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CC1120: Measurement issue? Can't boost RX performance with FEM

GGA
Genius 3985 points
Part Number: CC1120
Other Parts Discussed in Thread: CC1101, CC1190

Hi,

we use a Sky FEM as single PCB device usually for boosting TX in certain applications to up to 30dB (SKY 66119).

This chip has RX boost too, but when I try to gain more RX sensivity I fail:

Typical test bed: RF generator -> CC1120 -> BER test   --->> ~ -120dBm (transparent mode)

The FEM has a RX gain of ~15dB, this is what I can verify if I measure the output in a test bed like:
RF generator -> FEM -> spectrum analyzer (SA) --->>> the input at SA is 15dB higher the the output of the generator, this is what I expect. E.g. -125dBm input, -110dBm output

But in verify mode:
RF generator -> FEM -> CC1120 -> BER test the BER is the same BER than without the FEM, ~ -120dBm
Thats where I struggle: do I oversee something or any other hint?
Or: to what extend a FEM can boost sensivity in real world?

thx.

  • 0
    TI__Guru**** 317180 points

    I would have expected a 3 dB improvement in sensitivity.

    Have you tested using packet mode to see if you get the same result there?  

  • 0 in reply to TER
    Genius 3985 points

    TER,
    I didn't test packet mode yet.

    Where is my logic error: you expect 3dB, but I gain 15dB in RX. My specci clearly show -115 if I put -130 into the FEM.
    I understand, just from my RF experience,  that the FEM can't achieve 120+15 = 135dB, but what (physics ?) is the real life limitation?

  • 0 in reply to GGA
    TI__Guru**** 317180 points

    Cascaded Noise Figure. 

    Meaning that you can't look at just the gain but also the noise figure of the FEM and the LNA in CC1120 to see how much you will gain. 

    As an example: On CC1120 we gain ~3 dB adding CC1190 in RX, for CC1101 we gain close to 6 dB doing the same since the noise factor of the CC1101 LNA is higher. I haven't done any actual calculations but I have seen adding an external LNA typically gives 3 dB. I assume that you can go lower using a LNA with a very low NF but then the current consumption will be high.