> can i use PIC18F452/10bit ADC/ to detect 50Khz sound
> with sampling frequency 100kHz?

I think even under the best conditions that's pretty much on or
past the limit (according to Table 17.1 of the 18F452 manual)

If you're wanting to just detect the presence of 50kHz rather
than measure frequency, how about something like an LM567
or a similar notch filter to produce a there/not there signal for
the PIC ?

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