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Original question:

CC1310: RSSI value differentiation analysis

CC1310: RSSI Offset and AGC and AGC_REF Level by adding LNA

Yuning Wang
Prodigy 30 points
Part Number: CC1310
Other Parts Discussed in Thread: CC1190

Tool/software:

Hi, 

We are adding a LNA on CC1310's RX side. 

Assuming the gain of the LNA is 15 dB, what's the correct RSSI offset we should change ? 

In the case  (uint32_t)0x00FB88A3  without LNA , RSSI reading is -40 dBm 

In the case  (uint32_t)0x00FB88A3  with LNA , RSSI reading is -25 dBm.

We thought the RSSI value should represent the real loss over the air, the the AGC level would change it accordingly by CC1310 itself. 

Thus we changed it to (uint32_t)0x000A88A3. By doing this, the RSSI reading turned to -40 dBm with LNA now.  

My questions are : 

1. Is my modification of RSSI offset correct with an external LNA ? 

2. should I change the AGC_Ref Level by adding an external LNA or only changing the RSSI offset is enough ? 

Thanks

  • 0
    TI__Mastermind 18665 points

    Hi,

    1. The RSSI Offset should be adjusted similar to what is described in Section 2.3.2 of SWRA542 (Using CC1190 Front End with CC13xx Under EN 300 220): https://www.ti.com/lit/swra542
    2. If the PHY settings are otherwise unchanged, the AGC_REF level should not need to be modified. However, for a sanity check it is worth running a PER vs Input Level test using the new setting(s).

    Regards,

    Zack

  • 0 in reply to ZC
    Prodigy 30 points

    Hi Zack, 

    1. The function of RSSI_offset is only for reading the correct value ? or it did affect something inside the algorithm ? 

    2. What is the PHY setting you meant here ? 

    Thanks

    Yuning 

  • 0 in reply to Yuning Wang
    TI__Mastermind 18665 points

    Hi Yuning,

    1. Yes, it corrects the RSSI value read. Other parameters in the algorithm, such as the AGC reference level (AGC_REF) are dependent on the RSSI value so it does affect the AGC algorithm.

    2. In this context I mean all other PHY settings (data rate, deviation, RXBW, etc.), not any specific setting (the output power level can of course be changed) i.e. if you add an external LNA and do not modify any other setting apart from adjusting the RSSI offset, this should be all that is required.

    However, it is still recommended to double-check the performance as recommended previously.

    Regards,

    Zack

  • 0 in reply to ZC
    Prodigy 30 points

    Hi Zack, 

    Regarding the 1st question, actually I'm a little bit confused about the RSSI_offset value. 

    from the Section 2.3.2 of SWRA542, it changed from 0x00FB88A3 to 0x000188A3. In another document, swra682 ( chapter 14 ), the two bits are the offset in two's complement. 

    FB means -5  ( from the sample code,  "Set RSSI offset to adjust reported RSSI by +5 dB " )

    01 means +1, so 0x000188A3 should offset the reported RSSI by -1 dB.   

    please correct me if I'm wrong. so the gain of the FEM (CC1190) is about 6dB, when the FEM is used together, the RSSI reading should be 6 dB lower than the reading from the default setting (0x00FB88A3), is that correct ? 

    Thanks

    Yuning. 

  • 0 in reply to Yuning Wang
    TI__Mastermind 18665 points

    Hi Yuning,

    An example that may help clarify this, from https://e2e.ti.com/support/wireless-connectivity/sub-1-ghz-group/sub-1-ghz/f/sub-1-ghz-forum/856174/ccs-launchxl-cc1350-rssi-to-dbm-equation-after-reading-from-register-0x4004-517c : 

    For the CC1310LP in the 868 MHz band the offset should be adjusted by +5 dB, and the offest written through the override is therefore -5:

    // override_phy_rx_rssi_offset_5db.xml

    // Rx: Set RSSI offset to adjust reported RSSI by +5 dB (default: 0), trimmed for external bias and differential configuration

    (uint32_t)0x00FB88A3,

    In the 433 MHz band, the RSSI should be adjusted by -2, and the ofset is therefore +2:

    // override_phy_rx_rssi_offset_neg2db.xml

    // Rx: Set RSSI offset to adjust reported RSSI by -2 dB (default: 0), trimmed for external bias and differential configuration

    (uint32_t)0x000288A3,

    As you write, the adjusted RSSI offset changes from the default +5 (0x00FB88A3) to -1 (0x000188A3), i.e. 6 dB lower than the standalone CC1310. Your design may need to be further adjusted, but that is the recommended starting point and the procedure you should follow. As you can see from the plot in SWRA542, this is what gave a linear RSSI vs Input Level curve when measured for that design.

    Regards,

    Zack