You can configure a digital output channel to output a signal with a set frequency and period. Use a control on the front panel of a VI to control the on time (also called the active time or duty cycle) of the pulse. Pulse-Width Modulation Example

Refer to the Pulse Width Modulation (Host) VI in the labview\examples\CompactRIO\NI Scan Engine\Advanced\Specialty Digital Configuration - PWM\Specialty Digital Configuration - Pulse-Width Modulation Mode.lvproj for an example of pulse-width modulation. The example uses an NI 9474, but you can modify it to use a different digital output module.