it would be easier to just use a tone detector IC. this is the typical method for most of the modulated IR receivers I have seen, as well as ultrasonics. you will then get a nice clean digital output from it.
you may also wish to use 40kHz as your modulation frequency, as that is by far the most common frequency to use (38kHz is also common)
and it is what is used in many TV remote controls (which allows you to test your circuit easily using a known-working TV remote)