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matching nrf24 to 100ohms balanced

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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  • 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.

  • Close, but not quite right. The component values would be half you show for the topology you have chosen.

    Also, since you are within an octave or so of SRF for these components the actual value will be a little less. I normally use the MuRata Chip S-Parameter & Impedance Library app to look up real impedance values. Though most respectable chip manufacturers also publish their real component data.

    So that being said, the real series inductance should be about 7.5nH/2 or the most available standard size will be 3.9nH. You are generally better off buying components that comply with the standard number scheme then trying to buy something that is the precise mH.

    Then the best approach for the cap is a 1.5pF straight between the two legs. Just skip ground. The way you have it the effective impedance is twice what you thought it was.

    The stripline doesn't have to be a diff pair. Just treat it as two 50ohm lines that are coplanar with ground. and stick them next to each other close enough that you still have room to place the series components.

    You are going to need to keep this match really close to the nRF for it to be correct. You should be using 0402 or 0201 parts.

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  • Close, but not quite right. The component values would be half you show for the topology you have chosen.

    Also, since you are within an octave or so of SRF for these components the actual value will be a little less. I normally use the MuRata Chip S-Parameter & Impedance Library app to look up real impedance values. Though most respectable chip manufacturers also publish their real component data.

    So that being said, the real series inductance should be about 7.5nH/2 or the most available standard size will be 3.9nH. You are generally better off buying components that comply with the standard number scheme then trying to buy something that is the precise mH.

    Then the best approach for the cap is a 1.5pF straight between the two legs. Just skip ground. The way you have it the effective impedance is twice what you thought it was.

    The stripline doesn't have to be a diff pair. Just treat it as two 50ohm lines that are coplanar with ground. and stick them next to each other close enough that you still have room to place the series components.

    You are going to need to keep this match really close to the nRF for it to be correct. You should be using 0402 or 0201 parts.

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