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I am making my first transceiver using SX1261/2 and I am trying to understand the pinout of the antenna (input and output) pins.

At page 101 I see a typical schematic and I want to know more about why more than 1 pins are used for the output antenna (pins RFO and VR_PA) and same question for the input antenna (pins RFI_N and RFI_P) and why there is also -what looks like- a matching network between the -each- two pins?.
-Additionally, how is this "technique" called and how do I find the values of the components between those pins (or how do I figure the components) for my application?

Application Design of the SX1261 with RF Switch

What I expected from designing a transceiver was to have 1 pin for the output antenna, where I need to design a matching network.
And similarly 1 pin for the input antenna where I needed to make a matching network for as well.

I can understand the rest, the RF switch IC along with the matching networks for each (input/output) function cases.

Christianidis Vasileios
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1 Answers1

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VR_PA is the power amplifier regulator output. It provides a variable bias voltage to the PA (via RFO) through RF choke L1. C1 and C2 are bypass capacitors.

The voltage is variable to set the output power level. You can see fig. 4-4 in the spec sheet to see the relationship between VR_PA voltage and output power.

On the input, it looks like the two inputs may be to allow for a balanced input connection, although the schematics show a single ended connection. I don't see anything in the spec sheet that really addresses it though.

Update: found this document which explains on page 21 that the receiver inputs are differential and the LC network is a balun to convert a single ended input to differential, so pretty much what I suspected.

GodJihyo
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  • The document you mention says "external pull-up inductor", which should not make sense, an inductor could oscillate there (based on my recent question: https://electronics.stackexchange.com/q/660611/252145) so you must be right this is a rf choke, but it could be a ferrite bead as well right? Also in this document they dont mention the use of RFI_P, which is the input pin and it should not need extra power biasing. – Christianidis Vasileios May 27 '23 at 17:36
  • @ChristianidisVasileios A choke is just a specific use of an inductor. They've been used in a way similar to seen here since the days of vacuum tubes. The type of choke used will depend on the frequency and circuit, it could possibly be a ferrite bead but they'd probably have used a different schematic symbol if that's what they intended. As for RFI_P, it doesn't need bias and none is shown. L6 and associated capacitors are chosen to give a 180 degree phase shift between RFI_N and RFI_P. – GodJihyo May 27 '23 at 18:03
  • Yes my bad I meant "...dont mention the use of RFI_P, which is the input pin along with RFI_N and RFI_N should not need extra power biasing, since it is antenna input.". So 180 degree phase shift between these two pins, but this is not mentioned in the datasheet or the document you sent me, so I guess why this 180o shift is needed is a question for another post. – Christianidis Vasileios May 27 '23 at 18:17
  • @ChristianidisVasileios It needs a 180 degree phase shift because it's a differential input, so one input is inverted from the other and that's 180 degrees. They don't say that explicitly in the data sheet because it's kind of a given. – GodJihyo May 27 '23 at 18:26