Re: [Discuss-gnuradio] USRP spike

[Vincent W]

On 08/12/2010 02:36 PM, Brian Padalino wrote:
> On Thu, Aug 12, 2010 at 1:56 PM, naveen nischal
> <address@hidden> wrote:
>>> problem though is a spurious spike of about 17db which appears at
>>> whatever center frequency we tune to in the spectrum. we think that this
>>> might be the problem as that might be jamming the signal which was
>>> supposed be about the same db level. The point of notice for us is that
>>> this spike is always there even without the antenna connected.
> 
> Terminate your antenna input.  Does it still show up?
> 
> Chances are you have a DC offset at the ADC that needs to be removed.
> This will happen if the DDC in the FPGA isn't required to resolve any
> frequency offset due to the limitations of the LO in the RF chain.
> 
> One way to mitigate this is to tune a little bit away from your signal
> of interest, then mix your signal of interest to baseband, and filter
> off the DC component.
> 
>> Thanks,
>> Naveen

Hi,

I think I am encountering a similar problem, and would very much appreciate your
feedback, bearing in mind have an experimental physics background. I'm using the
WBX board on the USRP2, and notice unusual signal spikes when I tune to a
specific frequency. In some cases, there are multiple spikes that show symmetry
about a given frequency, but disappear when tuning to that frequency. In others,
there is a single spike that seems to follow the centre signal.

I've included some screenshots of the output from usrp2_fft. The first set of
pictures, dealing with 99MHz, 100MHz, and 101MHz, illustrate the first kind of
spikes:

        - If I tune into 99MHz, I see large spikes at 96MHz, 100MHz, and a 
small spike
at 104MHz (bracketed in black).

        - If I then tune to 100MHz, I see a small spike at 100MHz (and I'm not 
sure if
this is an artifact), but outside of that, everything looks very reasonable and
is what I expect.

        - Finally, if I tune into 101MHz, I see large spikes at 100MHz and 
104MHz,
along with a small spike at 96MHz.

The position of the anomalous spikes is the same when tuning to 99MHz and
101MHz, but their magnitudes seem to be mirrored about 100MHz. I think this has
something to do with the DDC; at 99MHz, it's negative, at 101MHz it's positive,
but I'm not sure how this helps.

The second type of problem is very similar to the one previously discussed in
this thread.

If I tune into 1.134GHz, I see a spike at the centre frequency, of 1.134GHz and
another spike at 1.136GHz, both around 1.135GHz. However, when I tune into
1.135GHz, the two spikes disappear, to be replaced by a single spike at 
1.135GHz.

I'm not quite sure how to proceed, or what to reference. It certainly seems that
these spikes are spurious, and I'd love to get rid of them, but I'm not sure 
how.

Regards,

Vincent

usrp2_99M_spike.jpeg
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usrp2_100M_spike.jpeg
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usrp2_101M_spike.jpeg
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