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I'm trying to understand an antenna impedance matching / balun network that I came across in the receiving signal path in the schematics for the STEVAL-FKI868V2 S2-LP Evaluation Board, shown in the picture above.

RXP / RXN are the balanced LNA input pins of the transceiver IC, which has an input impedance that is given in table 17 in its datasheet: S2-LP Input Impedance

The anennta's characteristic impedance is assumed to be 50 Ohms. Now obviously, C14, C17 and C16 are just DC blocks and large enough to be neglected. C13, C18, L3 and L5 form an LC balun. C15 is not fitted.

But what exactly are L4 and C38 (marked red) for? How did the designers come up with their values? Is there a name for this kind of circuit?

Especially C38 looks odd to me, it makes the whole balanced line asymmetric. Any hints are greatly appreciated!

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  • \$\begingroup\$ L5C38 act as a pass band while L5 L4 C13 must be a Tx trap filter. Run it on a SYM in Falstad.com/afilter \$\endgroup\$ Feb 21, 2020 at 0:50

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They're for matching as well. The apparent asymmetry could be due to layout considerations, but it's worth noting you're going from unbalanced to balanced, so some asymmetry might be expected on general principles (note that ground planes have a different inductance than traces).

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