Generating sinewave from a pulse train by using a micro-controller
You can use a PWM and a low pass filter. If you have a microcontroller that can do a 10-bit PWM at 16MHz that gives you a PWM frequency of about 15kHz so you could use a low pass filter with a cutoff of a few hundred Hz. 

You will have to figure whether determining the next value of PWM and the resulting flat spots on the sine waves from the limited resolution (you could dither) are acceptable and possible with the micro you have in mind. 



You could also use a DDS chip (Direct Digital Synthesis) that contains a sine lookup table in ROM, phase counter and a DAC. Analog Devices has a wide range of such products. 



Or, use a DAC connected to the Microcontroller or an FPGA, which could allow extremely high quality synthesis. We use a system like that to generate extremely pure sine waves (24-bit DAC) in the tens of Hz for cryogenic measurements.