[Discuss-gnuradio] frequent phase slip with the new digital.costas_loop_cc

[Kyle Zhou]

I adapted my codes from gnuradio-3.3 to the new 3.6 version.
One of the changes is the new digital.costas_loop_cc is now implemented based 
on control_loop.
The costas loop is used for phase recovery of a QPSK signal.
But I noticed that with the new version, the output experiences a lot of phase 
slips. 
To be exact, the phase at the output rotate 90 degree from time to time.
I changed it back to the old gr.costas_loop_cc and this did not happen.
The setting for the old loop is alpha=0.01, beta=alpha*alpha/4, 
max_freq=2*pi*0.1, min_freq=-max_freq.
The setting for the new loop is loop bandwidth = 2*pi/100

The new version is only good when SNR is very high, say 13dB+

Any has encountered the same problem? or am I doing something wrong?
KZ