I am currently using the flex2400 board and I feed it with some designed
signals from a sophisticated signal generator. I analyse the spectrum of
interest by fft and pass all information to a file sink. I plot the file
in Matlab to evaluate the results.
As I feed the USRP with a single sine tone with a frequency of 2.444 GHz
an amplitude of -50dbm I saw on my plot a nice peak at 2.444 GHZ but
second peak at 2.452 GHZ but attenuated by 15 db’s when daughter board
tuned to 2.452 Ghz. To verify the accuracy of the signal generator I
connected it to a high quality spectrum analyser. The spectrum analyser
verified that the output of the signal generator is a clean peak without
side peaks. However, the usrp_fft.py tool from the gnuradio examples
the same phenomenon including the second peak.
The parameters I use in my application are :
Decimation factor 8
complex samples at 16bit I and 16 bits Q each
fft size 64 ( corresponds to 125 kHz bin resolution)
My first idea points to the effect of the second mixing in the DDC from
remaining frequency offset after the analog mixing in the daughterboard
the centre frequency as close as possible to baseband.
When the tune method is set to the centre frequency of 2.452 Ghz, the
mixes with 2.448 GHz and the DDC with -4MHz. By mixing with a cos wave
two peaks, one at (f-f0) and one at (f+f0), but both with half signal
strength. The resulting peak from mixing with the double frequency
now explain the appearance of this side peak in my plot.
- Why is the second peak attenuated? If it is a result of mixing it
as high as the original signal?
- If the assumption of the two peaks is correct, why are the assumed
real measured peaks mirrored in other configurations (other signal
and center frequency of the usrp)?
- The flex2400 is able to tune to every frequency between 2400 and 2500
in steps of 1 MHZ. Why can I not tune the flex directly to the centre
frequency without another mixing stage in the DDC? The DDC frequency is
allways between -2 and -5.5MHz. Would this effect disappear if no second
stage mixing is needed?
I found almost no documentation about the configuration of the DDC.
filters are implemented and what are the parameters used in the logical
of mixing, decimating and low pass filtering?
Is there any way to avoid this physically not existing signals and if
there a detailed explanation why this phenomenon occurs in an irrational
seams so) way?
I am very grateful for any advice,
Perhaps Matt and Eric are the experts in this matter. So this question
specially directed to you.
Thanks a lot