On Oct 13, 2010, at 8:45 PM, Drew Read wrote: > We're having some problems with transmitting using the WBX board on a > USRP1 (not an old one). > We have a flow graph with a NULL source going into a NBFM, then > through a multiply_const and into the USRP. > At low frequencies (100MHz) the signal looks ok, but when we get up > passed 500MHz we can see/hear that what should be an unmodulated > carrier also has a single side-band (and is audible as a tone when > demodulated) . And by increasing the const in the multiply_const, we > can see our wanted signal getting stronger while the side-band remains > there at a constant level.
That sounds (and looks) like an issue that I encountered yesterday with my USRP + WBX. I was using it to generate some test signals in the 1.5-1.7 GHz range, and I found that there was a constant spur about 1-2 kHz above the transmitter's center frequency. I also found that I could reduce its effects by playing with the signal level, transmitter gain and an external attenuator. For now, I think I can work around it by simply generating my test signals at an offset from the transmitter center frequency and adjusting the center frequency to put them back where I want them, thus moving the spur out of the middle of my test signal. > We've had a play with manually adjusting the DC offset of the USRP > sink, which seems to change the size of the side-band a bit, but we > don't really know what to do with that. I also found a bit of the transmitter center frequency bleeding through when I modulated it with no DC component. I haven't tried to mitigate this yet, but my workaround for the spur should also let me ignore this little bit of unwanted carrier for the time being. I presume that it's due to a small DC offset error in the DACs, based on the my observation of what appears to be a small (sub-LSB) DC error in the receiver ADCs. In my receive path, I found there to always be a received component at the tuned receiver frequency, and I found that I could null it out by adding a small (magnitude < 1.0) complex constant to the USRP source output before it passed to the rest of my receive chain. Over the course of a few hours and a power-cycle, I found that I had to readjust the constant once or twice. This little error is pretty negligible when receiving larger signals, but it was annoying while I was trying to look at some pretty weak signals. I may be able to swamp it out with some external gain, but I didn't have an LNA handy yesterday to try that. For the time being, I'm going to try to ignore these DC components in both the receiver and transmitter by simply offsetting the LO a bit from the signal that I want to transmit or receive. I have to do some more tests tomorrow using my USRP as a signal generator, so I hope this will work. In the longer term, I'm interested in looking into whether there's a better approach to trimming out any DC offsets in the DACs and ADCs. I'm also interested in learning more about that unwanted spur in the transmit output (particularly since it's quite a bit larger than the component that appears to be caused by a small DC error). > We've tried several USRPS and several WBX boards and seen the same > problem on all of them. > We don't see it on the USRP2. Interesting. -- Mark J. Blair, NF6X <n...@nf6x.net> Web page: http://www.nf6x.net/ GnuPG public key available from my web page. _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
