On Wed, Jun 8, 2011 at 11:24 PM, Morgan Redfield <redfie...@gmail.com> wrote: > On Wed, Jun 8, 2011 at 7:20 PM, Marcus D. Leech <mle...@ripnet.com> wrote: >> On 06/08/2011 09:51 PM, Morgan Redfield wrote: >>> >>> Ok. I found another problem in my code. The transmit_path has a >>> multiplier in it that was set to 200 (it could go up to 32768). This >>> was for the original USRP, but I think UHD needs a signal with >>> magnitude<=1. It seems like the signal was probably clipping. I >>> changed the magnitude of the multiplier and fiddled with the gain >>> parameters a bit. I'm now able to get the standard boxcar that I >>> expect for OFDM. There's still a spike at the center frequency though, >>> I don't see a gap. >>> >>> Thanks, >>> Morgan >> >> If you're doing these tests over-the-air, is it possible that there's a >> signal on TV channel 44 around you? >> Channel 44 starts at 650MHz. >> >> >> >> -- >> Marcus Leech >> Principal Investigator >> Shirleys Bay Radio Astronomy Consortium >> http://www.sbrac.org >> >> >> >> _______________________________________________ >> Discuss-gnuradio mailing list >> Discuss-gnuradio@gnu.org >> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >> > > When I'm not transmitting, I don't really see anything above the noise > at the center frequency of 650MHz. There is a small spike (a few dB or > so) right at the center frequency, but I think that's due to the USRP. > Other messages on this list seem to indicate that it isn't abnormal. > > That spike is only very noticeable when I'm transmitting. I'll try > changing the tx frequency and playing around with the gain tomorrow > and let you know how that goes. > > Thanks, > Morgan >
I found that centering my FFT on a frequency that's offset from what I'm transmitting at will remove that central spike. I was able to finally see the gap in the center of the OFDM boxcar and adjust that. It looks like in my setup I have an offset of about 6kHz. My OFDM signal never seems to be more than about 10 dB above the noise floor though. When I bump up the gain or tx-amplitude, everything gets raised by the same amount. I'm still not able to demodulate packets, and I think this is why. Do you have any advice about this? Thanks, Morgan _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
