Hi,

— 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,

-20math.log10(self.fft_size)-10math.log10(power/self.fft_size)).Adib

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).

Best Regards,

Firas