DQPSK works only at certain center frequencies

Hi,

I’m using two XCVR2450 on USRP2s.
When I’m testing gnuradio-examples/python/digital/benchmark_tx(rx).py,
it works fine with DBPSK modulation, regardless of center frequency.
But with DQPSK, receiver receives messages only when operating at 2.45
or 2.40 GHz.
With D8PSK, receive nothing.
I have changed interpolation, gains, and bitrate, but neither solves
that problem.
Please help me!
Is anybody who has problems with DQPSK or D8PSK?

Kim.