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# Pi filter for impedance matching

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Hellow,
I build the following pi filter to match 100 ohm source to 1000 ohm load, I did the calculations based on RF Circuit Design Book

As u can see at the center freq fc = 30MHz & Q=15 I had 3.771dB, I was surprised by where this value? this is amplification!!!!

The second question is: If i want to match 1000 ohm source to 100 ohm load, Simply replace the capacitors, one must place the other, when i did so and simulate i had the following:
[img width=680 height=478]
Look!! -9.8dB at the center freq.
thanks alot

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A pi filter can be used to match loads? Most impedance matching that I've seen match the source and load impedance. Although depending on the application, pi filters can play a role in impedance matching.

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my question is why and how filter amplify signals??

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I'm surpised at the results. I know that mismatching impedances can cause standing waves, and it may depend on where you probe on the transmission line. But I think there would still be enough signal loss, and it would seem hard to get that good of a measurement to make a comparison.

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A series LC circuit is a low impedance at resonance. The signal current is high. The voltage across its capacitor and the voltage across its inductor is higher than the input signal voltage.

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This is a "second order" circuit. You took a bode plot over a frequency range. Things are going to change over frequency. Have you solved the differential equation for this circuit to find the poles and zeros? It's also what is called a "passive circuit"... so, unless you hit a resonant frequency (as AG pointed out) in your sweep, this thing can only attenuate.

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