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 capacitor 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.
Addition: I used SmartRF Studio to set my custom board into continuous Tx mode, and to set a LaunchPad for continuous Rx. With ~1m seperation, the receiver was seeing between -35dBm and -60dBm, and then when I powered the Tx down, the Rx dropped to -100dBm. This means the custom board is transmitting something. Does this provide any extra insight?
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.
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.
Now I've figured out how to work my SDR, here is a better resolution graph for each device.
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?