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(I hope this is not regarded a "shopping or buying recommendation" question - in my opinion it is not).

In order to split a larger project into more tractable and flexible sub-blocks I create two PCBs (motherboard/daughterboard).

This is a very common setup in the digital world (PAM2-PAM4) and there are high-end connectors for high speed digital (Samtec, FMC).

However, in my project I also need to "flawlessly" connect RF signals between the boards. In summary - these connections have to be made between the boards:

  1. ground plane, power (easily solved)
  2. high speed digital, ~100 MHz, ~50 I/Os (easily solved with Samtec connector)
  3. Analog clock, ~200 MHz
  4. 4x RF signals, up to 3 GHz

For 3-4 I would use SMA connectors - if it would not be a daugher board. However, I would prefer a more solid connector - ideally just one for 1-4.

I found RF board-to-board connectors but they tend to be single signals. Even if I could get an RF connector with multiple signals, physical alignment together with an FMC connector for the digital lines is painful.

So I am really looking for a board-to-board connector solution that faithfully connects 1-4.

If I can use high-speed digital connectors also for the RF signals - are there any issues or disadvantages using them compared to SMA, e.g. insertion loss, return loss/matching, isolation? Or stated differently: Why are these connector usually not used for carrying RF signals?

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The same connectors that can handle multi-gigabit digital signals can handle gigahertz RF.

In fact, gigabit digital signals are generally more challenging to handle, since they tend to have much wider bandwidth than RF signals.

However, if you think you will find any connector with 0 insertion loss, whether for RF or digital, you will end up disappointed.

Why are these connector usually not used for carrying RF signals?

  1. Because in general RF is a much smaller part of the overall electronics industry than digital is.

  2. Because most RF applications don't need dozens of signals in a single connector.

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  • \$\begingroup\$ That would be the most perfect and desirable scenario. But could you elaborate on this? What about impedance mismatch in this connector vs. SMA/RF connector? What about ground discontinuity? Insertion loss is not the biggest concern ... but what is it compared to SMA? (I think SMA connector is 0.1-0.2 dB) \$\endgroup\$ – divB Feb 6 '18 at 8:51
  • \$\begingroup\$ @divB, please edit your question to ask what you want to ask. \$\endgroup\$ – The Photon Feb 6 '18 at 15:42
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    \$\begingroup\$ @divB, The FMC connector is just one example of the SEARAY product line. If you go to the SEAF page on samtec.com, you can find numerous test reports showing the S-parameters for different combinations of SEAF and SEAM parts, single-ended and differential signals, and recommended signal and ground pin arrangements. \$\endgroup\$ – The Photon Feb 6 '18 at 17:27
  • \$\begingroup\$ Great pointer! I updated my question - hopefully a little bit more clear. I guess the one thing I am missing is: Is there anything that I am not thinking of that could cause problems compared to using SMA connectors? Or stated differently: Why does SEARAY seem to be used exclusively for digital (PAM2/PAM4) signals and never for carrying RF signals? \$\endgroup\$ – divB Feb 6 '18 at 19:38
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Since you mentioned Samtec, they have custom solutions with 50Ohm coaxial connections.

A more standards-based approach would be to use SMP or AFI connectors.

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