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CC1200: GSM 900MHz perturbation

Quentin_F
Prodigy 100 points
Part Number: CC1200


Hello,


we have an application with CC1200 + SE2435. We have done some test to see if a GSM signal (900MHz, 0dB) disturb the sensitivity in Rx mode. We are using a smart RF Eval Board with CC1200 EB in Tx, another personal board which permit us to add attenuation and our PCB in Rx. GSM signal is injected just before our PCB.

The parameters are :

  • Frequency : 869.8375 MHz
  • 9.6kbps
  • RX filter BW : 25 kHz
  • Deviation : 2.3841

Our results are :

  • Sensitivity without GSM signal in Rx mode : -106 dBm
  • Sensitivity with GSM signal injected : -83 dBm

There is a SAW filter in the LNA which work fine, we have checked that part and it's ok we can see on the analyzer that the signal at 900MHz is filtered.

Is there a solution to resolve this without putting more space between GSM antenna & ISM antenna in our future design?

Thanks,


Quentin.

  • 0
    TI__Mastermind 39860 points
    The CC1200 has a finite blocking and at 31 MHz I would guess this is 80 dB with your PHY. When you add an LNA the blocking performance will be degraded and the CC1200 + SE2435 without SAW probably has a blocking of 70 dB (again just a guesstimate). Adding a SAW will improve the blocking and assuming 20 dB attenuation at 31 MHz offset your results makes sense. That is, the practical sensitivity will be interferer (in dBm) - blocking (in dB).

    I believe SE2435 has a bypass mode. Do you see any improvement if you use this option? (I know this will degrade sensitivity, but interesting to know how much SE2435 degrades the blocking).
    What kind of SAW filter is being used? There are some narrowband options (e.g. NDK and Murata) that might improve the attenuation at 31 MHz offset.
  • 0 in reply to Sverre
    Prodigy 100 points

    Thank you for your answer.

    Here the results for sensitivity in bypass mode of the SE2435

    • Without GSM : -97 dBm
    • With GSM : -80 dBm

    So there is still a difference when we have a GSM signal.

    The SAW filter is this one : http://www.farnell.com/datasheets/1751707.pdf

    Quentin.