[Discuss-gnuradio] Pulse on FSK Preamble

hello Everybody,

I am trying to recover my fsk signal with the Muller&muller block. 

Here is my configuraiton:

Sample rate of USRP = 240K

bitrate of signal = 9600bps

receiving the signal from the USRP,

 apply a low pass filter -> quadrature demod ->rational resampler -> m&m sync block

My signal contain a preamble of 100ms of 1010 (0x55). after my Muller&Muller block, I can see the recovering working for my signal but not for my Preamble. I have pulse on the preamble which sometime are visible on the data part as well.

https://ibb.co/hfLt62K 

https://ibb.co/xqRkRP7

 I have tried also with Polyhpase clock sync block but this one took to much time to be syncronised with my preamble.

Can someone help me by explaining this pulse? I porbably do not use correctly the sync block ( I have left the default value except for the omega=sps ). Can someone redirect me to a documentation to use it correctly?

let me know if I have to give more information.

Thank you in advance for your help

--

Samuel Verdon