. Your
    mike cable will have three conductors, i.e. two signal wires, and one
    screen.

    Robin
     
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