[USRP-users] Band power measurement.

Mike McLernon Mike.McLernon at mathworks.com
Sun Nov 28 22:38:51 EST 2010


Hi Vlad,

For a simple signal such as a sine wave, you can use the Spectrum Scope block to essentially do the measurement for you.  The attached model, mMeasureSignalPower_10b.mdl, is meant to be run in the R2010b version of MATLAB.  The sine wave source is at 100 Hz, with an amplitude of 1.  With that amplitude, the power is 0.5 W, assuming a load impedance of 1 ohm.  That 0.5 W translates into 27 dBm.

Note the FFT plot of the signal:

[cid:image001.png at 01CB8F4C.43DB67D0]

The two signal spikes at +/-100 Hz are at 24 dBm, giving a total power of 27 dBm.

If you are using a more complicated signal than a sine wave, then you can simply reckon the Simulink signal values as having units of sqrt(watts).  In that case, you can get the power Px of signal x in dBm according to

Px (dBm) = 10*log10(var(x)) + 30.

Hth,
Mike


From: usrp-users-bounces at lists.ettus.com [mailto:usrp-users-bounces at lists.ettus.com] On Behalf Of Vlad Stoianovici
Sent: Sunday, November 28, 2010 8:37 AM
To: usrp-users at lists.ettus.com
Subject: [USRP-users] Bandpower measurement.

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 procesure 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.


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