Electronics > RF, Microwave, Ham Radio

Why are my center frequnceys off on my LC filters?

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Laurencewilliams:

--- Quote from: koffp on June 24, 2022, 11:27:12 pm ---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
L1:https://www.digikey.com/en/products/detail/kyocera-avx/HLC02320HTTR/3528327
C2:https://www.mouser.com/ProductDetail/Kyocera-AVX/0201ZK4R0BBSTR?qs=k2FEcAV%252B92Yoi7sJsjfPBA%3D%3D
C3:https://www.mouser.com/ProductDetail/ATC-Kyocera-AVX/600F240JT250XT?qs=AdTq2PFIamn48Yj%2Fkca%252BNQ%3D%3D
L2:https://www.mouser.com/ProductDetail/Murata-Electronics/LQW18AN34NG80D?qs=hNud%2FORuBR3cm0mPuhqMJw%3D%3D
C4:https://www.mouser.com/ProductDetail/Kyocera-AVX/0201ZJ3R5BBSTR%5C500?qs=%252Br8cYUmWfSeE47%2FWazW4EQ%3D%3D

135 MHz 3rd order cheb:
C1:https://www.mouser.com/ProductDetail/ATC-Kyocera-AVX/600F820FT250XT?qs=AdTq2PFIamkUclHcJDFISw%3D%3D
L1:https://www.mouser.com/ProductDetail/Murata-Electronics/LQW15AN17NG80D?qs=hNud%2FORuBR3E4YmylizoPA%3D%3D
C2:https://www.digikey.com/en/products/detail/kemet/CBR06C609CAGAC/3473936
L2:https://www.digikey.com/en/products/detail/pulse-electronics-power/PA2607-231NLT/2267054


Note:
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

--- End quote ---

It sounds like you encountered an issue with your LC filter designs not matching the desired results. This could be due to several factors, such as parasitic elements, process variations, and measurement errors.

One possible solution to reducing the impact of parasitic elements is to increase the trace width and thickness. This increases the inductance of the trace, reducing its parasitic capacitance, and reducing its resonant frequency. However, increasing the trace width and thickness also increases the parasitic inductance and resistance, so it is a trade-off.

Another solution is to implement an active filter, such as an op-amp based filter, instead of a passive filter. Active filters have the advantage of having better control over their response and higher stability, although they are typically more complex and consume more power.

It's also worth considering the impact of process variations. The parameters of the components you used, such as the inductors and capacitors, may have variations due to the manufacturing process, and this can affect the overall performance of your filter. To minimize the impact of process variations, you can use higher-quality components or implement design margins to account for the variations.

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