Electronics > RF, Microwave, Ham Radio

My BPF filter wont work when I try to implement it

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koffp:
No matter what I do I can't get my 2 BPFs to work in real life.. One is a 5th order cheb from 500-570 mhz and the other is a 3rd order cheb from 500-540 mhz. I'm a new engineer and this is my first time actually implementing a filter i designed..

I calculated them correctly and the simulations matched up but when I hook it up to the spectrum analyzer and the VNA it shows the pass band being way off centered at about 350 mhz instead of 535 mhz for the 5th order and all the way down at 70 mhz for the 3rd order. The engineer who was here before me had a lot of year experience and his filters were also getting their center frequency skewed off from the 500s to the 300s(mhz) so it seems like there is some essential PCB layout design I am missing...

I'm at my wits end, I don't know what to do and i'm getting embarrassed in front of my coworkers by how long this is taking me to figure out. I know that my pcb trance geometry isnt perfect( not final revision) and that the parts have tolerances but i simulated it with the worst case senerio in terms on tolerances and it still worked.. I understand my bandpass not looking great but i don't understand it being skewed off by 200 mhz, especially since the engineering before me made the same mistake..

If anyone can notice anything i'm doing wrong it would mean the world to me. Thanks[/img]

Odysseus:
Certainly, your layout seems unsuitable, assuming I'm interpreting your PCB screenshot correctly. A solid ground plane is usually required for good RF performance.
That said, I don't actually think your layout is the main culprit. More likely you used inadequate inductors with insufficient Q or SRF for your passband. Are any of them ferrite core? Some of them are so small I can barely tell they're present in your photo. They should all be size 0805 or larger with an air core.
Also, the design of your 3rd order filter is odd. You can see the same 70MHz peak in your own simulation screenshot.

koffp:
odysseus

Thank u for you're explanation, I hope that's the problem.. I had to use these part beacuse of current avaliablity.. I didn't pay attention to the Q. I will try just ordering the through big hole version of them and pay attention to the Q.. or do u think it's better to just do microstrips. If microstips can u recommend an affordable

Odysseus:
A microstrip filter isn't necessary for 500MHz. You just need appropriate inductors and maybe a better layout.
As an alternative to ordering through hole parts, you could wind some inductors by hand.

koffp:
This is gonna go into a board that will be used in production so winding it by hand is not an option lol, thanks

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