I'm able to vary the transmit power with the set_tx_gain API. As long
as the requested gain value is within the range returned by
get_tx_gain_range, I measured that requesting a x-dB gain change
actually causes a x-dB variation in power. All good for relative
values, but now I need to know the absolute power numbers. Any API
available for that? If not, please validate/correct the following logic.

The max tx power for my B210 model is given as 10 dBm in the spec
sheet below.
http://www.ettus.com/content/files/b200-b210_spec_sheet.pdf
(It actually says that it's at least 10 dBm in a single RF channal,
but let's keep it simple and assume that the max tx power is exactly
10 dBm.)

In all my absolute power level computations I rely on the following:
10 dBm level would be achieved if set_tx_gain was used with the max
value returned by get_tx_gain_range and all the IQ samples to be
transmitted were saturated. For example, if my IQ samples have an
average power 20 dB below saturation (to account for PAPR), then I'm
actually transmitting at -10 dBm even when the max gain value is used.

Thanks,
Dario

Thank you Marcus, very useful insight on the closed-loop control.
On a somewhat related topic: is the max tx gain (90dB on the B210)
something that we should stay away from? I see that very high tx gains
cause degradation of the rx signal when the board is used in
full-duplex mode, even when the center frequencies are far apart.

Thanks,
Dario

On Fri, Mar 20, 2015 at 9:10 AM, Marcus D. Leech via USRP-users
<usrp-users@lists.ettus.com mailto:usrp-users@lists.ettus.com> wrote:

 On 03/20/2015 12:05 PM, Dario Fertonani via USRP-users wrote:

     I'm able to vary the transmit power with the set_tx_gain API.
     As long as the requested gain value is within the range
     returned by get_tx_gain_range, I measured that requesting a
     x-dB gain change actually causes a x-dB variation in power.
     All good for relative values, but now I need to know the
     absolute power numbers. Any API available for that? If not,
     please validate/correct the following logic.

     The max tx power for my B210 model is given as 10 dBm in the
     spec sheet below.
     http://www.ettus.com/content/files/b200-b210_spec_sheet.pdf
     (It actually says that it's at least 10 dBm in a single RF
     channal, but let's keep it simple and assume that the max tx
     power is exactly 10 dBm.)

     In all my absolute power level computations I rely on the
     following: 10 dBm level would be achieved if set_tx_gain was
     used with the max value returned by get_tx_gain_range and all
     the IQ samples to be transmitted were saturated. For example,
     if my IQ samples have an average power 20 dB below saturation
     (to account for PAPR), then I'm actually transmitting at -10
     dBm even when the max gain value is used.

     Thanks,
     Dario

 Your logic is essentially correct, except that you cannot know the
 exact output power levels without actively measuring them.  RF
 amplifier gain varies
   on a device-to-device basis, and on a frequency basis, and on an
 ambient-temperature basis, and on a power-supply stability basis.

 In systems with active power-control management, they often place
 a directional coupler in the TX path and use closed-loop control
 of the RF output
   power by measuring some of the TX energy, and knowing precisely
 what the coupling coefficient is of the directional coupler, and
 transfer function
   of any power-measuring equipment hung off the coupler.

 But for gross estimation, I see no flaws in your logic.




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