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I am looking to use a SX1280 LoRa IC with a Abracon ACAG0801-2450-T antenna. However, I am a little confused about the impedance matching.

The SX1280 reference design uses this as the matching circuit:

enter image description here

With component values: enter image description here enter image description here However, it does not provide any specifications or part for the antenna.

The antenna uses this as the matching circuit:

enter image description here

From the below diagram from the datasheet, it seems as if the pi matching network matches to 50 ohms.

enter image description here

Should I use both these matching networks connected to each other, just one of them, or some other combination?

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  • \$\begingroup\$ I expect ANT1 in your schematic implies a 50 ohm antenna but, SX1280 doesn't say so explicitly and, your antenna data sheet doesn't say it is matched to 50 ohms with the 4.3 nH and 1.5 pF capacitor. Bit of a dilemma really. \$\endgroup\$
    – Andy aka
    Commented Nov 28, 2022 at 20:57
  • \$\begingroup\$ What part does the reference design use for ANT1? \$\endgroup\$ Commented Nov 28, 2022 at 22:34
  • \$\begingroup\$ @Andyaka I have edited the question with a few more details. From the layout diagram on the antenna datasheet, it appears that the the inductor and capacitor match the antenna to 50 ohms, but I am not 100% sure. \$\endgroup\$
    – jb0
    Commented Nov 29, 2022 at 0:46
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    \$\begingroup\$ @Jasen The datasheet does not specifiy a part or specifications for ANT1, but in reference designs it is a connector such as this 50 ohm connector. \$\endgroup\$
    – jb0
    Commented Nov 29, 2022 at 0:48

1 Answer 1

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The SX1280 reference design doesn't provide an antenna matching circuit because each antenna is different, and there is no "generic matching". If one uses a SMA connector and good sleeve dipole antenna, no matching is needed for example.

The matching circuit provided in the reference design achieves 3 things:

  • match the output impedance of the SX1280 (not 50 ohm) to 50 ohm,
  • filter TX harmonics,
  • and isolate the DC voltage present on the RFIO from the DC voltage of the antenna, which can be 0V in case of a "shorted" antenna like a PIFA, with C3 (100pF value is a giveaway).

Keeping in mind that you may have to adjust the antenna matching circuit component values depending on your exact PCB and product (PCB size, presence of plastic enclosure, etc, will influence your antenna behavior), I see 3 ways to address your question:

  1. Keep the circuits separated: keep the reference SX1280 matching circuit as-is, just add the antenna matching circuit "right" of C3. Bonus: by removing C3, you have a "50 ohm point" where you can solder a coax pigtail to either do conducted measurements on the SX1280, or VNA measurement of the antenna + matching.
  2. Same as above, but use the integrated 2450FM07D0034 from Johanson Technology (the DC blocking is build-in) + the antenna matching circuit.
  3. Combine both matching circuits: the antenna you've picked is not DC-shorted, so C3 can go away, you can combine C2 with the 1.5pF antenna matching capacitor (2.7pF total) and put the 4.3nH inductor in series. You won't have a "point" where impedance is 50 ohm, so fine-tuning the antenna with a VNA might be a bit more complicated.
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  • \$\begingroup\$ Thank you! I had no idea parts like the 2450FM07D0034 existed. \$\endgroup\$
    – jb0
    Commented Jan 10, 2023 at 3:27

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