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nRF24l01+ 2450BM14A0002 + RFX2401C + 2450BP14D0100. Using VDD_PA pin for controlling PA/LNA.

Fadi Bunni

Hello Community,

So I am building a RF system from scratch based on the nRF24l01P transceiver with a PA/LNA, using a balun for impedance matching and a band filter for filtering unwanted frequencies.

 

As stated in the data sheet the PA/LNA RFX2401C on page 5, can be set in 3 different modes: Shutdown(0,0), RX Active(0,1) and TX active (1,x). I know that I don't care about the shutdown mode, since I only need the RX and TX active modes. As seen on the schematics, the RXEN is always pulled up to high, this means that I can control the TX active, RX active, modes by changing the TXEN from 0 to 1. For this I want to use the VDD_PA pin to control the TXEN, such that when I am transmitting from the transceiver the VDD_PA will be set to high which in turn will set the TXEN to high, meaning the PA/LNA will be set to TX active mode (as in power amplification mode) and if I set the transceiver in receiving mode the VDD_PA will be set to low, which in turn will set the TXEN to low, meaning the PA/LNA will be set to RX active mode (as in LNA mode). Now my question is, will this above described method work? or do I need to use a external GPIO from a MCU to control the TXEN pin? The datasheet on nRF24l01P does not explain when and if the VDD_PA goes high or low?

Furthermore, what do you think about the schematics, anything that pops out and needs to be changed or added?

Thank you for your time.

Regards
Fadi Bunni

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  • 0

    Hi,

    A long long time ago (in a galaxy not so far), we did a prototype board that show how it can be possible to connect a PA to the nRF24L01. You can find it attached, page 1 must always follow reference layout, page 2 depends on your PA design, maybe you can use it as reference. The VDD_PA pin will go high during transmission (start ~50us prior), and low during reception (and idle). Please be aware that the VDD_PA high is 1.8V (not VDD).

    Best regards,
    Kenneth

    nRF24L01 and SE2425U_schematics.pdf

  • 0 in reply to Kenneth

    Nice, just what I wanted to hear, saves me some pins and programming time. 1.8V is good the PA needs >1.2V. So no problem.

    Now that I have you. I have a question about grounding multiple transceivers, on one PCB with the following 4 layers: RF/signal, Ground, VDD, Signal. How should the grounding be for 4 transceivers? A star type of typology, where each transceiver is connected to their own groundplane which in turn is connected to a common groundplane (where the MCU is also connected) or should all transceivers be connected to the same groundplane. 

    Thank you for your valuable time.

    Regards
    Fadi Bunni

  • 0 in reply to Fadi Bunni

    A common ground plane is typically the safest option, since ground currents can be hard to control if you start splitting ground planes.

    On a 4-layer pcb, make sure inner layers have cut-out on inner layers for the radio matching network, as the radio matching layout (circuitry connected to ANT1/2 and VDD_PA) has been designed for a 2-layer board. I suggest you create a new case for a proper review once you have made a layout.

    Best regards,
    Kenneth

  • 0 in reply to Kenneth

    Hello, i have build it and tested it, and it works perfectly, up to 2300 meters as of now, going to test with a high gain antenna, and hopefully reach 5-7km rage :) 

    Thanks for your help.

    Regards
    Fadi

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