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Problem with radio frequency custom board. Balun mismatch?

angel

Hi Nordic,

I have an application that perfectly runs in the nrf51DK but that does not work in my custom board. In my custom board everything else works (i2c, leds lighting, etc..) except for the communication related stuff. I think I am having some problems with the antenna. The balun I am using is 2450BM08B0003, which is the one recommended in this page for the nrf51422. As a detail, the components in the board have been placed by hand and soldered with oven reflow.

Here I attach the parts of the design that have to do with the communication.

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Any idea on how to debug my problem? I have no idea what is going on. I have been reading some other people have the exact same problem, but I have yet no idea of a proper solution.

PS: Sometimes we can transmit data from our custom board to the nordic developer kit. But not the other way around, we cannot receive data. Is this usual?

Thanks for your help!

  • 0

    It doesn't appear you have implements any matching between the antenna and the balun. I would assume some type of matching would be needed.

    What is the trace length of the antenna? The total length should be ~31-32mm for a quarter wave antenna at 2.4ghz.

    Your antenna may be around that length but then you introduce some parasitics because of the via which connects the antenna to the feedline / balun.

    This via might be causing added reflections but hard to say without measuring it.

    Here are some things you can try.

    You can verify your RSSI by using the nrfConnect tool of your smart phone. Basically have your custom board advertise while you listen with you smart phone place directly next to the board. Normal you would expect rssi any where between 30-50 if the units are directly next to eat your; if the lower number being better. In your case you might see values much higher like 70-80's which would confirm your matching findings.

    To correct the matching if you don't have a network analyzer you can measure the antenna trace length to see how close it is to 31-32mm. If you are less than, remove some of the solder mask on the antenna and add length by solder copper tape to make up the delta. If you are greater than try trimming the antenna by cutting the trace to account for the delta.

    The above might see not work because of the via. So what you can try next is to remove the via from the circuit. Basically make two cuts, one disconnecting the feedline from the via and the other disconnecting the antenna from the via. On remove some solder mask from the feedline and then add on copper tape in order to get the correct trace length.

    if it is matching you should see some type of change.

    Assuming you have already looked into the balun soldering and chip soldering.

  • 0

    Also are those traces (gray lines) underneath your antenna? Another thing you would need to check to the impedance from the feedline to the balun and then the balun to chip. determine the transmission line impedance of ANT, ANT1 and ANT2. Those could be causing your mismatch as well.

    If you have access to an network analyzer let me know. Problem is easy to resolve with one of those.

  • 0 in reply to Michael Antwih

    Hi Michael, thanks for your fast and complete reply! Really helpful!

    We have measured the length and it is around 24.025mm if I measure it until it is connected to the balun. The thing is that if I do the same measurement with the nrf51 dongle, I get around 27.8mm. But nothing is close to 31 or 32mm, not even in the devices that work perfectly (nrf51 dongle)

    Here is a picture of other antenna in our design (the previous one was slave, this is the master)

    http://imgur.com/a/tRUtf

    In this case we have a ground wall, as you can see.

    To get this ~31-32mm, from where to where should we measure?

    Thanks!

  • 0 in reply to Michael Antwih

    Yes, those traces are underneath. Do you think that they will interfere importantly in our communication?

    We may have access to a signal analyzer, but I am not sure yet. If we had access, what is the procedure we should follow to test the antenna? Thanks a lot again!

  • 0 in reply to Michael Antwih

    Even though the dk board measures 27.8 it has a matching network before the balun which compensates in order to get the correct frequency response.

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