simulink pulse generator block

hi, im configuring the block to generate pulses at period of 20usec and 50% duty cycle, 0 phase delay. the output of the block is enterd to a scope block. the simulation parameters are "fixed step" (runga-kuta) with max step size of 1usec. after running the simulation, when i watch the scope, i dont see the freqency and the duty cycle i have configured. why? regard ofir