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RF section of USB dongle reference design vs. Keyfob reference design for CC2540

Kevin K
Prodigy 125 points
Other Parts Discussed in Thread: CC2540

Hello,

Does anyone know why there is such a large difference in electronic design between the RF antenna sections on Keyfob as compared to the USB dongle for the CC2540? The USB RF section only has two parts plus antenna whereas the Keyfob has 9 parts plus antenna. It looks like the Keyfob is doing more filtering but why is this not needed in the USB dongle design?

Thanks,

Kevin

  • 0
    TI__Guru 57865 points

    Hi,

    On the keyfob, the antenna matching is made with individual components, whilst the USB Dongle has the matching network integrated in a chip. The latter solution takes less area on the PCB but can not be matched with same accuracy as using individual components.

    Best Regard

  • 0
    TI__Guru** 106705 points

    Hi Kevin,

    The RF port of the CC2540 is differential so to be able to connect it to a single ended antenna a balun must be used. On the CC2540 USB dongle this is realized by a single component solution from Johanson Technologies, 2450BM15A0002. On the keyfob discrete components are used instead. Additionally there are a couple of discretes between the antenna and balun for antenna matching and filtering on both designs.

    The benefits with the integrated solution is: -better surpression of harmonic radiation (low pass filtering), -reduced footprint (PCB area), -ease of use. The disadvantages of the integrated balun is: -higher insertion loss (~1 - 2 dB higher than properly designed discrete solution), -cost

    /Fredrik

  • 0 in reply to Fredrik K
    Prodigy 125 points

    So it seems like the largest downside is the loss of about 1.5db of signal (from what I can tell from the specs for the balun). The cost I suspect is not all that different - the balun costs 25 cents from Digikey in quantity and all the other parts for the discrete solution are probably more (although I didn't price it out). Would you agree?

    Also, why in the USB design do they have a 0 ohm resistor connected to the antenna? Is this needed for some RF issue?

    Thanks!

  • 0 in reply to Kevin K
    Expert 1685 points

    I'm assuming the 0 ohm resistor is there in case the antenna needs to be disconnected.  This can easily be done by depopulating the ohm and tacking on a lead to a connector or something.

  • 0 in reply to Jonathan Cohn
    Guru 31280 points

    The series resistor to antenna is generally a part of an additional filter for fine tuning

    the characteristic impedance of an antenna to a balun, commonly there a footprint

    for a parallel grounded capacitor (the capacitor isn't being assembled), just before the series resistor.
     

    On the side note, a discrete solution is cheaper.