Re: [Discuss-gnuradio] Spectrum Sense: Making Sense of the data

[Johnathan Corgan]

address@hidden wrote:

> This means the output from m.data is from the center frequency to the
> maximum frequency and half way it flips to the minimum frequency and
> increases back to the center frequency.
>  
> My question basically is does anyone know if our findings in the graph
> above correct? And if not can anyone shed some light on what order the
> output is in.

The comment refers to "bin 0 == DC." This means the first value is the
passband center frequency, which becomes DC after downconversion by the
USRP.

The first half of the array contains the positive baseband frequencies,
which corresponds to the passband center frequency out to the maximum
passband frequency.

The second half of the array contains the negative baseband frequencies,
which correspond to the lowest passband frequency up to the passband
center frequency.

This is exactly what your graph looks like.

This format is due to the output format of the underlying fftw library
we use to calculate FFTs.  You can rotate the values by swapping the
first and second halves of the array and end up with it going from
lowest passband frequency to highest passband frequency.

-- 
Johnathan Corgan
Corgan Enterprises LLC
http://corganenterprises.com