limeRFE and limeSDR-USB

Projects
1

I bought the limeSDR-USB TypeA and the limeRFE housed in the acrylic case as can be seen on Lime Microsystems | LimeRFE

I also bought a 10 pint ribbon cable (adafruit 1675) to connect the two boards together via the 10 pin connectors.

The limeRFE PCB is oriented in such a way that the heatsink is on top. However, the 10-pin connector on the limeRFE is on the opposite side of the heatsink so it is now on the bottom.

But the 10-pin connector on the limeSDR USB is on top.

So the 10-pin connectors on the two PCBs are on opposite sides.

I must twist the adafruit 1675 ribbon cable 180 degrees to be able to connect the limeSDR USB and the limeRFE. But not only that, the 10-pin connector on the limeRFE is placed extremely close to the corner of the PCB / standoff of the acrylic case so in addition to twisting the cable 180 degrees I have to bend it around the corner post/ standoff.

I read someone suggesting that doing a z-fold to get around the corner post is the way to go, but then someone else said that performing a z-fold to get around the corner post and then twist the ribbon cable can damage the ribbon cable, because the internal wires are very thin.

Overall, I don’t understand this design choice and would love to know what lime micro systems were thinking in terms of how a user would reliably connect a limeSDR-USB to the limeRFE with the 10-pin connectors being on the the opposite sides of the two boards.

Why could they not have put the 10-pin connector on the same side as the heat-sink ?

In the LimeRFE_1v31_QuickStart.pdf the photo of the limeSDR and the limeRFE shows the limeRFE as a naked PCB, with the heatsink down, and with the 10-pin connector on the same side as the 10-pin connector on the limeSDR mini, that is on the up side, which makes perfect sense to me, except having the heatsink resting on a table top does not seem entirely sound either. What was the thinking here ?

2

The acrylic enclosures are designed with lab use in mind and in such cases USB connectivity is envisaged. Whereas the SDR FPGA GPIO control of LimeRFE option was intended more for use in solutions where both would be integrated into a single enclosure supplied by the customer.

3

Thanks for the reply. Since I am new to electronics packaging I wonder if you answer two quick follow-up questions.

  1. Metal or Nylon Hardware
    I am planning to house the limeRFE and the limeSDR plus a bi-directional coupler (so I can utilize the SWR subsystem of the limeRFE) in a Hammond Extruded Aluminum case ( Access to this page has been denied. ), and I wonder, should I use metal or nylon M3 screws, nuts, and standoffs to mount the limeSDR and the limeRFE in the enclosure ? My thinking was I should use metal hardware to use the enclosure as a ground plane to get the best EMI shielding (Faraday Cage) but a colleague of mine said I should use nylon hardware, and that we should rather isolate the boards as much as possible to prevent shorts, etc. The acrylic cases for the limeSDR and the limeRFE comes with nylon hardware. So for comparison I looked up Ettus Research and they seem to be using metal hardware in their SDRs. I plan to use external antennas mounted to the case via bulkhead connectors so I am thinking this means the enclosure will already be part of my RF ground system, so I should use metal hardware.
    What do you think ?

  2. Cooling
    a) If I use a fan, do I need the heat-sink on the limeRFE or can the heatsink be removed to save space. Not sure if this is a good idea.
    b) Can you recommend a cooling solution, a specific fan for example ?

  3. We have been discussing if the two PCBs could be vertically stacked or need to be mounted side by side. And if they can be mounted vertically, is it recommended to order them in a certain way, that is, should the limeRFE be placed above or below the limeSDR, and what is the recommended distance separation between the two PCBs ?

Anyway, grateful for any tips. I am a complete newbie to this type of work.

Thanks.

4

1. Metal or Nylon Hardware
If you do not want to pay special attention to possible shorts between metal parts at different voltages, it is safer to use nylon hardware (screws, spacers, and separators).

If you plan to use a metal enclosure, special care must be taken. The enclosure should be large enough to prevent parasitic coupling with RF lines on the board, but also designed to be small enough so that any self-resonant frequencies are far away from the operational band.

2. Cooling
We do not recommend removing the heat sink.

If you decide to do so anyway, we advise paying attention to which amplifier on the board you are using and installing a fan for that specific area.

We cannot recommend any specific fan because we have not tested the board under such conditions. Please test your assembly (LimeRFE without the heat sink and with a fan) in a controlled environment where the board can operate for a longer period under supervision.

3. Stacking Boards
The general rule is that the separation between RF boards should be large enough to minimize coupling.

This separation can be reduced if the LimeSDR board is placed below the LimeRFE board. The LimeRFE board has most of its RF traces on the top layer (heat sink is in bottom layer).