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matching nrf24 to 100ohms balanced

iot_newb

I want to use the nrf24l01+ with the se2436 apmlifier, but the input of the amplifier requires an input of 100ohms balanced, while the nrf24 has a balanced output of 15ohm+j88ohm according to the datasheet.

Can I build a matching circuit to directly connect these to devices or is it recommended to first transform the signal to 50ohms unbalanced and then back again to 100ohms balanced.

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  • 0

    Thanks for the input.

    You are correct the impedance of the nrf24 is 15-j88 and therefor the load impedance should be 15+j88. And if my calculations are correct I would need a 3.9nH in series and a 1.5pF in paralell to get the proper matching at 2.4Ghz.

    image description

    According to the datasheet of the nrf24. ANT1 and ANT2 needs a DC path to the VDD_PA, should I connect that through the balun DC connection of the SE2436 as shown in the figure above?

    I've edited the figure according to your recommandations.

    I'm controlling the switching between TX_Mode and RX_Mode with CE and VDD_PA. The amplifier is in RX_Mode as long as the nrf is active, CE = high. When the nrf switches to TX_mode the VDD_PA goes high, and switches the amplifier to TX_mode. At this point both CTX and CRX is high, which is a state not specified in the datasheet, but tests has shown that the amplifier really is in TX_mode.

    image description

    Here is a picture of the layout, the nrf on the left and the amplifier to the right. I've been using 0402 inductors and capacitors.

  • 0 in reply to iot_newb

    The way you describe the logic seems fine.

    I assume this is a two layer board. It's not great that you had to split the ground plane, but I see why you did. Make sure the top and bottom are stitched together well around the split.

    You should increase the width of the traces as much as possible in the RF. Your traces are way, way too small. You will never get to the size required for it, but if you make them as thick as is possible you will minimize the error.

    Ideally you would match it correctly when you had the artwork since it won't be perfect. But I get the feeling you don't have a VNA.

    The good thing is, the way the match is done the fix for the parasitics will be a decrease in the size of the inductors and an increase in the size of the cap. Pretty trivial. But I bet it ends up close even without re-visiting it.

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  • 0 in reply to iot_newb

    The way you describe the logic seems fine.

    I assume this is a two layer board. It's not great that you had to split the ground plane, but I see why you did. Make sure the top and bottom are stitched together well around the split.

    You should increase the width of the traces as much as possible in the RF. Your traces are way, way too small. You will never get to the size required for it, but if you make them as thick as is possible you will minimize the error.

    Ideally you would match it correctly when you had the artwork since it won't be perfect. But I get the feeling you don't have a VNA.

    The good thing is, the way the match is done the fix for the parasitics will be a decrease in the size of the inductors and an increase in the size of the cap. Pretty trivial. But I bet it ends up close even without re-visiting it.

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