Design And Development Of X-Band Hybrid Bandstop Filters

This report presents two designs of hybrid BSF for X-band (8 – 12 GHz). The BSF is widely applied in microwave communication such as in radar and satellite. It is used at the RF receiver for protection against strong interference and blocking signals. Furthermore, it is also used to reject high-power signals outside the receiver band and is usually located at the front-end of receiver prior to pre-amplifier. Waveguide filter offers low loss and is well known for high-power and millimeter-wave applications. In contrast, it comes with bulky size, non-planar, hollow structure and high-cost of fabrication process compared to the other planar circuit design. Due to the non-planar structure, it is difficult to integrate waveguide with the other planar circuit. The main focus of this research is to produce hybrid BSF by integrating two planar circuits by using simple microstrip transition. The first design is hybrid notch BSF which consists of two-part of filter network that is integrated with the impedance inverter. SIW BPF is used as a two-port network and impedance inverter is represented by four-port directional coupler. The SIW BPF is initially designed from rectangular waveguide filter. This waveguide BPF is then converted into a planar circuit by implementing the SIW technique. This method reduces the overall size of the filter due to the dimension being inversely proportional to of the substrate. The second design is the hybrid reflection-mode BSF. It is constructed from the same structure of impedance inverter as the previous design, but it is connected to the even and odd-mode of SIW BPF. All simulations have been conducted using the HFSS and ADS software. In order to validate and prove the concept and theory, all simulated designs are fabricated and measured using VNA. The standard PCB with low cost fabrication process is used. Dimension of both filters is quite similar but the hybrid notch BSF produces a lower loss compared to hybrid reflection-mode BSF. Both designs have successfully transformed from BPF (Chebychev response) to BSF (Inverse-Chebychev response). The hybrid notch BSF has an insertion loss of about 3.83 dB and a return loss of 9.1 dB. Meanwhile, the insertion loss and return loss for hybrid reflection mode BSF is 5 dB and 12 dB respectively. This shows that hybrid notch produces lower loss compared to hybrid reflection-mode BSF. The design method also proves that SIW filter can be easily integrated with coupler (planar circuit), using simple transition methods such step impedance, compared to conventional waveguide. Step impedance is used to ensure a field matching between the microstrip line and SIW over a broad bandwidth. Probe feeding transition can be use for integration between waveguide and planar circuit, but it needs a metal short block with a quarter-wavelength on the substrate. This contributes to the complexity of the transition and requires another accessories in order to complete the transition