Duty cycle to voltage without an external capacitor

[powerelec]

I have an incoming variable frequency (1 KHz to 20 KHz) and variable duty cycle pulse. I need to convert the duty cycle into an analog voltage between 0 and 1. I do not have the luxury of RC filtering the pulse using an external capacitor.

What is the simplest way to implement this function on chip?

 

[powerelec]

Anyone want to take a shot? If you feel the question is missing details - do let me know. I have some implementation ideas but I would like to hear some ideas first to get a discussion going.

 

[BradtheRad]

Perhaps it is possible to use some sort of capacitance on-chip?

Or else, can you have a counter circuit increment from a clock, but only when pulses are present during a time window?

Is software on-chip an option?