Derek is spot on. 0 dB into any USRP is screaming loud. Most of the units are very sensitive, so it's perfectly fine to err on the side of too much attenuation to start and dial the gain as you need. It depends on the USRP type, but signals much above -15dB can physically damage the hardware if you set the gain too high.
On Wed, Mar 23, 2016 at 9:35 PM Derek Kozel <derek.ko...@ettus.com> wrote: > Hello Tom, > > The input power to the USRP daughterboard should not be above -15dBm. I > would recommend putting as much attenuation as you have available inline > and raising the gain until just before a timesink shows the signal > clipping, as Andy says having the maximum attenuation set will be raising > your noise figure. > > Regards, > Derek > > > On Wed, Mar 23, 2016 at 4:28 PM, Andy Walls <a...@silverblocksystems.net> > wrote: > >> >> > From: >> > Tom Golden >> > Subject: >> > Re: [Discuss-gnuradio] Ettus N210 >> > GMSK 9600 >> > Date: >> > Wed, 23 Mar 2016 13:14:18 -0600 >> > >> > ______________________________________________________________________ >> > Here's my flow-graph along with a snapshot of the constellation and >> > FFT. >> > >> > >> > Thanks!! >> > -Tom >> >> Hi Tom, >> >> A couple of things: >> >> a) The polyphase resampler isn't going to work well without a filter >> definition in the taps field. >> >> b) The constellation sink will not display anything meaningful without >> sample timing synchronization; it is not useful at its current position >> in the flowgraph. The constellation sink also doesn't display anything >> useful for an FSK modulation normally; (G)MSK being an exception, if >> treating it like a PSK modulation vs. FSK. >> >> c) You don't have any coarse or fine frequency synchronization. That >> will cause you major problems, if trying to treat GMSK as a PSK >> modulation. It will cause you minor problems, if treating GMSK as an >> FSK modulation. >> >> d) Timing recovery blocks usually want a signal that has peaks (which >> you get by putting the signal through a matched filter), and those peaks >> should nominally be scaled to +/- 1.0. You don't have a matched filter >> or an AGC before the Clock Recovery block. >> >> e) The USRP's 0 dB gain setting is actually the USRP inserting the >> maximum attenuation it can (e.g. 37 dB of attenuation). That can kill >> your signal to noise ratio. You may want to consider adding "gain" as >> long as the time domain signal doesn't look clipped (sometimes hard to >> tell with FSK). >> >> f) You may wish to look at what Nick Foster's gr-ais does to demodulate >> a 9600 baud GMSK AIS signal. It will probably give you a nice starting >> point; just ignore the stuff about correlating to a preamble. >> https://github.com/bistromath/gr-ais >> >> If you share your datafile somewhere, I might be inspired to whip a >> flowgraph that works on it. :) But that could rob you of the learning >> process. >> >> Regards, >> Andy >> >> > >> > On Wed, Mar 23, 2016 at 1:01 PM, Marcus D. Leech <address@hidden> >> > wrote: >> > On 03/23/2016 02:48 PM, Tom Golden wrote: >> > Hi, >> > >> > I'm a novice gnu radio user. I'm using gnuradio with >> > an Ettus N210 cabled to a modem transmitting GMSK >> > 9600bps. This is just for a test to verify the modem >> > transmit bits. >> > >> > I'm having issues with resampling. The N210 clock >> > can't be set to a multiple of 9600, so I'm attempting >> > to resample. I've tried various mechanisms but the >> > output after resampling to 96000 is too noisy to >> > successfully decode bits. I've tried the GMSK demod >> > block as well as the combination of Quadrature >> > Demod->Clock Recovery MM->Binary Slicer - and neither >> > works. >> > >> > I've also played with the Polyphase clock sync but I >> > don't see any noticeable difference. Can anyone >> > recommend a solution? >> > >> > Thanks!! >> > -Tom >> > >> > For a first step, it would be useful for you to share your >> > flow-graph with the list. >> >> >> >> _______________________________________________ >> Discuss-gnuradio mailing list >> Discuss-gnuradio@gnu.org >> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >> > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > -- Very Respectfully, Dan CaJacob
_______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
