The Design Of Sharp Cut-off Micro-strip Low Pass Filter With Broad Stop-band

As a frequency-selecting device,the low pass filter not only can be installed in the front end of the system receiver,but also can effectively limit the system's effective work bandwidth,thus protecting the post-amplifier from the influences of interference signal.It can also be installed in the front of the system's band-pass filter so as to achieve the purpose of eliminating band-pass filter's spurious passband and restraining the amplifier' s harmonic signal.The micro-strip low pass filter has the characteristics of small volume,light weight and low production cost as well as convenient for integration.It is widely used in microwave test system,communication system and radar system.At present,it has become the research hotspot in the field of microware integrated circuit.In order to get the performance of sharp cut-off,broad stop-band and miniaturization filter,this paper introduces the micro-strip T-shape branch with open circuit,Hilbert fractal Defected Ground Structure(DGS),the reverse T-shape shunt stub and other units and also makes an improvement for the design of traditional high-low impedance micro-strip low pass filter and the micro-strip low pass filter with shunt branch line,which has got a good filtering performance.Firstly,the method of high impedance transmission line unit to load the shunt reverse T-shape branch is adopted in this paper,thus constituting the band stop filtering branch circuit,restraining the spurious passband and decreasing the filter' s area.Taking the 7-stage Chebyshev ' s low pass filter as an example,the design and test are conducted in this paper.The experiment result shows that the cut-off frequency of reverse T-shape shunt branch filter is 5.06GHz.The maximum insertion loss of inside transmission band is lower than 0.95dB and within the 5.61?13.6GHz stop-band restrain,it exceeds 21dB.However,when comparing with the traditional loss pass filter with open circuit stub line,the area of filter decreases 20%.Secondly,on the basis of micro-strip low pass filter with open circuit stub line and by replacing series connection of transmission line for the micro-strip T-shape stub with open circuit,a 7-stage open circuit micro-strip T-shape stub low pass filter with a cut-off frequency of 3.5GHz is designed in this paper.Then the T-shape stub band-stop filtering branch is implanted into the interior of the low pass filter.In accordance with the spurious passband which appears in low pass filter,the corresponding structure of T-shape stub to restrain the spurious passband is designed in this paper.As the simulation result shows,the return loss(RL)of improved open circuit micro-strip T-shape stub low pass filter transmission band is lower than-28dB.The return loss of inside transmission band is lower than 0.5dB and the return loss of the outside transmission band is more than 30dB,which has reduced nearly 10%of the volume when comparing with the traditional loss pass filter with open circuit stub line.In order to further improve the performance of low pass filter,the 5-stage dumbbell-shaped DGS micro-strip low pass filter with cut-off frequency of 5GHz is taken as an example in this paper and its design is aloo improved in this paper The Hilbert fractal technique is introduced in this paper and the dumbbell-shaped DGS structure unit is replaced by DGS Hilbert structure unit,thus the volume of the filter can be reduced greatly and the transitional band of the filter can be decreased a lot.In order to make up for the change of transmission line' s impedance which are brought by the DGS unit,the micro-strip transmission line with gradually varied impedance is designed at the input and output end of the filter,which has greatly improved the filter's impedance matching performance.The stimulation result shows that the return loss(RL)of improved fractal Defected Ground Structure(DGS)low pass filter is lower than-18GHz within the 0-5.3GHz transmission band.Within the 6-23GHz,the stop-band restrain has reached-20 dB,which can effectively restrain harmonic ware of many times and the size of the filter is 23.6mmX9.5mm.When comparing with the filter with dumbbell-shaped Defected Ground Structure(DGS),the volume of the filter decreases 10%.As the experiment result shows,the improved designs of such three kinds of low pass filter all achieve the performance of sharp cut-off and broad stop-band as well as low loss and miniaturization.