I have a working ZigBee system based on ATMEGA256RFR2. (Datasheet) On page 33 you can check the RF system of this IC.

Now I want to use a FEM module to increase the performance and I found the SKYWorks SKY66114-11 which is designed for this exact purpose. (Datasheet)

On the ATMEL chip there is a balanced output of RFP and RFN which normally would go into a Balun and then to the final antenna.

However I could not find any documentation on how to connect those unbalance RF pins of ATMEL to the SKY IC's standalone pin.

I'm not sure if I would need the balun or not and what would be the recommended matching circuit.

Looking at an Atmel Application Note (AN42140): They are connecting an RCB256RFR2 board (Datasheet) to an SE2431L (Datasheet). There are only one balun between the two of them. No resistors, no capacitors. Is it okay? Connecting them together without any matching circuit?

What do you think?


RFP and RFN form a balanced output so, if you require an unbalanced input for the FEM, use a balun to combine the two outputs. The FEM data sheet should tell you the input impedance and the Atmel data sheet should inform you of the range of suitable load impedances it expects.

  • \$\begingroup\$ FEM states that it needs "50 Ohm RF source". In AVR's datasheet I could not find any reference of the load impedance. However I have a fear that if I use the balun it might saturate the FEM's amplifier input. \$\endgroup\$
    – Daniel
    Apr 7 '18 at 10:30
  • \$\begingroup\$ You can use a resistive attenuator between balun output and FEM. Just two resistors suitably chosen like 50 ohm in series with the balun output and 100 ohm to 0 volts. \$\endgroup\$
    – Andy aka
    Apr 7 '18 at 13:48
  • \$\begingroup\$ For example here is an image which is not a ZigBee IC but a Bluetooth one which is quite similar. They grounding the RFN and connecting directly RFP to the SKY. digikey.it/-/media/Images/Article%20Library/TechZone%20Articles/… \$\endgroup\$
    – Daniel
    Apr 7 '18 at 15:15
  • \$\begingroup\$ If the atmel data sheet says you can do this then you have a perfect solution. As the schematic comes from digikey I would suspect that using a single unblanced output is fine but, as always, check the data sheet to see what it says. \$\endgroup\$
    – Andy aka
    Apr 7 '18 at 16:45

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