Please do not get me wrong. I believe that the work from Ettus and the contributors to GNU-Radio are revolutionary! The equipment with the interface to GNU-Radio is not only priced at a level that is affordable to many amateurs and hobbyists, it opens up a brand new world for... (the list is too long to list here)
In fact, I am so impressed that I desire to contribute to the community. First by contributing to these forums and second by eventually posting projects to CGRAN! My intent was to let Gaetano know of the potential for spurious signals so that he can properly select a center frequency that is free of these little nuisances. Again, I am impressed with the level of commitment and the work that the team puts into to improving an already great product. Evan Merewether -----Original Message----- From: discuss-gnuradio-bounces+evan=syndetix....@gnu.org [mailto:discuss-gnuradio-bounces+evan=syndetix....@gnu.org] On Behalf Of Marcus D. Leech Sent: Friday, December 23, 2011 7:54 AM To: discuss-gnuradio@gnu.org Subject: Re: [Discuss-gnuradio] DC component > I have noticed that there are some fixed frequency spurious signals in my > N210. These spurious signals are probably associated with the harmonics of > the clock. If your DC component is at some nice even frequency like 2GHz, I > would suspect a spurious signal to be the cause. > > Evan > > Spurious signals are a virtually-inevitable aspect of modern receivers/transmitters. Many of us are used to radios that are "purpose built", and probably don't realize that most such radios have their own problems with spurious signals ("spurs"), but that they get "tweaked" in the design phase to move those (inevitable) "spurs" outside the operational envelope of the particular application at hand. Your radio have a CPU? Move the fundamental of its clock frequency so that the fundamental and harmonics fall outside of your band of interest. But in a radio whose "band of interest" lies anywhere from DC up to a few GHz, that's a very tall order. The good news is that most of the time, these "spurs" are quite weak, and are generally overwhelmed by any actual signal coming in from the antenna at the the same frequencies. For modern wideband modulation schemes, an in-band spur that's 30-40dB below your nominal incoming signal make essentially no difference to the receive SNR. For narrowband weak signals that are coming in just above the noise floor, it might be a different story. I've attached a plot of 50MHz of spectrum (thanks to the latest 50Msps/sc8 support in UHD) around 1.420GHz, with the receiver input terminated in a 50 Ohm lab-grade termination. You can clearly see spurious signals spaced every 5MHz, and a stronger one right at 1.40MHz. The 5MHz may be from the ethernet clock, not sure, but the stronger spur at 1.4GHz is very likely due to an even harmonic of the 100MHz master clock. Even though this "spur" at 1.4GHz is 40dB "out of the noise", in most applications the signals themselves will *dwarf* that spur. The other spurs, across 50MHz of bandwidth are no more than 20dB "out of the noise". They don't worry me that much, even for applications like radio astronomy, where the signals are really weak. Placing a low-noise gain chain ahead of the receiver, with enough gain to "swamp" the receiver spurs is all I need to make these go away. It's true that a $40K laboratory-calibrated receiver like an (Agilent, R&S, etc) spectrum analyser will likely have fewer "spurs". But if you open one of those up, you'll notice a lot of individually shielded sub-assemblies--that's not just for show. They'll also do tricks like spreading the clocks for the control/monitoring CPU, shifting clocks around, to make the "spurs" move away from the current band of interest. And for the most part, things like laboratory spectrum analysers are "deaf as a post"--they aren't designed, generally, to be "off-air" receivers, so they tend to be less sensitive to their own "spurs". -- Marcus Leech Principal Investigator Shirleys Bay Radio Astronomy Consortium http://www.sbrac.org ----- No virus found in this message. Checked by AVG - www.avg.com Version: 2012.0.1901 / Virus Database: 2109/4697 - Release Date: 12/22/11 _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio