# Design of a Compact Ultra-Wideband Microstrip Bandpass Filter
## 4 Measurement and Simulation Results  
The photograph of the manufactured filter is shown in Figure 9a, and its characteristic curve is measured by the vector network analyzer (VNA) 3672D of Ceyear, Qingdao, China. The comparison between measurement and simulation results is shown in Figure 10. The measured 3 dB bandwidth ranges from 10.215-15.375 GHz, RL in the passband is better than 18 dB, roll-off rates are 85.80 dB/GHz and 10.74 dB/GHz respectively, and the insertion loss at the center frequency of 12.795 GHz is 0.58 dB. Measurement results are in good agreement with simulation, and errors in manufacturing and measurement result in some differences in curves. Figure 8: Simulation of S parameters when the DGS slot width \(W_{9}\) is varied from 1.4 mm to 2 mm, with a step of 0.2 mm increment. Figure 10: Measured S parameters of the proposed UWB filter. Figure 9: Photograph of (**a**) the fabricated UWB filter and (**b**) measurement scene. Considering that the length of the quarter-wavelength terminal-open stubs corresponding to low-frequency transmission zero is long, it is folded to reduce the size in the design. The frequency corresponding to the transmission zero is basically consistent with the simulation results, but the stopband suppression characteristics are slightly worse than the simulation results due to the multilayer design and the need to add RO4450F prepreg for processing. After processing, it is necessary to remove the glue residue including spilled resin and other wastes, which can be treated by CF\({}_{4}\)/O\({}_{2}\) plasma etching and alkaline potassium permanganate. In addition, the low-frequency resonance point in the passband has a certain frequency shift, which may be due to the fact that when laminating, the prepreg between the two substrates will be squeezed into the holes which leads to an inconsistency between the overall thickness of the device and the thickness of the simulation, and also affects the performance of the blind holes structure. Before measurement, the vector network analyzer must be calibrated with a calibration kit of 31123A (Ceyear, Qingdao, China). During the measurement, as shown in the Figure 9b, the processed filter needs to be connected to SMA connectors and then connected to the vector network analyzer through connecting wires, and the vector network analyzer also needs to be preheated and calibrated. Port 1 of the VNA Ceyear 3672D connects to the input terminal of the filter under test, and port 2 connects to the output terminal of the filter under test, with a frequency sweep range from 8.8 GHz to 17.4 GHz. Errors may also be caused by such preparations not being carried out well. SMA connectors and connecting wires also have a certain insertion loss, which will affect the measurement results. To reduce the gap between measurement and simulation results, the vector network analyzer needs to be fully warmed up before measurement. The calibration kit is adjusted to the corresponding size of the SMA connector for calibration, and then the observation results are measured several times. Meanwhile, the proposed filter has a low and flat group delay matched with the tendency of insertion loss, and group delay in the whole passband is less than 0.1 ns, as shown in Figure 11, which ensures the stability and integrity of the signal in the process of transmission. Table 2 shows a comparison between the proposed filter and other designed filters. The UWB filter [2] designed based on traditional methods has a wide bandwidth, but its in-band return loss is large. In Ref. [7], an ultra-wideband bandpass filter is designed by using coupled branch lines, and the odd and even modes of the filter are analyzed by using equivalent circuits. Ultra-wideband and good stopband harmonic suppression are realized. The structure in [13] uses liquid crystal polymer technology to reduce the insertion loss to 0.4 dB, and more transmission zeros are obtained, but the return loss is still large. In Ref.