[Discuss-gnuradio] GRC and gr_fft_vcc function showing different signal power in dB

[Hemant Saggar]

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 MHz.  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