current of the amplifier. Because of that requirement the filter is a combination of a 3rd-order low-pass and a 1storder high-pass. Its schematic is shown in Fig. 9. Active Filter To receive an interference-free data signal a filter is required. Because a very steep filter is needed and the µC − FSF + Figure 9. Schematic overview of the passive filter with amplifier. + FSF − Next Stage Forward Transmission (dB) Figure 10. Schematic overview of one stage of the active filter reception path is unidirectional, an active 8th-order Sallen-Key low-pass filter with a cutoff frequency of 150 kHz is chosen. Fig. 10 shows one of the four filter stages. These are identical to each other with only varying component values. The following amplifier has a gain of 37 dB. B. System Performance To show the channel characteristics, the system is modeled in the simulation environment LTSpice. The developed hardware is measured with a vector network analyzer. As a result of the simulation and measurement with and without IPT system, the forward transmission and the group delay of the whole channel is shown in Fig. 11. It can be seen that simulation and measurement are matching very closely. At the frequency of 500 kHz the power signal of the active IPT system can be seen. The pass band is chosen from 20 to 120 kHz because within this range the group delay is almost constant with values between 11.8 and 9.5 ns for the simulation and 12 and 8.5 ns for the measurement. In addition the variation of the passband attenuation is very small and constant. In the simulation it is 0.5 dB without and 3.2 dB with active IPT. In the measurement it is 1.1 dB and 2.2 dB. 0 −20 −40 Data Transmission −60 −80 −100 104 105 Frequency (Hz) (a) fz fs 106 fz fs 106 Group Delay (µs) 25 20 15 Data Transmission 10 5 0 104 105 Frequency (Hz) (b) Simulated Simulated with IPT Measured Measured with IPT Figure 11. Resulting (a) Forward transmission and (b) group delay of the complete communication channel. 28 IEEE CIRCUITS AND SYSTEMS MAGAZINE THIRD QUARTER 2019