Hardware design for microstrip parallel coupled BPF

[Lokesh Kumar]

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]

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

 

[Lokesh Kumar]

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

 

[biff44]

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]

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....

 

[biff44]

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

 

[no1cobra]

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。