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.
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
periodic antenna hooked up to the USRP2, with the u2_rev3-20100603.bin
image and txrx_wbx_raw_eth_20100608.bin firmware. I see these anomalous
even if I write the raw usrp2 output to a file, with no processing, read
octave using read_complex_binary, and plot it. I opted to use the
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
One of the first things I did, and where I noticed this, was when
FM receiver. That exercise is what gave me the first inkling that those
aren’t FM bands or stations, and, when centred at 100MHz, may not even
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
(DC-2.2GHz for now). My initial target is more modest - I would like to
wideband spectrum scanner. To this end, I’m quite interested in the
amplitude across a large frequency range. However, it’s been very
because I keep having to work around these artifacts when taking the
samples that eventually make up the wideband composite plot.
I don’t have a Faraday cage or RF anechoic chamber. I tried,
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
not much difference).
As evidence for the other kind of spikes, the ones that follow you
please see the attached pictures, centered around frequencies of 1134
1135MHz. These are the spikes that follow the centre frequency around.
The problem around 100MHz is very similar to what has been previously
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