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I am working on a design with multiple ESP32 on one board. The selected ESP32 does not have internal antenna, instead it has an IPEX connector for external antenna. On the device we would like to have SMA connectors for external antenna.

The RF path would be: ESP32 IPEX -> 50 ohm coax -> main board ipex -> mainboard SMA connector -> external antenna

Questions:

  1. Will I need any matching circuit on the main board?
  2. Will there be any interference between 50 ohm coax "air" wires from multiple modules?
  3. The mainboard has lot of components and has a small form factor and it will be a single side assembly. Therefore there are not much space to arrange nicely the ESP32 modules. In one of the concept I can put the modules to the side of the board but then the SMA connectors will be far. In the second concept I can put it as close as possible, but not all modules will be at the side of the board. The ESP32 design guideline does not state anything about placing a module, with external antenna, into the middle of a board. Or placing them close to each other. Will I have problems with any of the design below? Which concept is better?

design 1 design 2

Any recommendation for my issues are welcomed!

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    \$\begingroup\$ Assuming those are UFL to UFL cables, then I would not recommend it. I had a similar design and it got lots of problems both with signal losses and with EMC. Also, UFL is a very bad connector mechanically and breaks if you happen to sneeze in the same room. Meaning the less UFL connectors you have, the better. A less bad solution is to use UFL-to-SMA cable assemblies where the SMA is panel mount. \$\endgroup\$
    – Lundin
    Sep 1, 2021 at 11:44
  • \$\begingroup\$ Thanks for the comment. Yeah I think change the concept and leave out the UFL to UFL cable. And use UFL to SMA connector as suggested below. \$\endgroup\$
    – D_Dog
    Sep 2, 2021 at 9:46

2 Answers 2

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Will I need any matching circuit on the main board?

Yes Tx port must be matched to traces and antenna impedance.

Will there be any interference between 50 ohm coax "air" wires from multiple modules?

Yes it is possible, Tx on one may interfere with Rx on another with > 60 dB difference in signal levels. Network Analyzer Crosstalk tests or BER testing will tell how much interference is tolerable on intended routing, grounding, and cable type.

Semi-rigid copper cable is better, just as done in spectrum analyzers with small tubing.

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Will I have problems with any of the design below?

Yes Clock path, data esp. 80 MHz data harmonics, and RF must be controlled impedance. Guard tracks or vias must be used to reduce crosstalk.

Also, all RF and high speed data must be shielded from interfering with crystal which could control RF frequency with noise sidebands. Same for Vdd ripple for high current data bursts , must keep < 80mVpp worst case conditions.

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Since you're not using an on-board antenna, then you don't need to worry about placement. Put the modules where you want. As for #1 or #2 - whatever works best for you.

As for interference between the coax, that shouldn't be too much of an issue. You'll need to pay atention to the layout around the pcb SMA connectors to ensure your tracks between the IPEX and SMA connectors are 'fenced' ie surrounded by vias to contain the RF and that the coplanar waveguide is dimensioned for 50Ohm.

It might be easier/cheaper just to use IPEX->SMA bulkhead cables assemblies. Your call.

I should ask why three WiFi modules?

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  • \$\begingroup\$ I cannot tell you the application, but I can give you an example for a different one. If you would like to maintain three different network on three different rooms you can use one router with three antenna. The antennas can be placed separately. And still you can enable/disable each network dynamically. I let your imagination go through the use case :) And thanks for the comment! \$\endgroup\$
    – D_Dog
    Sep 2, 2021 at 9:51

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