AD9694 SYSREF signal doesn't work

Hi,
Now I'm using AD9694 in subclass 1 mode. I find that when I don't input the SYSREF, the LMFC is also generated after I config the chip through spi. It is like that the chip working is independent of SYSREF.

I observe the LMFC by outputing LMFC to FD_A pin.
Reg 0x120 value is 0x04.

I want to know which can result in the problem?

Thanks.