Forum: GNU Radio GRC and gr_fft_vcc function showing different signal power in dB

Hi all,

I am new to USRP and I started by trying to calibrate the received power 
in
USRP by giving a known sine signal. I gave a signal of *100.2 MHz at -30 
dBm
* to USRP which sampled it at 1MHz then passed it to a 1024 point FFT 
scope
in Gnu Radio Companion.  I saw a level of about* -52 dB at 100.2 MHz*.

I also modified the *usrp_spectrum_sense.py* to find FFT of same signal
using gr_fft_vcc function and then print the magnitude squared value to 
a
csv file. When I took 10*log10(value) the plot showed me about* -30dB 
level
at 100.2 MH*z.  Following are my queries

1) Is the power level in GRC -52 dB or -52 dBm? Accordingly why is the 
-22
dB loss? ( My antenna and channel gain were set to 0dB)

2) Why are fft squared amplitude in dB different here and which one is
correct?

Regards,
Hemant

On 10/17/2012 05:21 AM, Hemant Saggar wrote:
> at 100.2 MH*z.  Following are my queries
>
> 1) Is the power level in GRC -52 dB or -52 dBm? Accordingly why is the -22
> dB loss? ( My antenna and channel gain were set to 0dB)
>

The WX gui FFT plotter is scaled for dBfs.

0 dBfs is equivalent 1.0 sample counts, which is fullscale.

> 2) Why are fft squared amplitude in dB different here and which one is
> correct?
>
See ./gr-blocks/lib/nlog10_ff_impl.cc

This uses the gr_fft_vcc and does various scaling compensation

-josh