# GRC FFT Plot

Hi guys,

I transmitted this -30 dB signal (Image 1) using a usrp: The spectrum
analyzer displays -50 dBm (Image 2) as its reading. My receiver end
picked
up the signal with the same peak power at -30 dB (Image 3). I’m
confused. How
do I convert the dB power in the GRC to the received signal of the
spectrum
analyzer in dBm? How do I convert the Power (dB) of the FFT to dBW or
dBm?

Best,

*Gerome Jan M. Llames *
Engineering Research and Development for Technology (ERDT) Scholar
University of San Carlos - Technological Campus
Nasipit Talamban, Cebu City, Philippines, 6000
Mobile: +639271525124
Email: [email protected]

“Design is not just what it looks like and feels like. Design is how it
works.” - Steve Jobs

I transmitted this -30 dB signal (Image 1) using a usrp: The spectrum
analyzer displays -50 dBm (Image 2) as its reading. My receiver end picked
up the signal with the same peak power at -30 dB (Image 3). I’m confused.
How do I convert the dB power in the GRC to the received signal of the
spectrum analyzer in dBm? How do I convert the Power (dB) of the FFT to dBW
or dBm?

Just remove 20 …

1. Send in a CW tone from a signal generator of known power (dBm)
2. Read the level from GR FFT (dB)
3. Figure out the amount you need to offset (dB)
4. Multiply your samples by a constant value of 10**(offset_dB/20.0)
5. The GR FFT will now be calibrated to an absolute power (dBm).

Note if you change any settings, technically you would have to go
through
this again.

Lou

Jan wrote

Best,

*Gerome Jan M. Llames *

View this message in context:
Sent from the GnuRadio mailing list archive at Nabble.com.

I misread you question and thought you were receiving with the USRP.
Same
procedure, just don’t let the transmitted signal go past +/- 1.0 or you
will
clip the DAC. In practice keep it backed off 6 dB or more to allow for
crest factor.

Lou