Hello guqunlin, Thank you for contacting National Instruments. This puluse's duty cycle is limited to discrete values: 50 %, 33 %, 25 %, 20 %, 16.6 % etc.You can refer to the FP-CTR-502 and cFP-CTR-502 Operating Instructions (http://digital.ni.com/manuals.nsf/websearch/FD58CA0682B21E5B86256D4A0062510A) to find the reasons.
You might want to consider using the (c)FP-PWM-520 which has 12-bit duty cycle resolution (4,096 discreet values from 0 to 100 percent). You could also use the (c)FP-PG-522. This module is limited in the resolution of the duty cycle as well. To set the duty cycle, you specify the on time and off time of the pulse.
Also, you can refer to this link on discussion forums: http://forums.ni.com/ni/board/message?board.id=170&message.id=91147&requireLogin=False