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I know that USRPs are uncalibrated devices. In many exampls you can set the so called Tx or Rx gain (as in the picture below).

enter image description here

Now, I know that this does not correspond to the real power of the transmitter, nor is it a "gain" of the antenna - the most appropriate term here would be "transmitter channel gain", right?

So I would like to know how this transmitter channel gain process is implemented, how it is calculated, and how it works in practice (since a particular gain value doesn't correspond to a particular power level).

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The term "gain" is pretty appropriate. It's a number describing the output power relative to some arbitrary base power.

So I would like to know how this transmitter channel gain process is implemented,

Depends on the USRP (or if you're using a modular model, on the daughterboard), but speaking generally: A mixture of selectable/bypassable amplifiers, variable gain amplifiers and adjustable attenuators.

how it is calculated,

Not calculated. Set by configuring the things mentioned above.

how it works in practice (since a particular gain value doesn't correspond to a particular power level).

It sets how much stronger or weaker your amplification is. Simple! You add 3 dB of gain, you get twice as much output power. Of course, that's approximate, and only within the linear range of all the signal chain.

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  • $\begingroup$ Thank you! I'm using NI USRP 2922 $\endgroup$
    – MagicMan
    Feb 2 at 16:41
  • $\begingroup$ And dautherboard use HMC624LP4E attenuator $\endgroup$
    – MagicMan
    Feb 2 at 16:44
  • $\begingroup$ The NI USRP 2922 is an Ettus N210 with a SBX daughterboard, IIRC, and that indeed contains two HMC624LPE (one for RX, and one for TX). Good detective work! $\endgroup$ Feb 2 at 18:53

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