Electronics > RF, Microwave, Ham Radio

Why are my center frequnceys off on my LC filters?

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I recently created 2 LC cheb constant K filters, on for 3rd order 115-155 Mhz and one for 5th order 496-560 Mhz. When made the actual physical PCBs I got different results, the first one centered at 125 Mhz instead of 135 Mhz and the second centered at 470 Mhz instead of 530 Mhz, theiur bandwidths are about correct though. Has anyone else come across this problem? I made sure to use silicon RF components with high self resonant frequency and high q around 500.. I dont know what else I could do to negate parasitic impedance, I made the traces as efficient as possible, only thing left would be to change the trace thickness(im using 8 mils) and the trace substance(im using standard copper).

I'm about to resort to attempting calculating micro-strip dimensions by hand and manually drawing them in orcad since my company doesnt have ADS or anything like that..

Could it be something wrong with my parts??

Here are the parts:

530 Mhz 5th order cheb:
C1: https://www.mouser.com/ProductDetail/ATC-Kyocera-AVX/600S120FW250T?qs=ZTdx6reOWK%2FXUH41hO8rGw%3D%3D

135 MHz 3rd order cheb:

I realize my sober is sloppy, escecially on the 5th order cheb butthose are old picture, I dont have the pictures but I made other cirucits were my solder was perfect and I managed to move the cneter frequncy from 470-475Mhz but thats not even close to 530 Mhz which I need it to be

Here are sum more relevant pics

How can you be sure you have the correct element values?  Simulating a circuit is all well and good but since your results are so close to what you want, just trim the values and you should be good to go.  The error you obtain isn't very great, and is probably attributable to component tolerances.

David Hess:
Isn't the parasitic capacitance from the layout enough to explain the shift in frequency?

Because your layout is not appropriate. Why ?
-There is no VIA around GND Connections and you should place VIA stitching all around. This is important
-Components are too close to GND side copper pours. They will create stray capacitances. Especially high impedance nodes are very sensitive to these stray capacitances. ( Midpoint of L-C series circuits )
You should place spaces on the main line.( almost open )
-Component Tolerances play very important role on the Filter's Response therefore you should use trimmer capacitors or variable inductors where possible.
-You have to simulate whole circuit with a EM simulator with realistic and modelled components
-You have also to do the calibration on the same substrate with through line. ( for S21)
-Monte Carlo analysis will give you an insight about shifts on frequency axis.
-You used thermal relief for GND connections of the connectors. This is wrong. Tie them directly.
-Finally you should open the copper pour just underneath of the connectors' pins and main line because it will create parallel stray capacitor between nodes and GND. It will badly shift the center frequency.


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