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CC1200 RSSI Offset

ECH12
Expert 1195 points
Other Parts Discussed in Thread: CC1200

Hi,

I saw in one of the replies the following:

"CC1200 stand-alone has either 99 or 81 as RSSI offset value (i.e. this is the value subtracted from the RSSI reading to convert from decimal to dBm) depending on MDMCFG1.DVGA_GAIN setting. You get the appropriate MDMCFG1.DVGA_GAIN setting from SmartRF Studio. If you add a range extender with 15 dB gain the RSSI offset will be 114 or 96."

I understood from the specifications that I need to calibrate the value. From the above answer it seems that the offset is known. I currently don't need the exact offset value and good estimation will be enough. Can I use the pre-calculated values?

If I need to calibrate the RSSI offset, can I use the CC1200 development kit for that purpose? Meaning, can I connect CC1200EM to my board with wire and inject signal to measure the RSSI offset?

Thx

  • 0
    TI__Mastermind 39860 points

    The RSSI offset is measured on CC1200EM. If you do not need the exact offset you can use either (99 + ext LNA gain) or (81 + ext LNA gain)

  • 0 in reply to Sverre
    Expert 1195 points
    Thx for the reply,

    Can you please explain what do you mean by:

    1. RSSI offset is measured on CC1200EM
    2. + ext LNA gain

    I am not sure I fully understand it. Don't I need just to set the offset to be 99? When to use 99 and when to use 81?

    Thx
  • 0 in reply to ECH12
    TI__Mastermind 39860 points
    The offset of 99 and 81, which we provide in e.g. SmartRF Studio, is measured on CC1200EM. This is the offset for CC1200 stand-alone.

    Assuming you add an external LNA you need to change the RSSI offset. The external LNA has a certain gain and let's call this "ext LNA gain". The offset is then either (99 + ext LNA gain) or (81 + ext LNA gain)
  • 0 in reply to Sverre
    Expert 1195 points

    We took the CC1200 design from TI reference design. Does it mean that we expect to have same RSSI offset? When I should use 99 and when 81?

    I see in SmartRF that it shows 81 on the continues RX view but 00 on the gain offset register. Why 81 was selected?

    Thx for your help!!!

  • 0 in reply to ECH12
    TI__Mastermind 39860 points
    RSSI offset depends on the MDMCFG1.DVGA_GAIN setting. Examples from SmartRf Studio: For 1.2 kbps MDMCFG1.DVGA_GAIN = 0 and RSSI offset is 99. For 38.4 kbps MDMCFG1.DVGA_GAIN = 1 and RSSI offset is 81.

    In Studio the RSSI is calculated in SW as (RSSI reading from register - RSSI offset).

    If you want the CC1200 to handle the correction write 99 (or 81) decimal into AGC_GAIN_ADJUST. The RSSI reading will then be adjusted automatically so that the RSSI reading from register corresponds to the actual input power level.

    Please refer to the CC1200 User Guide (section RSSI)
  • 0 in reply to Sverre
    Expert 1195 points
    Will the RSSI be adjusted also for the RSSI reported in the appended status data?

    Why Studio use the RSSI adjustment in software and not using the register offset (at least this is what I would expect)? Any drawbacks when using the RSSI offset in register?
  • 0 in reply to ECH12
    TI__Mastermind 39860 points
    Yes

    There are two ways of doing the adjustment; SW or through register. We used the former in Studio, but there are no drawbacks in using the latter.