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.
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?