Re: [Discuss-gnuradio] question regarding usrp_fft.py

[Firas Abbas]

Hi,


--- On Sat, 7/11/09, adib_sairi <address@hidden> 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,
> -20*math.log10(self.fft_size)-10*math.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