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CC2540 MiniDK Dongle Rx power

Victor Ting
Prodigy 60 points
Other Parts Discussed in Thread: CC2540

Hi all,

This is my 3rd post and my previous 2 questions haven't received any answer.  Maybe they're stupid questions, so if they are, please let me know.

Anyway, I have one more question if anyone is able to reply:

On the CC2540 MiniDK Dongle, I've been trying to measure the received signal strength and it seems that the reported RSSI saturates at -33dB even with the keyfob right next to the dongle and transmitting at full power (4dBm).  I'm fairly certain the reported RSSI is correct (i.e. no offset) since my tests with changing the Rx sensitivity on the dongle is noticeable.  With the regular sensitivity, the link is lost when the RSSI reaches around -85dBm and with high sensitivity, the link is lost at around -95dBm.

I'm guessing the saturation is due to the dongle and not the keyfob because the RSSI saturates with the keyfob set at 0dBm output power and switching to 4dBm has no effect on the receive RSSI value.

I checked the Application Note for selecting antennas and for the Meandered Antenna design, I didn't see anything that could account for such a large loss.

Is this -33dB loss normal for these boards? Is there a design that can improve the link budget?

 

Thanks,

Vic.

  • 0
    TI__Genius 10955 points

    Victor Ting said:
    On the CC2540 MiniDK Dongle, I've been trying to measure the received signal strength and it seems that the reported RSSI saturates at -33dB even with the keyfob right next to the dongle and transmitting at full power (4dBm).  I'm fairly certain the reported RSSI is correct (i.e. no offset) since my tests with changing the Rx sensitivity on the dongle is noticeable.  With the regular sensitivity, the link is lost when the RSSI reaches around -85dBm and with high sensitivity, the link is lost at around -95dBm.

    The saturation on -33 dBm is expected with standard gain mode. If you look at the CC2540 data sheet on page 7, you see that the useful RSSI range is typically -90 to -35 dBm for standard gain mode. This means that if the RSSI is above -35 dBm, a value around that (-33 in your case) is reported.

    The sensitivity you observe is also reasonable. The data sheet (Page 4) lists a typical sensitivity of -87 dBm for standard gain and -93 dBm for high gain.

    Victor Ting said:
    I'm guessing the saturation is due to the dongle and not the keyfob because the RSSI saturates with the keyfob set at 0dBm output power and switching to 4dBm has no effect on the receive RSSI value.

    The saturation happens in the receiver, so yes, it is due to the dongle with your setup having the dongle as the receiver.

    Victor Ting said:
    Is this -33dB loss normal for these boards? Is there a design that can improve the link budget?

    This is not a loss! It is just a limitation in the RSSI estimator. The actual RSSI is significantly higher than -33 dBm. Therefore, this observation has no impact on your link budget. Note that the receiver works correctly even if the RSSI is saturated.

  • 0 in reply to hec
    Prodigy 60 points

    Great!  Thanks for your answer on the RSSI.  It makes sense.  The reason I expected an RSSI value closer to zero was due to a post by Karl when he was discussing the measured RSSI value in another CC2540 design:

    "I believe the default power setting is 0 dBm, so if you are seeing much less than this, then the problem is likely hardware related."

    I suppose I misunderstood the context.

    Thanks,

    Vic.