5.2. Digital Filters
A received LRUT signal sample was used to estimate the filter performance. The signal
and its FFT spectrum are plotted in Figure 6. The signal frequency was 44 kHz. The
system is switched from transmitting to receiving after a certain time. After the
transmitting, the receivers are switched on. They firstly pick up some of the transmitting
signals from the other adjacent transmitting channels. These explain the flat signal
followed by large amplitude signals appearing at the beginning of the signal trace. The
spectrum shows the 3rd and 5th harmonics signals at 136 kHz and 215 kHz
respectively. A band pass FIR filter was designed for this signal. Figure 7 shows the
filtered signal and its spectrum. It can be seen that the 3rd and 5th harmonics at high
frequencies are suppressed by the designed filter.