Hello Tim,
I found a solution without changing your code. I just send a "volume" that is related to the frequency (5...10Hz -> Vol=1; 10...15Hz -> Vol=2 and so on). Over all, I have pulses with ~1ms width in the frequency range from 5 to 100 Hz. Frequencies less than 5Hz are forbidden, because the pulses are getting longer than 1ms. At frequencies higher than 100Hz the pulse length is getting smaller. But this is acceptable.
This was my goal 
Thank you very much for your help.