LimeSDR USB 1.4: Circuit Board Layers/Stack up and length of FX3_DQ/CTL wires

Hello everyone,

I really like the discussions here so far, this forum is great and does a great job in helping to better understand the LimeSDR board.

I would like to adjust the pcb layout and have it manufactured at a later point. In order to not influence the HF related material stack up and impedance’s as well as signal paths I have some short questions.
(LimeSDR board rev. 1.4)

  1. Layer materials
    Unfortunately I was not able to find the exact material of all layers. If I open the Layer Stack Manager I can see that the upper 2 cores and the Prepreg in between are specified as FR-4. Do you know what exact type of FR-4 material is currently used on the different layers? (I haven’t found the specification of the remaining layers in the Stack Manager.)

  2. RF layers
    As far as I understand the HF parts are impedance controlled and solely placed on the top and bottom layer. Does this enable me to adjust the thickness/material stack up of the other layers in order to ease the manufacturing process for my distributor?

  3. Via’s
    As far as I could investigate all via’s are through holes (which makes it easier to manufacture the board) and there are no buried via’s or similar included in your design. Just want to check if I see that correct? :slight_smile:

  4. Length of DQ and CTL wires (FPGA to FX3)
    The PCB note in the schematics says that the FX3_DQ and FX_CTL wire lengths shall match. As far as I understand this is because of the timing requirements of the signal pairs. However, the length of DQ and CTL type wires are different, although all DQ (and also CTL) wires themselves are nearly the same length. My question is where the requirement comes from and if you might which delta in wire length is tolerable? (I would like to change the signal paths for my redesign.)

Thank you very much for your help! :slight_smile:
Best regards

1 Like

Paging @Zack :slight_smile:

Are there any infos available @Zack?
I am also wondering about the choice of the board material and unusual thickness for the top and bottom layer.

Thank you very much. :slight_smile: