About uhd_fft.py

GNU Radio
1

Hello

I’ve just successully installed gnuradio and connected my new USRP n210.
I
run uhd_fft.py to test my hardware and it all works well, I activity in
the
spectrum.

The thing is, whatever I set the center frequency, all I see is a
carrier
at that center frequence at -50dB and “noise” at other frequencies at
-70dB. I have included 2 screenshots with center frequencies of 2.4G and
2.4001G, the situation is the same in both of them.

I thing there is something really wrong with this. Besides, I remember a
few time when I ran uhd_fft.py file, I saw noise at -110dB and several
carriers at different frequencies. But that carrier at center frequency
was
there too.

I appreciate any help

Thanks

2

The thing in the center is caused by a lot of things, like IQ
imbalances and the distribution of noise towards zero etc. We all get
it when the gain is high. Do you have something you are trying to
detect? Do you have an antenna connected to the right port? What
front-end board are you using? Try 100MHz and look at some local radio
stations.

3

Orkhan -

What exactly do you think is wrong?

Cheers,
Ben

4

Orkhan -

Oops! Ignore my last message.

Will you please send me your serial number, date of purchase, and
purchasing name / organization, off-list?

Cheers,
Ben

5

Andrew

Sorry for the late reply. I am using SBX daughterboard.

The attachments I included earlier are inaccurate and seems like I was
see
that output because I was terminating uhd_fft file using ctrl+c

Right now I have my noise level at -110dB and I can see different
carriers.
The only ambiguity I am facing is that as I change by center frequency
by
0.0001GHz I see some carriers gone and different ones emerged at various
frequencies.

This is happening in every band.

6

They might be internal noise from the USRP, turn up the gain input to
drown them out. That’s what I do.

~Andrew

7

I am sending two screenshots of ffy, one with center freq 2.40002G and
the
other 2.40005

Please note the difference between them

I ran the calibrations too

Does anybody have an idea about how I can correct this? - Or what is the
source of it?

Here is my uhd_usrp_probe output:

/
| Device: USRP2 / N-Series Device
| _____________________________________________________
| /
| | Mboard: USRP-N210-REV4
| | hardware: 2577
| | mac-addr: a0:36:fa:25:37:df
| | ip-addr: 192.168.10.2
| | gpsdo: none
| | serial: EDR17SFUP
| |
| | Time sources: none, external, external, mimo
| | Clock sources: internal, external, mimo
| | Sensors: mimo_locked, ref_locked
| | _____________________________________________________
| | /
| | | RX DSP: 0
| | | Freq range: -50.000 to 50.000 Mhz
| | _____________________________________________________
| | /
| | | RX DSP: 1
| | | Freq range: -50.000 to 50.000 Mhz
| | _____________________________________________________
| | /
| | | RX Dboard: A
| | | ID: SBX (0x0054)
| | | Serial: E3R13X6XS
| | | _____________________________________________________
| | | /
| | | | RX Subdev: 0
| | | | Name: SBX RX
| | | | Antennas: TX/RX, RX2, CAL
| | | | Sensors: lo_locked
| | | | Freq range: 400.000 to 4400.000 Mhz
| | | | Gain range PGA0: 0.0 to 31.5 step 0.5 dB
| | | | Connection Type: IQ
| | | | Uses LO offset: No
| | | _____________________________________________________
| | | /
| | | | RX Codec: A
| | | | Name: ads62p44
| | | | Gain range digital: 0.0 to 6.0 step 0.5 dB
| | | | Gain range fine: 0.0 to 0.5 step 0.1 dB
| | _____________________________________________________
| | /
| | | TX DSP: 0
| | | Freq range: -250.000 to 250.000 Mhz
| | _____________________________________________________
| | /
| | | TX Dboard: A
| | | ID: SBX (0x0055)
| | | Serial: E3R13X6XS
| | | _____________________________________________________
| | | /
| | | | TX Subdev: 0
| | | | Name: SBX TX
| | | | Antennas: TX/RX, CAL
| | | | Sensors: lo_locked
| | | | Freq range: 400.000 to 4400.000 Mhz
| | | | Gain range PGA0: 0.0 to 31.5 step 0.5 dB
| | | | Connection Type: QI
| | | | Uses LO offset: No
| | | _____________________________________________________
| | | /
| | | | TX Codec: A
| | | | Name: ad9777
| | | | Gain Elements: None

8

Orkhan Badirkhanli wrote in post #1051249:

I am sending two screenshots of ffy, one with center freq 2.40002G and
the
other 2.40005

Please note the difference between them

I ran the calibrations too

Does anybody have an idea about how I can correct this? - Or what is the
source of it?

Here is my uhd_usrp_probe output:

Hi Orkhan

I having the same issue and I can’t find a reasonable explanation for
this. I suppose this is some kind of aliasing?

I noticed that this effect only occurs when you change the central
frequency in the GUI.

Filipe

9

Orkhan -

On Tue, Mar 13, 2012 at 5:53 AM, Orkhan Badirkhanli
[email protected]wrote:

I am sending two screenshots of ffy, one with center freq 2.40002G and the
other 2.40005

Please note the difference between them

I ran the calibrations too

Does anybody have an idea about how I can correct this? - Or what is the
source of it?

Can you be more specific about what you want to correct? Is it the IQ
imbalance that is causing you trouble?

Have you tried increasing the gain? You said you ran the calibration
scripts - will you copy/paste the terminal output from when you run
uhd_fft.py?

Cheers,
Ben

10 
I ran the calibrations too

uhd_fft.py?

Cheers,
Ben
Indeed at low gain levels–with the low-order bits hardly being
stimulated at all, you’ll get “weird” FFT results–exactly like the
scalloped images
that Orkhan showed. This gets worse with higher decimations, because
there’s less and less of anything in the signal, due to filter
effects. You need enough gain to make sure that you’re “lighting up”
enough bits, even after filtering for the FFT to “make sense”.

This is easy to see. Terminate the input to your RX. Turn the gain
down. Use high decimations. You’ll get the scalloped FFT output.