Re: [Discuss-gnuradio] sampling artefact because of two step mixing?
Hi Don, the problem is that the second peaks are not at DC in baseband. For example: I generate a sine tone at 2.444 Ghz. Then I tune the usrp to centre frequency of 2.448 GHz. Now I can see the peak at -3 Mhz in base band. Ok, this is correct because what Iam seeing is the original sine. When I tune the USPR to centre frequency 2.450 Ghz I see an attenuated peak at 1 MHz in baseband what correspond to 2.451Ghz. This second peak is 6 Mhz above the original sine. If I rise the gain in the usrp_fft.py tool there appear more undesired peaks but I am still feeding the USRP with olny one tone. If I decrease the gain to eliminate all secondary peaks my original signal gets too weak and it is less than 10 db over the noise floor. I am trying several setting to find out what is the reason of all this. The last try was: 1 sine tone at 2.488 Ghz feed into USRP The result was: one peak at 2.488 Ghz and one alt 2.432 GHz (20 dB's lower) Now the secondary peak is 56 MHz away from the original tone!!! Please, can anyone help me? I am getting desperate... Luis On Wednesday 18 October 2006 20:12, you wrote: I have observed a similar phenomenon with the LFRX daughterboard. It seems there is often (or always?) a peak at DC in the baseband (USRP output) spectrum, regardless of the tuned frequency of the USRP. I suspect it is due to rounding down in the CIC decimation filter. Adding a value of e*complex(1.0,1.0) where 0.5 = e = 0.8 to the USRP output will cancel it. -- Don W. - Original Message - From: Luis Simoes [EMAIL PROTECTED] [snipped] 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. ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
[Discuss-gnuradio] sampling artefact because of two step mixing?
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
Re: [Discuss-gnuradio] sampling artefact because of two step mixing?
I have observed a similar phenomenon with the LFRX daughterboard. It seems there is often (or always?) a peak at DC in the baseband (USRP output) spectrum, regardless of the tuned frequency of the USRP. I suspect it is due to rounding down in the CIC decimation filter. Adding a value of e*complex(1.0,1.0) where 0.5 = e = 0.8 to the USRP output will cancel it. -- Don W. - Original Message - From: Luis Simoes [EMAIL PROTECTED] [snipped] 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. ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
