[Discuss-gnuradio] How to use the SNR block in the GRC

[intermilan]

Hi all:
    I found these is a block gr_probe_mpsk_snr in the grc which is used to measure the snr of the bpsk or qpsk signals. But I
do not know how to use this block.In my flow graph, I put the bpsk modulated data input this block,and put the output of the block to a file_sink.
But there is nothing in that file.So I hope someone can tell me how to use this block.
Thanks in advance.
inter