you can probably use COUNT to do the job.

Detecting 7.5Khz with +/- 10% => from 6.82 KHZ to 8.25 Khz

so what about the following
Code:
' Pic 16f84 at xtal 4 Mhz. WDT off PTU off' 
' Detecting 7.5Khz with +/- 10% => from 6.82 KHZ to 8.25 Khz
'
FI VAR PORTA.3 'input freq
SO VAR PORTB.0 'OUPUT LED
W0 VAR WORD    'VARIABLE W0

INIT:
    count fi,100,w0 ' Count pulses for 100ms
    IF (W0>682) AND  (w0<825) then PULSE
    GOTO INIT

PULSE:
    HIGH SO 
    PAUSE 500
    LOW SO
    PAUSE 500
    GOTO INIT
END