For these specific products, the generation engine is designed in such a way that the minimum on time is constant, but the max/min duty cycle changes with frequency. Figure 1 shows three signals that have an on time of 8.25nS. Since the three signals have different frequencies, there is a different minimum duty cycle for each dependant on the minimum on time spec. The 30 MHz signal can have a duty cycle as low as 24.75%, but the 40 MHz signal can only have duty cycles 33% or greater without violating the minimum on time spec.
An example of how this value is displayed in the Specifications of the PXIe-5433 compared to the PXI-5404 can be found below:
Minimum Duty Cycle = (100% x Minimum On Time)/T_period
Maximum Duty Cycle = 100% - Minimum Duty Cycle