Re: [Discuss-gnuradio] USRP + RFX2400 - Emitting power control

[Matt Ettus]

Arthur wrote:

> And my questions are:
> - What is the ""exact maximum"" of the amplitude to reach the specified
> maximal power of 17dBm (50mW)? (Is that 16000?)

You need to understand the realities of practical RF systems.  The gain 
and maximum power will vary:

- with different modulation systems
- from unit to unit
- over temperature
- over frequency
- with how well matched your antenna is
- with how much compression you are willing to tolerate

This is not just true for the RFX2400, but all RF systems.

It is impossible to say that maximum power exactly some level,or that 
maximum power occurs at a specific baseband amplitude.  All of those 
factors come into play.  If you absolutely must get the maximum power, 
then you need to calibrate the power vs. amplitude in your particular 
situation.  The other option is to choose an amplitude that produces 
good signal levels across a wide range of situations.  10,000 to 16000 
is usually ok.

> - By the way, what is the relationship between amplitude and power? Is
> "power = coefficient * (amplitude)2"? Or is the power only proportional
> to what the board can/decides to provide as Voltage * Intensity at the
> antenna?

Yes, power will be proportional to the square of the baseband 
amplitudes.  If you double the amplitude, you will get 6dB more output 
power, unless you are in compression.

> - In the o'scope utility provided by the sources, what are the two
> signals I can observe when using 1 card/1 antenna? Are these two signals
> Q and I? What are the y-axis units? Is it an expression of the power or
> of the amplitude (like sqrt(2)*amplitude, but with one zero missing)?

Those are amplitudes of the complex quadrature baseband received signals.

Matt