Always a spike on the FFT at the center frequency

GNU Radio
1

Whenever I run the userp_fft.py script (USRP1, Ubuntu 10.04 and 10.10),
there’s almost always a large spike (10-20 db above the noise floor) at
the
center frequency. This is the same for all the daughter boards I’ve
tried
(WBX, LFRX, and BasicRX).
Why is this? What am I missing?
Thanks.
-William

2

On Wed, 2010-12-01 at 16:16 -0500, William C. wrote:

Whenever I run the userp_fft.py script (USRP1, Ubuntu 10.04 and
10.10), there’s almost always a large spike (10-20 db above the noise
floor) at the center frequency. This is the same for all the daughter
boards I’ve tried (WBX, LFRX, and BasicRX).
Why is this? What am I missing?

DC offset. It’s normal. If it’s a problem just tune off center so your
frequency of interest isn’t at DC.

–n

3

On 12/01/2010 01:16 PM, William C. wrote:

Whenever I run the userp_fft.py script (USRP1, Ubuntu 10.04 and 10.10),
there’s almost always a large spike (10-20 db above the noise floor) at
the center frequency. This is the same for all the daughter boards I’ve
tried (WBX, LFRX, and BasicRX).
Why is this? What am I missing?

This is due to truncation in the USRP. If you add 0.5 to the signal, it
should go away.

Matt

4

Matt E. wrote:

This is due to truncation in the USRP. If you add 0.5 to the signal, it
should go away.

Matt

Dear Matt,
how and where does the truncation is done? is it in the FPGA (DDC) ?

regards,
Adib

Mohd Adib Sarijari
Universiti Teknologi Malaysia
www.fke.utm.my
www.utm.my

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5

On 12/05/2010 11:22 PM, adib_sairi wrote:

Dear Matt,
how and where does the truncation is done? is it in the FPGA (DDC) ?

At the output of the DDC.

Matt