Questions about routing LVDS signal at 125Mhz

Discussion in 'The Projects Forum' started by maxg31, Mar 14, 2018.

  1. maxg31

    Thread Starter New Member

    Mar 13, 2018
    2
    0
    Hello everybody ! I'm a young technician without a lot of experience in routing signals a then I have several questions to ask you ...

    Here is my project : I have to convert and distribute a 125MHz clock in lvttl to 4 systems which accepts only lvds (Redpitaya). There will be around 40cm between the source and the receivers.

    I find this IC from ti to convert lvttl in lvds : http://www.ti.com/product/SN65LVDS105

    Then I have to make a pcb to route the signals and here are my questions :
    - How can i calculate the width, the thickness and the separations between the traces to get a 100 ohm line ? Does the frequency matter in this calculation ?
    - I saw odd and even mode for differential lines impedance, which one matter for the impedance matching ?
    - Is it necessary to connect the gnd of my pcb to the systems which receives the lvds signal ?
    - I thought make a 2 stacks pcb one for the signals and the other for gnd to make microstrip, is it the right solution ?
    This is my first conception for "high" frequency signals, i'm a little bit confused between all the parameters which matter so if you can help me I will be very thankful.

    Ps : I am from france then forgive my bad english :D
     
  2. tribbles

    New Member

    Jun 19, 2015
    26
    2
    I've routed USB3 signalling quite happily, which runs at a much higher frequency than 125MHz!

    You need a microstrip differential calculator - such as https://www.allaboutcircuits.com/tools/edge-coupled-microstrip-impedance-calculator/

    There's a number of parameters, and generally, you just change the numbers until you get the right value. Some of them you'll need to get from your PCB manufacturer - such as the dielectric, trace height, and layer thickness (i.e. the thickness of the board between the microstrips and the ground plane).

    In this particular calculator, trace height is the "Trace thickness", layer thickness is the "Substrate thickness", and the dielectric is the "Substrate dielectric".

    The problem is that when you start playing with the figures, then you may find something which will make 100 ohms, but you'll find that the PCB manufacturer can't make it because the traces are too thin, or the separation between the traces are too thin. So you'll then need to make it fatter, or larger separation. I normally start off with my manufacturer's minimum trace width, and then see what the separation needs to be. If it's too small, increase the separation, and then see what you need to do to the trace width to make it fit.

    The value you're after is the 'Differential' impedance.

    The ground will be connected to both systems.

    And a 2-layer PCB will generally have a thicker substrate - it is possible to do it with 2 layers (I've seen plenty of USB2.0 boards [90ohm] which are two-layer). Personally, I would be using 4-layer.

    Example values which I've taken from the PCB manufacturer I use:

    - 35um trace thickness
    - 1.5mm substrate height (based on a 2-layer board)
    - 0.65mm trace thickness
    - 0.15mm trace separation
    - 4.7 substrate dielectric

    Gives 100ohm differential impedance.

    Note that you don't need to have the whole trace at these values - if you have a chip with close pins, then you can have a short stub where you have whatever you need to fit the chip, and when you've got far enough away from the chip, then you go to the 'right' settings.

    If you have a 4-layer board, then the trace width can be thinner (a 0.25mm substrate height allows a 0.335mm trace width).
     
  3. BR-549

    AAC Fanatic!

    Sep 22, 2013
    4,007
    1,030
    Is the source and all receivers on the same board?
     
  4. maxg31

    Thread Starter New Member

    Mar 13, 2018
    2
    0
    Hello, thanks a lot for your responses !
    @tribbles you gave me a lot of usefull informations it is great !
    @BR-549 No the receivers and the source are wired and it will be around30-50cm between them.
     
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