Electronics > RF, Microwave, Ham Radio

Why are my center frequnceys off on my LC filters?

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Bud:
Component values may change with frequency. You can use a VNA to measure each type of components used across the design frequency range. You can simulate until cows come home but this practical measurement will give you real world insights.

Joel_Dunsmore:
Late to the game, but in case someone else looks:

Every SMT capacitor needs to replaced with a series LC where L is on the order of 2.5 nH (+-50% I'd guess) and every inductor needs to be replaced with a parallel LC where C is on the order of 0.2 pF; and every resistor (not that there is one in a filter) is replaced with a series RL, shunted by C, of the same value.  These typically cause a filter center freq to shift down 30-50% in freq, in rough numbers.  All the L's appear bigger as do all the C's, so everything goes lower.  Berkeley's EE 142 has this as a principal lab (at least when I co-taught it).

Hamelec:
maybe you should think about if there is a really need for filters - maybe a lowpass and a highpass would be enough..?

MathWizard:

--- Quote from: BigBoss on June 25, 2022, 03:30:26 am ---Because your layout is not appropriate. Why ?

-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 )


--- End quote ---
Would one possible fluid dynamics equivalent be like in a piping system, if in the pipes with the greatest flow restriction, you were adding and taking away indentations in real time, of the pipes, which would make the water pressure to rise and fall accordingly  ? Would that be like stray capacitance in a high impedance section ?

ejeffrey:
The hydraulic analogy for electronics is... only qualitatively helpful in the best of situations.  Applying it to parasitics in a multi-stage bandpass filter is unlikely to be helpful at all.

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