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## Re: [Discuss-gnuradio] Bandpower measurment USRP2

 From: Vladutzzz Subject: Re: [Discuss-gnuradio] Bandpower measurment USRP2 Date: Sun, 28 Nov 2010 15:53:19 -0800 (PST)

```Marcus,
First of all thank you for your reply!
I have some questions about your very much appreciated explanations:
After using the complex-to-mag-squared block, should I consider the
coefficients as being in W or mW (should I use 10*log10 or 30 + 10*log10 to
get the power value in dBm)?
What is the function of the single-pole-iir_filter? What does it actualy do?
I'm asking this so I will know how to calculate its parameters. Does it give
the frequency bin based signal band, a more rounded appearance?
Again thank you for your help!
I hope other people will also find this useful.

===================================================================

Marcus D. Leech wrote:
>
> 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.
>> Thanks!
>>
>>
> 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
>
>
>
> _______________________________________________
> Discuss-gnuradio mailing list
>
>

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