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Re: [Discuss-gnuradio] GR, USRP, and GPIB measurements

From: madengr
Subject: Re: [Discuss-gnuradio] GR, USRP, and GPIB measurements
Date: Fri, 15 Aug 2014 21:41:28 -0700 (PDT)

You don't need to take the FFT to get the power.  See my attached example (in
GR 3.7).  Just take the output from the USRP, complex_to_mag_squared,
integration with decimation to 1 Hz, 10*log10() + k, and probe at 1 Hz rate.

I'm using an N210 + WBX with gain=15 dB.  That gives me a calibration factor
of k=-34.8 dB.  I can vary power of my signal generator from -50 dBm to -10
dBm with less that 0.1 dB error in the calculated power.





Gayathri Ramasubramanian wrote
> Hi
> Thank you for your note.
> I am trying to get the power of received signal using the blocks you had
> suggested previously.
> I have attached a flow graph snapshot ('uhd_fft_DK.grc.png").
> I am using the UHD:USRP Source to receive the signal which in turn sends
> the signal in the foll path
> "complex to Mag^2" block --> "integrate and decimate" block -->  "10 log
> +k"  block---> "signal probe" block.
> However there are errors mainly referring to the in-compatible I/O sizes
> of
> source (complex to Mag ^2 block) and sink (Integrate and Decimate block).
> The I/O size of "Complex_Mag^2" block is said to be 4096, while the I/p
> size of i"ntegrate and decimate" block is 4. ==> error as not compatible.
> Should I change the Vector length? I am not quite sure?
> The error propagates to other blocks if I change anything.
> Kindly help solve this.
> I have also attached another flowgraph (flowgraph screenshot.png) which
> works but it doesn't have the "integrate and Decimate" block or the
> Log10+K
> block but it works and receives the signal (developed by someone else
> previously in our lab). Now since the "Log10+K" block is missing, does it
> mean the output is given in mwatts instead of dBm?
> I cant seem to use this version on as the "probe_signal_vector" block does
> not seem to be supported in GRC which is the version in my system.
> I tried using simple probe_signal block instead but again faced the I/o
> size incompatibility error?  Is there a work around for this "probe signal
> vector" block?
> Also could you explain briefly how to approach and solve there I/o size
> errors.
> Look forward to your response
> Thanks
> Gayathri

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