About the -ve represenation on FFT graph

GNU Radio
1

Sorry if the question was re-posted.

Can anyone reply this entry level question?

In complex data representation such as the attached image by grc,
the LPF cut the frequency in both direction(+/- freq)
which the center frequency is 0Hz

http://www.fungkai.com/~howie/sdr/complex_1_basefreq0.gif

When I set the base frequency to a large positive value such as 10MHz,
the cutoff shows on both side apart from the center frequency too.

http://www.fungkai.com/~howie/sdr/complex_1_basefreq10.gif

What’s the frequency lower then the center frequency represent?

2

On 02/27/2011 07:57 PM, rono wrote:

http://www.fungkai.com/~howie/sdr/complex_1_basefreq0.gif

When I set the base frequency to a large positive value such as 10MHz,
the cutoff shows on both side apart from the center frequency too.

The baseband frequency parameter only changes the value printed on the x
axis. For example: If you had a usrp source tuned to frequency X, you
would also set the baseband frequency to X → for display purposes.

-josh

3

Thanks for your kindly reply.

Can you explain more that, both sides from the center frequency of the
input frequency has been cut by a low pass filter.

Doesn’t a low pass filter allow the lower frequency to pass?

Thanks,
Rono

4

On 02/27/2011 10:21 PM, Howard Wong wrote:

Thanks for your kindly reply.

Can you explain more that, both sides from the center frequency of the
input frequency has been cut by a low pass filter.

If you want to filter around 0 to f, then use the complex bandpass
filter. Its the same FIR filer with different filter taps.

Doesn’t a low pass filter allow the lower frequency to pass?

Correct. For a complex baseband signal, this is all frequencies between
+/- cutoff_freq

-josh