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CC2640: RSSI offset

Kota Suzuki
Intellectual 270 points
Part Number: CC2640
Other Parts Discussed in Thread: CC2650,

I have an issue when reading in the RSSI value using the command "HCI_ReadRssiCmd."

The steps I take are as follows:

1. Send "HCI_LE_TransmitterTestCmd" from CC2650 to CC2640 (5dbm).

2. Use "HCI_LE_ReceiverTestCmd" on CC2640.

3. Use "HCI_ReadRssiCmd" on CC2640 and get a -30 dbm signal.

Is there an offset value I am not aware of, or some sort of calculation I need to do to convert the 5dbm signal to -30dbm?

Thank you in advance.

  • 0
    TI__Guru 56590 points
    Hello Kota,
    Please refer to an old post of mine:
    e2e.ti.com/.../1828052
  • 0 in reply to Eirik V
    Intellectual 270 points
    Thank you for your input.
    I have created a custom PCB and would like to subtract the offset to calculate the loss as stated in your old post, but I do not know the actual value of the RSSI offset of CC2640. Is it stated somewhere in the source code, or is there a way to calculate the offset?
  • 0 in reply to Kota Suzuki
    TI__Guru 56590 points
    You don't need the offset. HCI_ReadRssiCmd will supply the correct RSSI received at the input RF pins.
  • 0 in reply to Eirik V
    Intellectual 270 points
    Sorry for my lack of knowledge, but as I have stated in my original post, the difference of dbm is 35 (5dbm input, -30dbm RSSI output), which leads me to assume the loss is 35dbm. Is this assumption correct, or am I misunderstanding the whole concept of RSSI?
  • 0 in reply to Kota Suzuki
    TI__Guru 56590 points
    Hello Kota,
    The main part of that loss is path loss (natural decrease in power as a signal is "stretched" isotropically and propagate through all directions in as if inflating a balloon. 35dB of loss from transmitter to receiver seems reasonable for typical setup. There is also some loss in the antennas (depending on efficiency) and the matching circuits between the antenna and RF pins. You can use the excel sheet in this post to calculate the expected path loss:
    e2e.ti.com/.../375823
  • 0 in reply to Eirik V
    Intellectual 270 points
    Thank you for your response Eirik, the excel sheet will be a lot of help.

    I may have left out important details when explaining my situation.
    On one end, I use the CC2650DK to send a trasmitter test, and on the other I have my custom PCB which uses the CC2640 chip to use the receiver test. Since my custom PCB is not authorized by the Radio Law, I need to use a cable in between the chips in order to send bluetooth signals only through the cable. That being said, the cable itself is no more than a foot, and since I am directly connecting the 2 chips, I assumed the loss would be fairly low. Would the loss I am getting (5dbm trasmit, -30 RSSI = 35dbm difference) still be a viable result?

    Also, is the excel sheet provided in your previous response authorized by Texas Instruments? Can I say "Since this excel sheet is authorized by TI, the expected loss is x."?
  • 0 in reply to Kota Suzuki
    TI__Guru 56590 points
    Kota,
    The loss in the cable should ideally be only a few dB. But you can have a large mismatch loss depending on how you connect the coaxial cable and the specific impedance of your RF path. The RSSI dynamic range and saturation will limit the maximum signal strength that can be presented. Also you should be aware of the Absolute Maximum Ratings given in the CC2640R2F datasheet. The maximum Input RF level must not exceed 5 dBm.

    To get more reliable results and avoid potential damage to your device, you should either add a 30-40 dB attenuator between your two boards or reduce the output power:
    HCI_EXT_SetTxPowerCmd(HCI_EXT_TX_POWER_MINUS_21_DBM );
  • 0 in reply to Eirik V
    Intellectual 270 points
    Thank you for your input Eirik
    Even though my problem has not been solved (still a constant 30+ difference in dbm),
    since this post is dragging on, I will end it here, for now.
  • 0 in reply to Kota Suzuki
    TI__Guru** 106705 points
    Hi Kota,

    On the EM board in the DK there is a capacitor which is used to select either PCB antenna or the SMA connector. By default this is mounted towards the PCB antenna. This will typically give you a ~30 dB loss at the SMA connector.

    I guess you need to change the orientation of the capacitor. You can refer to the EM schematic for details.

    Cheers,
    Fredrik