'lo all. We are trying to use the USRP with the new gmsk demod block, and are encountering some weirdness. To generate our GMSK signal, we are using an Agilent signal generator. BT=0.35, 125 ksymbols/sec, 2 samples/symbol, with a PN15 as the data source.
We load the produced slicer.dat into Matlab and cross-correlate with the known PN15 sequence. Sometimes the output looks good: all our peaks are at 2^15 - 1 = 32767, indicating that all bits are being demodded correctly, as you can see here: http://img472.imageshack.us/img472/2766/fullpeakscb3.jpg Other times, we encounter "half-height" peaks, which go up to 2^14-1 = 16383, indicating that 25% of the bits are being demodded incorrectly. You can see this here: http://img441.imageshack.us/img441/7507/halfpeaksax7.jpg If you zoom in on the previous image, you can see that each xcorr peak actually consists of 2 high points, then a lull of 14 or so, then another high point. This pattern is always the same when the half-height phenomena is observed: http://img168.imageshack.us/img168/7657/halfpeakszoomeduy7.jpg Any idea what might be going on, or how to debug this? -Steven
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