USRP spike

GNU Radio
1

this spike is always there even without the antenna connected. the fft
is directly the output of the usrp source in the grc so, we are
presuming that the problem might be in the usrp motherboard .
Attached is the screenshots FM receiver spectrum. Did anyone have this
problem? How can we fix it?.

Thanks,Naveen

2

On Thu, Aug 12, 2010 at 1:56 PM, naveen nischal
[email protected] wrote:

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. the fft
is directly the output of the usrp source in the grc so, we are
presuming that the problem might be in the usrp motherboard .
Attached is the screenshots FM receiver spectrum. Did anyone have this
problem? How can we fix it?.

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

Hope this helps, and good luck.

Brian

3

Brain,

Thanks for the reply. We have tried terminating the antenna input, the
spike
still shows up. We also tried tuning a bit away from the signal of
interest and
mixing the signal of interest to baseband but it doesn’t seem to help,
the spike
just follows the signal all over.

Thanks,
Naveen

From: Brian P. [email protected]
To: naveen nischal [email protected]
Cc: [email protected]
Sent: Thu, 12 August, 2010 12:36:09 PM
Subject: Re: [Discuss-gnuradio] USRP spike

On Thu, Aug 12, 2010 at 1:56 PM, naveen nischal
[email protected] wrote:

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. the fft
is directly the output of the usrp source in the grc so, we are
presuming that the problem might be in the usrp motherboard .
Attached is the screenshots FM receiver spectrum. Did anyone have this
problem? How can we fix it?.

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

Hope this helps, and good luck.

Brian

4

Hi Naveen,

On Mon, Aug 16, 2010 at 2:05 PM, naveen nischal
[email protected] wrote:

Brain,
Thanks for the reply. We have tried terminating the antenna input, the spike
still shows up. We also tried tuning a bit away from the signal of interest
and mixing the signal of interest to baseband but it doesn’t seem to help,
the spike just follows the signal all over.

I expected the DC offset to still show up after terminating the
antenna input, but I don’t understand how the spike always follows the
signal unless something is actually there.

From your example, you have 435.300MHz as your center frequency which
“appears” to have a large signal present.

If you tune to 437.300MHz, do you see a strong signal at 435.300MHz
still - or does it “follow” your center tuning to 437.300MHz and
435.300MHz is “in the noise floor”?

Thanks,
Naveen

Brian

5

Brain,

Sorry my bad…your earlier technique worked. Thanks much

Regards,
Naveen

From: Brian P. [email protected]
To: naveen nischal [email protected]
Cc: [email protected]
Sent: Mon, 16 August, 2010 12:16:57 PM
Subject: Re: [Discuss-gnuradio] USRP spike

Hi Naveen,

On Mon, Aug 16, 2010 at 2:05 PM, naveen nischal
[email protected] wrote:

Brain,
Thanks for the reply. We have tried terminating the antenna input, the spike
still shows up. We also tried tuning a bit away from the signal of interest
and mixing the signal of interest to baseband but it doesn’t seem to help,
the spike just follows the signal all over.

I expected the DC offset to still show up after terminating the
antenna input, but I don’t understand how the spike always follows the
signal unless something is actually there.

From your example, you have 435.300MHz as your center frequency which
“appears” to have a large signal present.

If you tune to 437.300MHz, do you see a strong signal at 435.300MHz
still - or does it “follow” your center tuning to 437.300MHz and
435.300MHz is “in the noise floor”?

Thanks,
Naveen

Brian

6

On 08/12/2010 02:36 PM, Brian P. wrote:

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

7

Hi Naveen,

On Mon, Aug 16, 2010 at 6:01 PM, naveen nischal
[email protected] wrote:

Brain,
Sorry my bad…your earlier technique worked. Thanks much
Regards,
Naveen

Glad you were able to get it figured out.

Brian

8

On 08/16/2010 10:14 PM, Vincent W wrote:

there is a single spike that seems to follow the centre signal.
is what I expect.
this thread.
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 L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

9

On 08/17/2010 02:01 AM, Vincent W wrote:

As evidence for the other kind of spikes, the ones that follow you around,
please see the attached pictures, centered around frequencies of 1134 and
1135MHz. These are the spikes that follow the centre frequency around.

The problem around 100MHz is very similar to what has been previously reported
here: Re: [Discuss-gnuradio] Questions about WBX daugtherboard,
but I’m not sure exactly what the correct resolution is.

To follow up, in the case of those spikes that seem to follow the center
frequency, the solution appears to be increasing the gain until they go
away, or
else avoiding them entirely by setting it to the maximum value.

However, I still have problems with the spikes around multiples of
100MHz. As an
example, if I tune the WBX to 100MHz, everything appears fine, but if I
tune to
100.25MHz, as shown in the attached picture, I see all sorts of spurious
spikes
that do not go away - regardless of the gain setting.

Regards,

Vincent

10

On 08/16/2010 11:22 PM, Marcus D. Leech wrote:

Chances are you have a DC offset at the ADC that needs to be removed.

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.

Answering your questions:

For the 100MHz signals, what does your receive chain look like?

I’m not quite sure what you mean. My initial experiments use the
400-1000MHz log
periodic antenna hooked up to the USRP2, with the u2_rev3-20100603.bin
FPGA
image and txrx_wbx_raw_eth_20100608.bin firmware. I see these anomalous
signals
even if I write the raw usrp2 output to a file, with no processing, read
it in
octave using read_complex_binary, and plot it. I opted to use the
usrp2_fft
program for ease of use.

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.

One of the first things I did, and where I noticed this, was when
building the
FM receiver. That exercise is what gave me the first inkling that those
peaks
aren’t FM bands or stations, and, when centred at 100MHz, may not even
exist.

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?

Ultimately, one of my goals are to build a low frequency dielectric
spectroscope
(DC-2.2GHz for now). My initial target is more modest - I would like to
build a
wideband spectrum scanner. To this end, I’m quite interested in the
relative
amplitude across a large frequency range. However, it’s been very
difficult
because I keep having to work around these artifacts when taking the
individual
samples that eventually make up the wideband composite plot.

I don’t have a Faraday cage or RF anechoic chamber. I tried,
unsuccessfully, to
build the former using aluminum foil and wire mesh, but judging from the
wideband spectrum output of the usrp2, it didn’t do much. I can show you
pictures of the set up, and the graphs of the spectrum before and after
(there’s
not much difference).

As evidence for the other kind of spikes, the ones that follow you
around,
please see the attached pictures, centered around frequencies of 1134
and
1135MHz. These are the spikes that follow the centre frequency around.

The problem around 100MHz is very similar to what has been previously
reported
here:
Re: [Discuss-gnuradio] Questions about WBX daugtherboard,
but I’m not sure exactly what the correct resolution is.

Thank you very much for all your help, I look forward to figuring this
out.

Vincent