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What is the square wave signal at RF pins of a CC2538

Serdar Köse
Genius 3360 points


I probe RF pins of a CC2538EM.

Here is the output when it is in idle state.

Then I get the below output when I set the MCU to transmit 0x5 via RF Studio:

while I have this settings:

I think it is obvious that glitch look like signal is the data signal to be transmitted. What is the square wave that oscillates with 1.36 Hz? 

  • 0
    TI__Expert 3050 points
    Hi,

    Your post had been forwarded to the respective HW/RF expert.

    Best regards,
    OD
  • 0 in reply to OD
    TI__Mastermind 36395 points
    Without knowing the measurement setup and what exact points you are probing. My guess is that what you are seeing is the DC bias that is needed by the LNA and PA to generate the RF signals. This DC bias is supposed to be blocked before it gets to the antenna by a DC blocking capacitor.

    Regards,
    /TA
  • 0
    TI__Guru**** 317180 points
    In IDLE the PA and LNA will be off and the voltage on RF_P and RF_N will be undefined. When I did a quick measurement the voltage one the RF_P/ RF_N was constant low.
  • 0
    Genius 3360 points
    Thank you for the answers and explanations.

    I was probing at RF_P and RF_N pins just before the capacitor. I think I used "idle" as a wrong term. I meant that I don't use RF studio to transmit data, but I didn't count that the module was broadcasting .

    I use GenericApp of ZigBee Mesh 1.0 stack. When I load the Coordinator App, the pin output does not oscillate, but when I load the router App, it oscillates. Now I observe that it oscillates till the route node joins a network, then pin goes to logic low till the DC blocking cap.
  • 0 in reply to Serdar Köse
    TI__Guru**** 317180 points
    Using a scope with time resolution in the seconds range is not a good tool to find out what's going on. When in Tx the DC value on the pins will be in the range you see when the signal is not zero. When in Rx the signal most probably is low (if I'm not mistaken about the type of LNA used) So what you are seeing is the radio alternating between Tx, Rx and IDLE. The protocol used decides when the radio does what.

    What you you trying to find out by your measurements?
  • 0 in reply to TER
    Genius 3360 points
    I was just curios to learn why there was a DC signal on the RF pin.

    The main problem I am dealing is the impedance matching on my board. I discovered the AN068 a little bit late, I took sometime to study transmission line theory again after some years later to university. But I am still having a problem to use AN068 to CC2538EM. I would be very grateful if you could take some minutes to take a look at my below question.

    http://e2e.ti.com/support/wireless_connectivity/f/158/t/389223

    Thank you again.