I'm trying to build a tunable bandpass filter for 140 to 1500 Mhz, and the design requires that I move the signal from a PCB with an SMA connector to another mounted about 3mm above it. I'm using board to board connectors for the other connections, but I wasn't sure whether it was possible to match 50 ohm impedance (or to get close enough) with a connector like that.

My question is, how should I move the signal from one pcb to the other with 50 ohm impedance? Is such a connection even possible in that little space, or is my approach fundamentally flawed?

  • \$\begingroup\$ Do you have any freedom to change the board to board distance? \$\endgroup\$ – The Photon Feb 18 '19 at 4:24
  • \$\begingroup\$ I can adjust it by about a milimeter each way; but it has to fit into an aluminum case that I can't change (which is why I needed the two layers to begin with) \$\endgroup\$ – user201537 Feb 18 '19 at 4:29
  • \$\begingroup\$ How many other signals are you connecting between these boards. There's a number of mezzanine connectors capable of handling ~2 GHz signals with 3-mm stack height, but these typically have 40 contacts or more, so you might as well route your other signals over the same connector with your RF signal. Prices will be higher than 100-mil spaced headers, but probably below $10. One possible issue, these will be designed for differential rather than single-ended RF signals. \$\endgroup\$ – The Photon Feb 18 '19 at 5:32
  • \$\begingroup\$ I only have a handful of GPIO pins running between the two, but it would be more difficult to fit a single ended to differential converter on the lower board, only to convert it back to an unbalanced signal for the amplifier \$\endgroup\$ – user201537 Feb 18 '19 at 5:42
  • \$\begingroup\$ at 1500Mhz the wave-length is 200mm, why are you worried about 3mm? \$\endgroup\$ – Jasen Feb 18 '19 at 7:55

You have perhaps two possibilities:

  1. Use a short piece of small diameter \$50\Omega\$ coax cable, folded and soldered directly to the boards: however, while nice for a prototype, this solution poses problems from the point of view of engineering and production.
  2. Use small RF/Microwave connectors like this one: I use this kind of objects from the year 2000 and, while the devices I use are designed for cable to board connection, there are devices designed for board to board connection, with board distances as low as \$2.5\mathrm{mm}\$. Also, there are many producers who offer such kind of devices.

Edit: I updated the answer by adding a link to the mating connector, precisely a plug, in order to have a board to board plug/receptacle couple of connectors. I stress the fact that the device shown are not exclusively produced by a single manufacturer, but there are several high quality producers that offers such devices.

  • \$\begingroup\$ Thanks for your response! I only have to build one, so manufacturing isn't a big issue. By 50 ohm wire, do you mean coax like LMR-100? \$\endgroup\$ – user201537 Feb 18 '19 at 20:02
  • \$\begingroup\$ Yes, something similar, LMR-100 could be fine, depending on the space you have at hand. There are of course lower diameter cables: the ones I have used jointly with the SMD coaxial connectors described above are similar to this one: when needed for the construction of a prototype, I cut away a few centimeters out from the connectionless side and used for doing the work. It is something quick and dirt, but it nevertheless works fine. \$\endgroup\$ – Daniele Tampieri Feb 18 '19 at 20:19
  • \$\begingroup\$ @user201537 I changed my answer accordingly. \$\endgroup\$ – Daniele Tampieri Feb 18 '19 at 21:18

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