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LAUNCHXL-CC1310: Custom CC1310 board, radio not working

Part Number: LAUNCHXL-CC1310
Other Parts Discussed in Thread: CC1310

Hello All,

I have designed a custom PCB based on the reference design for the CC1310 LaunchPad.

www.ti.com/.../LAUNCHXL-CC1310

The uC is programming, and running code, when I attempt to send data through the radio, the software reports a successful transmit, but I am not able to get any data received from my LaunchPad, which 

I have programmed as the receiver.

I'm really not an RF expert, but I positioned the RF front end components as similarly as possible to those on the reference design, and requested that the board stackup be the same as that from the reference design.

After re-reviewing the BOM, I realise that for C21 in the reference schematic, the capaitor that connects RX_TX to ground, I requested a 0.1uF instead of a 100pF capacitor, I guess that will have a pretty severe impact.

I also realise that when providing my manufacturer with a BOM, I only specified the part type and value, e.g. Inductor 7.5nH, but I now see that in the TI BOM, there is much more detail, specifying RF inductor, non-magnetic core.

Please can someone tell me which components in the RF front end are likely to be the culprit of my non-functional radio? I don't mind re-soldering the whole front end if needs be, but I'd like to only change the parts that I need.

Which are the most likely causes of my problem? What can I do to get it working? and how much can I test this, all I have is a scope.

Thanks in advance.

  • Steve,

    The most common issue we see if that the Transmit frequency is not the same as the receive frequency. The best way to verify this is with a spectrum analyzer. But they new SDR radios on the market now can also provide you a good indication of frequency error. (here is a link www.amazon.com/.../ref=sr_1_1

    Regards,
    /TA
  • Thanks TA,

    Do you think that the incorrect C21 value, and that fact that I did not specify that the passive components have to be RF and within certain tolerances has ended up making the frequency slightly off, rather than not transmitting anything?

    Does this mean that If I try to get two of my custom boards talking to each other it might work?

    Regards,
    -Steve
  • P.S. Please can you tell me how I am to use the SDR you recommend, do I need to connect the output of my RF circuit to the input of the SDR, and the software will provide details?
  • - C21: This cap is mainly a DC block so hopefully the performance is not very dependent on this.
    - Have you tried transmit? Program your board to send a CW (tone) and set your LP in cont Rx using SmartRF Studio. If the transmitter work you should see the RSSI go up if you turn on the transmitter. If the boards are closer than ~1 m you should see and RSSI higher than -30 dBm.

    That said you should look into the possibility to rent a spectrum analyzer. Doing RF design without instruments could easily be more expensive than the cost for some instruments.
  • Thanks for the advice. As suggested, I programmed my custom board for continuous Tx, and had a LaunchPad set for continuous Rx. At 1m distance, I was getting very varied RSSI (Between -35 and -60), when powering down the custom Tx this dropped down to -100dBm

    So, what does this mean? My transmitter is clearly radiating. Is it likely that the incorrect value of C21 and the non-RF performance Inductors and capacitors are causing the issue, or could it be something else?

    Any help to demistify this is greatly appreciated.

  • Addition: I have tested two custom PCBs trying to communicate with each other, and they work. Using custom boards with SmatRF Studio, I get a much more stable RSSI of ~-20dBm, as shown enter image description here

    So, I know that the custom boards are functional, but will not communicate with a COTS development board.

    I used a Software Defined Radio (SDR) dongle as a spectrum analyser, and tested the results of the LaunchPad transmitting, and the custom board, and here are my results.

    LaunchPad as Tx enter image description here

    Custom PCB as Tx enter image description here

    I have tried swapping the incorrect value capacitor C21 for one of the correct value, and it makes virtually no difference to the results.

    My only thought on what to do next is to change the other components in the front-end to more closely match that in the BOM.

    Please can someone explain the reason for the behaviour I have seen, and provide some information about the difference between the custom PCB and the LaunchPad based on the graphs above?

    Anything to improve my understanding would be greatly appreciated.

  • Use the spectrum analyzer you have and put the boards into Constant Carrier (Unmodulated). Then zoom in on the spectrum analyzer so that you can see 10kHz resolution. You will then be able to tell if the center frequency is correct. My expectation is that both of you custom boards are either low or high in frequency, but the TI board is correct.

    Regards,
    /TA
  • Thanks for the advice. I am using a low cost SDR module as a Spec. Analyser, and the transmitting board is set to Constant Carrier (Unmodulated).

    I don't think I can get much better resolution with the hardware I have. It seems that there is a clear difference between the traces from the Analyser even at the resolution shown.

    I understand that as I have not used exactly the same components as on the LaunchPad that I have needed up with a different performance, but I don't quite understand what parameters of the components are different, and what effect this has on the signal.

    Are the specification of some of the components in the ront-end more important than others. I guess so as the fact that I had C21 of by a factor of 1000, and changing it has made no noticeable difference, means that it is less crucial than some of the other components. Are there any others that are less critical? Which are the most critical?

    Regards,
    -Steve
  • Hi,

    It seems I just hadn't got the right software. Here are some graphs with better resolution, zoomed in around the frequency of interest.

    The LaunchPad seems to peak at ~867.935MHz:

    Where as my custom boards seem to be more at ~868.053MHz:

    So whatever I have done has caused a sufficient difference in frequency. Is this really likely to be from the RF front end, or is it likely to be from something else? E.g. The oscillator being slightly off, the PCB copper being slightly different? Small differences in stack up?

    I didn't think that a discrete Balun could shift frequency, am I wrong? can the matching circuitry after the Balun cause a frequency shift? What is likely to be responsible for this?

    Many Thanks.