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Calculate the RSSI offset value for the cc1121

Trevor Hunt
Intellectual 440 points
Other Parts Discussed in Thread: CC1121, CC1120

Hi there,

How do calculate the "RSSI offset"  value for the Rssi when doing rssi readings on the cc1121. The datasheet (swru295a) makes reference to a "calibrated RSSI offset value" which is used to calculate the correct RSSI level when reading  the "RSS1" register. Any ideas?

 

Regards

T.Hunt

  • 0
    Genius 3370 points

    The calibrated offset is more than likely an empirical value. So you will have to generate this calibration value based on measurements of actual hardware.

  • 0 in reply to Timothy Barr
    TI__Expert 3595 points

    This thread:

    http://e2e.ti.com/support/low_power_rf/f/155/t/131734.aspx

    answers better than I can for the CC1120.  I am pretty sure the same value should be used for the CC1121.

  • 0
    TI__Genius 12735 points

    Hi Trevor

    In genral the RSSI offset value will vary slightly with different register settings (data rates, frequency etc.), however for the CC112x we have seen that the offset is pretty much the same over the different test cases. Depending on the accuracy you need you can either use a fixed RSSI offset like we do in SmartRF Studio or calculate the correct RSSI offset value for each independent case.

    In SmartRF Studio we use a RSSI offset value of 102 for all test cases. The accurate RSSI offset value for one specific set up can be found by input a signal of known strength to the radio when the AGC_GAIN_ADJUST.GAIN_ADJUSTMENT bits are set to 0x00.

    The value you get from the RSSI register is then the  actual rssi level + rssi offset.

    In other words you need to subtract the RSSI offset from the value read from the RSSI register.

    Example:

    Assume a -70 dBm signal into the antenna and RSSI[11:0] = 0x200 (32) when AGC_GAIN_ADJUST.GAIN_ADJUSTMENT = 0x00.

    This means that the offset is 102 dB as 32 dBm - 102 dB = -70 dBm.

    When the offset is known it can be written to the AGC_GAIN_ADJUST.GAIN_ADJUSTMENT register field (GAIN_ADJUSTMENT = 0x9A (102)). When the same signal is input to the antenna, the RSSI[11:0] register will be 0xBA0 (-70).