in this SPICE simulation two passband active filters resonant at the frequencies corresponding to the mark and space states are used in place of the phase lock loop (PLL) for data recovery. The input signal is an FSK signal read from a file. This signal is then applied to two bandpass filters tuned to 2 kHz and 4 kHz. The output from each filter is then rectified. Two low-pass filters, with passband and stopband edge frequencies of 100 Hz and 2 kHz, to filter the doubled frequency after squaring, are then connected to a comparator to recover the original data.