How to obtain VARIABLE FREQUENCY at output from pic16f72 ?

Hello Friends
I m an Electrical Engg. student. We have a 1 KW inverter kit. The microcontroller used in the inverter circuit is pic16f72 .we want to obtain variable frequency output from the inverter.

The present freuency of the ac output is 50hz. we want to obtain variable frequency between 25 to 50 hz.

so friends pls tell me how can we obtain variable frequency output.
In addition to variable frequency,we want to have variable volatge also.
so i think PWM technique would work...but is it possible??

i would b so greatful for any help..

Post your schematic, board layout, and source code that already exists.