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I’m trying to understand how the matching network functions on the radio side of a transmission line. As an example, here’s an excerpt from the Nordic nRF52840 DK schematics. As I understand it, when the radio is receiving, matching network B transforms the antenna impedance to 50Ω to match the 50Ω microstrip transmission line “TL”. Matching network A then transforms the radio to a 50Ω load for maximum power transfer. So far so good.

What happens when the radio is transmitting? The transmitter output impedance would be low to minimize power loss. How does it get matched to the transmission line? How do we avoid having to toggle between two different implementations of matching network “A”, one for receive, one for transmit?

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    \$\begingroup\$ Where did "the transmitter output impedance would be low to minimize power loss" come from? Is there a source of information for that? I'm not exactly sure that's true. \$\endgroup\$
    – mrbean
    Nov 14, 2023 at 15:09
  • \$\begingroup\$ @mrbean If the output impedance was inherently, say, 50Ω, then that means half the power would be dissipated as heat by the transmitter. Seems like a bad design. \$\endgroup\$ Nov 14, 2023 at 15:20
  • \$\begingroup\$ Have you read the datasheet and looked through any of Nordics material on that part? \$\endgroup\$
    – mrbean
    Nov 14, 2023 at 15:44
  • \$\begingroup\$ The issue is that you are expecting the circuit to meet incorrect assumptions are wondering why things don't make sense. The issue is that the datasheet and white paper material was never read and the information was never processed. I would suggest reading the material. The answer is in the datasheet and material from Nordic \$\endgroup\$
    – mrbean
    Nov 14, 2023 at 16:25
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    \$\begingroup\$ Sheesh. I love the help offered in these forums and I sincerely appreciate you taking the time to answer me, but the snide remarks that frequently accompany these answers are counter-productive. \$\endgroup\$ Nov 14, 2023 at 16:35

2 Answers 2

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It may be that the transmitter output impedance is low, but it'll usually be designed to drive a 50 ohm load through its specified matching network. Similarly, the receiver input impedance may not be 50 ohms (power transfer isn't necessary), but it'll be optimized to sense signals from a 50 ohm source through the same network. So, in the design, you present the transceiver with 50 ohms to get the expected performance.

The reason for 50 ohms is that the real challenge is to get the signal through the transmission line. While transmission line impedance is theoretically anything you want, in practice it gets exponentially difficult as you depart from 50 ohms.

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