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Let's say I have a microcontroller with an antenna output. The characteristic impedance of the pin for the antenna output is x+jy. I want to then connect it to a 50-ohm antenna.

This is the part that does not make sense to me when I see certain PCBs. If we do not know the characteristic impedance of the MCU's antenna pin, then we should place the matching network (pi filter) as close as possible to the MCU's antenna pin. From my understanding the impedance at C2 should be 50 ohm, hence we try to keep the matching close to the MCU's antenna pin because it does not matter what the characteristic impedance of the MCU's antenna pin is due to the trace from the MCU's antenna pin to C1 is so short. Now since we know at C2 we have 50 ohm, it is very easy to impedace match the trace to 50 ohm.

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However, I sometimes see the following PCB setup below. Now to me, this does not make since. If we do not know the characteristic impedance of the MCU's antenna pin then the trace from the MCU's antenna pin to C1 cannot be determined but I see many PCBs pace the matching network close to the antenna rather than the MCU's antenna pin.

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Basically in a nutshell, we can determine and control the impedance out of C2 but not from the MCU's antenna pin to C1, so why place the matching network close the antenna rather than the MCU's antenna pin?

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  • \$\begingroup\$ I'm not an RF person but I would imagine that the MCU's datasheet would have some guidance on design for proper placement. \$\endgroup\$
    – Colin
    Feb 27 at 16:13
  • \$\begingroup\$ Since you don't know the Z0 of the MCU and can't easily match the traces to it, place the network as close as possible to the MCU pads. \$\endgroup\$
    – Lior Bilia
    Feb 27 at 16:29
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    \$\begingroup\$ I agree with Lior Bila - close to the MCU output pin. But you really can't design the matching network if you don't know the Zo of the MCU. Note that if everything is 50 ohms (MCU output, trace, connector, antenna), you really don't need a matching network. \$\endgroup\$
    – SteveSh
    Feb 27 at 17:00
  • \$\begingroup\$ You have seen "many PCBs" do this, sure, but do you know, in each instance, that their transceiver ports are also impedance-matched or otherwise? You seem to be making the assumption that they have the same impedance as your present case, which won't be true in general. \$\endgroup\$ Feb 27 at 17:23
  • \$\begingroup\$ @TimWilliams that is true. Can I ask are there any benefits or drawbacks on where you place the matching network? For example either close to the MCU pin or close to the antenna? \$\endgroup\$
    – JoeyB
    Feb 27 at 19:53

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Match the components at each end of the line to 50 Ω, both the MCU AND the antenna. If one or both are already 50 Ω, or very close, then there's less work to do.

There are two aspects to matching

  • getting the real part of the impedance right
  • getting the impedance real

An MCU pin will often have a bond wire internally from die to lead frame, meaning that the output is inductive. This will need some matching at the pin, at least a capacitor.

If you go straight to a transmission line on PCB, then the correct match would only be obtained by a mismatch at the antenna being transformed through the length of line into the appropriate reactance at the MCU pin. This is bad practice, as you now need an accurately known length of line, as well as the correct antenna mismatch, or antenna match tuning. You should have the freedom to change your length of line, or to change the antenna. A mismatch through a long length of line has a much small bandwidth than one that's electrically close.

It's not mandatory to run all lines as 50 Ω, but it does make life easier if you do. You can break the line and connect your test gear directly, measure the antenna and MCU connections to the line directly, change the length of the line.

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