Re: [Discuss-gnuradio] Bandpower measurment USRP2

[Marcus D. Leech]

On 11/28/2010 08:38 AM, Vladutzzz wrote:
> Dear all,
> I would like to receive as many suggestions as possible on how to accurately
> measure bandpower with a USRP2 + WBX setup.
> I know I should use a block that does the square magnitude (FFT squared) and
> sum the resulting coefficients but after this I don't really know what the
> correct procedure is. I would like to have the value in dBm and I know I'm
> suppose to use 30 dBm + 10log10, but the resulting value is not the correct
> one.
> Please help me by offering your insight on this matter.
> Thanks!
>
> Vlad.
>   
The basic flow for power measurement is:

source-->bandpass_filter->complex-to-mag-squared-->
                         
single-pole-iir_filter-->calib_multiplier-->calib_offset-->log10*10


You'll need to determine your calibration constants by experiment, and
you'd need to determine
  what those should be for any given
bandwidth/center-frequency/gain-setting.

You can't simply apply a fixed formula--there are too many uncertainties
in the analog realm
  to make precision power measurement work without caibration
experiments.  The power
  seen by your detector (complex-to-mag-squared+filter) will be
proportional to:

                 GAIN*(system-noise+signal-power)

There's substantial uncertainty in the *precise* value of GAIN, due to
*inevitable, expected*
  part-to-part variability.  If you, for example, command the GAIN on
your daughtercard to
  65dB gain, the actual gain may vary by up to about 2dB, and such
uncertainties are
  generally frequency dependent.  RF amplifiers usually have somewhat
more gain at their
  "bottom end" than at the "top end" of their frequency range.  Further,
you don't know
  how much system-noise there is, at least, not precisely, which means
that for measuring
  very small signals, the total-power seen by the detector may be
dominated by system-noise.

So, you have calibrate, through experiment. If you're trying to make
precision power measurements,
  you're going to have to calibrate for each variation in your system
setup (gain settings,
  frequency settings, bandwidth settings).


-- 
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org