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A question about cross talk! :)

Discussion in 'General Electronics Discussion' started by PauloConstantino, Jan 30, 2017.

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  1. PauloConstantino

    PauloConstantino

    16
    1
    Jan 29, 2017
    Dear friends,

    I think I am getting cross talk on my circuit and I'd like your advice if possible. Basically it's a complex, dense circuit built on breadboards, and using jumper wires. This is a prototype circuit. It generates composite video on a television.

    The circuit works fine, but I've noticed that when the wires are close together, I get small black lines on some parts of the screen, but when I move the wires apart, the lines disappear. Is this cross talk? How does cross talk work? Is it mutual capacitance and inductance?

    Also, I am using the 74HC series of IC's. Should I ground unused inputs? For example on quad AND gates, if I'm only using 2 of the gates in the package, can I leave the unused inputs floating or should I ground them, even if they are not involved in the logic?

    Best wishes and thanks

    Paulo
     
  2. shrtrnd

    shrtrnd

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    Jan 15, 2010
    Sounds like you already know your problem.
    Shield your circuits, and if you're really serious about this, get them off the bread-board and onto a printed circuit board.
    Aside from stray currents within your circuit, your jumper wires are probably picking-up outside RFI.
     
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  3. PauloConstantino

    PauloConstantino

    16
    1
    Jan 29, 2017

    Fantastic thank you!! :)
     
  4. shrtrnd

    shrtrnd

    3,767
    487
    Jan 15, 2010
    Thank me if that solves all of your problems. Here's hoping your circuit design is not a part of the problem.
    Shielding is the key.
     
  5. OBW0549

    OBW0549

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    Jul 5, 2016
    Quite possibly. That is one of the possible symptoms of crosstalk.

    For relatively slow, low-power logic such as 74HC, capacitive coupling would be the predominant mode of coupling from one signal to another. Inductive coupling is possible in principle, but I doubt it would cause any problems all by itself.

    Basically, what happens when you have capacitively-coupled crosstalk is that a high-to-low or low-to-high logic level transition on one signal "bleeds over" onto another signal in the form of a narrow, brief "spike" of a few nanoseconds duration. This spike may or may not be large enough to cause problems; when it does, the problems can range from minor (such as your black lines or spots) to severe, such as false clocking of flip-flops, registers or counters, causing erratic operation.

    Connect inputs of unused gates either to ground or to Vcc. Unused inputs of logic elements that are being used (e.g., an unused input of a 4-input NAND gate of which only the first three inputs are used) should be connected to whichever logic level, ground or Vcc, that will not interfere with operation. Don't ever leave unused inputs floating.
     
  6. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    I think OBW explained that well.

    Cross talk can take two forms one will be dominant than the other, capacitive or inductive, but they are always both there. Which one dominates depends on the geometry of the circuit / wiring and circuit operation. It's quite a complex subject and very difficult to calculate accurately what's going to happen in a circuit. For PCB traces the dominant effect is inductive when they are side by side and capacitive when they are above or below each other. With a wire in a cable like a jumper wire it will have very low inductance and won't contribute much in the way of magnetic flux. As the rise time increases the capacitive coupling becomes even more dominant due to the effect of the inductance at increased frequency.

    Thanks
    Adam
     
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  7. HellasTechn

    HellasTechn

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    Apr 14, 2013
    What i say may be wrong but it sounds more like EMI to me rather than cross talk. Basicly we talk about the same thing in other words. this interfearence is caused by nearby cables.

    Putting your circuit on a properly grounded PCB with a good ground plane should eliminate the problem.
     
    Last edited: Feb 2, 2017
    davenn likes this.
  8. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Cross talk is a form of EMI.
    Thanks
    Adam
     
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  9. davenn

    davenn Moderator

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    Sep 5, 2009

    well yes .... but cross talk specifically refers to induction between adjacent cable pairs or channels within a communications system .... usually in telephone circuits
    and hence why in such long run cable circuits, cable pairs have twists in them.

    RF induction between various parts of a circuit is a whole nuther ball game and requires a very different approach to remedy
     
    Last edited: Jan 31, 2017
    HellasTechn likes this.
  10. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    That may have been the case 50 years a go before people started stealing specific terminology and using it for something else :). Unfortunately crosstalk is one of many. It is widely accepted and is printed in many EMC / EMI papers that crosstalk is a term used to describe EM interference from an aggressor source to a victim source by either capacitive or inductive coupling. They very rarely speak about the original term cross talk these days.

    Thanks
    Adam
     
    HellasTechn likes this.
  11. davenn

    davenn Moderator

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    Sep 5, 2009
    and still is today !!!


    That's not my fault if it isn't used correctly
     
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