Re: [Discuss-gnuradio] about uhd_fft.py

[Marcus D. Leech]

Orkhan -

On Tue, Mar 13, 2012 at 5:53 AM, Orkhan Badirkhanli <address@hidden> 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

 

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.

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

On Sat, Mar 10, 2012 at 4:26 PM, Andrew Davis <address@hidden> wrote:

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

~Andrew

On Sat, Mar 10, 2012 at 6:28 AM, Orkhan Badirkhanli <address@hidden> wrote:
> 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.
>
>
> On Thu, Mar 8, 2012 at 7:24 PM, Andrew Davis <address@hidden>
> wrote:
>>
>> 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.
>>
>> On Thu, Mar 8, 2012 at 8:47 AM, Orkhan Badirkhanli <address@hidden>
>> wrote:
>> > 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
>> >
>> > --
>> > Orkhan
>> >
>> > _______________________________________________
>> > Discuss-gnuradio mailing list
>> > address@hidden
>> > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>> >
>
>
>
>
> --
> Orkhan

--
Orkhan

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-- 
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org