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## Re: [Discuss-gnuradio] mathematics behind probe.level()

 From: Marcus Leech Subject: Re: [Discuss-gnuradio] mathematics behind probe.level() Date: Thu, 29 Aug 2013 17:00:21 +0000 (UTC)

It uses a single-pole IIR filter.  You can set the alpha value when you create the block, and also, there's a set_alpha() method that is a public method.

on Aug 29, 2013, dilip thapa <address@hidden> wrote:
Marcus,

Thanks for very nice explanation:
so I see the code of this block with the help of your reply, and understand that how It is computing power for given N samples.
i.e power= (sum of X**2[n])/N where n goes from 0 to N.

And the code contains “noutput_items ” in place of N.

Then what value of N is used here. and how can we change that from python code?

~Dilip
Sent from Windows Mail

From: Marcus Leech
Sent: ‎Thursday‎, ‎August‎ ‎29‎, ‎2013 ‎8‎:‎44‎ ‎PM

For a complex signal, the instantaneous power level is proportional to (I**2 + Q**2), which you then average.

This is precisely what probe_avg_mag_sqrd does.

And, you shouldn't refer to it as an "energy level", since you're measuring power, not energy.  Those are different (but related) concepts in physics.

Now reducing even further, remember that the ADC is sampling a *voltage* level.  The power flowing through a circuit is proportional to the square of the voltage.
The samples that are coming into the flow-graph are proportional to the voltage seen by the ADC, so in order to calculate the power, you have to square them.
Review Ohms law to see that this is true.

on Aug 29, 2013, dilip thapa <address@hidden> wrote: