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# Hardware design for microstrip parallel coupled BPF

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#### Lokesh Kumar

##### Newbie level 5
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.

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

hey
can you tell how to improve the return loss????

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.

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....

Very brave of you to try to make a 1.8 GHz coupled line filter on FR4!

### Lokesh Kumar

Points: 2
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

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。