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 >
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