high frequency optoisolator needed

here I am trying to design an analog project which counts the frequency.Circuit consist of an photo transistor optoisolator and some counters for counting number of cycles per second.For this 1 second i am using timer ic(555).Now my doubt is which is the best phototransistor opto isolator should I use inorder to count frequency till 16 Khz.As above 16Khz i would require prescalar. Thanking you in advance

With 16 kHz I think any 4N25 - 38 would be o'k. 6N136 guaranteed up to 1Mb, and 6N137 to 10 Mb, though price is also goes up.

Are you sure sir? Will 4N25 take 16Khz on itself? Is there any extra hardware required ?

Not sure, my guess is based on 2 usec rise and fall time in data sheet, 16 kHz = 66 usec, it appears there is x30 times difference between