Verilog-A coding, How can I write a Vpulse whose frequency is variable.

[Ronalren]

Hello everyone, Im trying to write a Verilog-A Vpulse whose frequency is supposed to be 50M at 0-15us, 51M during 15-20us and 49M during 20-25us. How can I do that? What kind of operators or expressions are suoopsed to be involved in the coding? Timer or $abstime or $realtime?
Thanks a lot for any answer!