i am fibished with ths job.. i did it using a voltage controlled oscilator integrated circuit called 8038. i amplified the output of the microcontroller using an octocoupler. i obtained the DC value of the pwm pulses using an R-C circuit.. then i fed this DC value to the 8038 integrated circuit. now by changing the duty cycle of the PWM signal i can change the frequency of the output signal. 8038 can produce sine-triangulae and square waves at the same time..