# GRC and gr\_fft\_vcc function showing different signal power in dB **URL:** \ **Category:** GNU Radio\ **Created:** [October 17, 2012, 2:22pm UTC](https://www.ruby-forum.com/t/grc-and-gr-fft-vcc-function-showing-different-signal-power-in-db/223777 "2012-10-17T14:22:24Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![aris](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/258eb7/32.png) [@aris](https://www.ruby-forum.com/u/aris)\ **Post date:** [October 17, 2012, 2:22pm UTC](https://www.ruby-forum.com/t/grc-and-gr-fft-vcc-function-showing-different-signal-power-in-db/223777/1 "2012-10-17T14:22:24Z")
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 ---
**Author:** ![Hemant\_Saggar](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/ebca7d/32.png) [@Hemant\_Saggar](https://www.ruby-forum.com/u/Hemant_Saggar)\ **Post date:** [October 17, 2012, 7:45pm UTC](https://www.ruby-forum.com/t/grc-and-gr-fft-vcc-function-showing-different-signal-power-in-db/223777/2 "2012-10-17T19:45:36Z")
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. > 1. 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