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Hardware design for microstrip parallel coupled BPF

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Lokesh Kumar

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microstrip parallel coupled bpf using ads

I have designed the BPF for the following specifiations
1)Central freq.= 1.8Ghz
2)Bandwidth= 25Mhz
3)Passband ripple= 1dB
4)60dB attenuation away from 60Mhz from centre frequency
5)Source and Load impedance= 50 ohm
I got this schematic and frequency response.As we are getting the spacing between lines 9-10mm and also getting S11= -6 to -7 dB, so can we get the same response after designing the hardware for this
.Also give me idea for improving the result so I can design the hardware.
 

biff44

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Well....of course not!

1) you have analyzed this with a perfect, lossless dielectric (Tand=0)
2) you have analyzed this with a perfect superconductor for the metal
3) your return loss stinks, and it will only get worse in real life
 

biff44

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Can not tell without looking at a polar plot of s11, but it looks like it might be designed for ≠50 ohms. If I were hacking away at it, I would start by tightening or loosening the coupling to the first and last resonators.
 

Lokesh Kumar

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well i m making hardware using Fr-4 substrate(Er=4.2,H=60mil,Tand=0.0009) i just wants to know that the spacing i m getting b/w the Coupled lines are ok??? means they are 9mm and while the coupled line width is just 3mm !!!! i have seen in many examples that spacing is always less than the width of coupled line....
 

no1cobra

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1.8GHz最好不要用FR4的基板,FR4的基板介电常数变化范围大,不稳定,而且损耗大。建议你用聚四氟乙烯的基板

Added after 27 minutes:

1、你的原理图(schematic)有问题,损耗角正切不能为理想值,电导率不能为无穷大
2、对于这种类型的微带平行耦合滤波器(microstrip parallel coupled bpf )一般用奇数阶,而不用偶数阶,因为偶数阶不稳定。
3、你的滤波器相对带宽(FBW)太小,仅是1.4%,这样的滤波器做出来后插入损耗(insert loss)会很大。而且,相对带宽太小的话,加工精度对它影响很大。
4、带外抑制你要求60dB,这个恐怕也很难做到,建议(advice)你用椭圆函数滤波器。
 

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