Hello everybody, 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 data in Matlab to evaluate the results. As I feed the USRP with a single sine tone with a frequency of 2.444 GHz and an amplitude of -50dbm I saw on my plot a nice peak at 2.444 GHZ but also a second peak at 2.452 GHZ but attenuated by 15 db's when daughter board is 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 any side peaks. However, the usrp_fft.py tool from the gnuradio examples shows the same phenomenon including the second peak. The parameters I use in my application are : Flex2400 daughterboard 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 the remaining frequency offset after the analog mixing in the daughterboard tuned the centre frequency as close as possible to baseband. When the tune method is set to the centre frequency of 2.452 Ghz, the flex mixes with 2.448 GHz and the DDC with -4MHz. By mixing with a cos wave we get 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 (f+f0) can now explain the appearance of this side peak in my plot. But: 1. Why is the second peak attenuated? If it is a result of mixing it should be as high as the original signal? 2. If the assumption of the two peaks is correct, why are the assumed and the real measured peaks mirrored in other configurations (other signal frequency and center frequency of the usrp)? 3. The flex2400 is able to tune to every frequency between 2400 and 2500 MHz 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. Which filters are implemented and what are the parameters used in the logical steps of mixing, decimating and low pass filtering? Is there any way to avoid this physically not existing signals and if not is there a detailed explanation why this phenomenon occurs in an irrational (it seams so) way? I am very grateful for any advice, Perhaps Matt and Eric are the experts in this matter. So this question is specially directed to you. Thanks a lot Luis _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio