Re: [Discuss-gnuradio] USRP spike

[Marcus D. Leech]

On 08/16/2010 10:14 PM, Vincent W wrote:
> 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
>   
For the 100MHz signals, what does your receive chain look like?  Keep in
mind that the FM radio band exists between 88MHz and 108MHz,
  so if you're trying to do "off air" experiments, you'll run into
trouble. If you're not doing "off air" experiments, what kind of
shielding do you
  have for your equipment, to protect it from the FM broadcasting band?



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