Simulation and analysis: The final circuit was simulated, where the junction capacitance of the PIN diodes was set to 0.2pF. Fig.3 shows the transmission coefficient when the diode is progressively turned on. It can be seen that when the PIN diodes are off, the proposed BPLF exhibits filterability. As the number of triggered-on PIN diodes increases, the isolation grows, showing better protection performance.
As for the response process, Fig.4 shows the waveforms of the input and output. It can be seen that the output power remains almost the same for 1W, 4W and 16W input levels, which means well flatness. Also, it can be observed that response time is at the nano-second level.
Experimental results: In Fig.5, the comparison of simulation and test results in the small-signal state is plotted. It can be seen that the return losses at the input and output ends are better than -15dB. The insertion loss at 8.2 to 9.0 GHz is about 3.5dB in passband. As a bandpass filter, its relative bandwidth reaches 9.3%.
The limiting measurement performance in different frequencies and the assembled BPLF with the dimension of 14.5mm*10.5mm*0.385mm are shown in Fig.6. The alumina surface film process is adopted for fabrication of the structure and micro-assembly technology is adopted to bond PIN diodes. The details are shown in Fig.7. The pulse width of tested high power signal is 1us and the period is 1ms. The results show that the threshold level of the BPLF is about 13dBm and the limiting level is less than 17dBm with 50W injection.
Conclusion: A concept of BPLF based on comb line structure has been presented and experimentally investigated, exhibiting dual selectivity of frequency and power, which is significant for receiver in the complex environment.
Fig 3  Frequency responses of structures when different number of PINs are triggered on
Fig 4  3 power levels input with their corresponding output