— On Sat, 7/11/09, adib_sairi [email protected] wrote:
i have some question regarding the y-axis in usrp_fft.py… it is
labeled in unit dB. so how does it convert from ADC value to dB?
Using the formula below.
second, what is this formula mean?
log = gr.nlog10_ff(10, self.fft_size,
This formula is used to convert the FFT output value to dB. This is
relative conversion and the values does not mean actual power because
the actuall power value depends on other factors such as RF gain used.
What is meant by relative, is that if the input signal power is (for
example) -20dBm and we measured it in this formula as (say) 50dB, then
when we change the signal power to -30dBm, we get 40dB from the output
of this formula. So, lowering signal power level by 10dBm result a 10dB
value reduction from the formula output.
So, what is the factors that effect dB conversion?. The first value is
the FFT size, so that when we change the FFT size, the formula should
compensate for this. Second we should compensate for the taps of the
window used for FFT (Hanning, Hamming,…etc).