How to make a filter?

You need band pass filters to get rid of the side lobes (frequencies) for pure 20khz from 35khz. Bats can hear 20khz and small human children can but I know of no adult human that can hear that. After all of the filtering and pre-amps, you need a final amp and a tweeter speaker. I bet dogs and cats can hear it.

How wide is the signal at 35KHz? If say just a few KHz, then you can multiply (modulate) with say a fixed 30KHz signal (carrier) and pass the result through a say 10KHz low-pass filter. A 35KHz signal would come out as a 5KHz tone, a 37KHz input would come out as a 7KHz tone.

To prevent aliasing errors, you should pass the input signal through a 30KHz to 40KHz bandpass filter first.

ANY HUMAN COULD NOT ABLE TO HEAR MORE THAN 15KC ! YOU ARE WASTE OF TIME TO TRY THAT. SIMPLE FILTER IS LC FILTER.