sd_ble_gap_tx_power_set variables

[vn2000]

I am using nRF52 DK to evaluate the TX power settings using sd_ble_gap_tx_power_set command:

sd_ble_gap_tx_power_set (BLE_GAP_TX_POWER_ROLE_CONN ,  m_conn_handle, tx_power);

I'm still unclear about this last  tx_power variable:

What is the range of tx_power that I can set?

Can it be a negative number like (-20)?

What does tx_power correspond to the actual power like mW?

What is the default value of tx_power at startup?

[Kenneth]

Check out:

https://devzone.nordicsemi.com/f/nordic-q-a/54562/location-to-set-tx-power-in-advertisement-and-connection-mode-also-change-them-dynamically-at-runtime/220720#220720 

[vn2000]

Hi Kenneth,

Thanks for the useful link. That clarified a lot.

However, there is one last thing that I'm still curious about is why the unit dBm is used.

dBm is usually associated with the sound level, whereas power mostly used Watt or mW as unit. I wonder how dBm is related to mWatt in this case.

[Kenneth]

You may find this useful:
https://ham.stackexchange.com/questions/1440/what-is-dbm-and-why-is-it-used-rather-than-watts 

[vn2000]

Thanks for the info.