# Matched LPF (without reflections)

## Main Question or Discussion Point

Hey,
I analyzed the circuit (matched LPF without reflections) hereby attached and produced a simulation of its impedance(freq.). I am now asked to explain why it is important to use a filter with such input impedance at high frequencies. If the filter's impedance at high frequencies were, say, 50ohms I could probably answer - "impedance matching! Thus benefiting from maximum power transfer etc.", but since at high frequencies the impedance approaches 0 I am not quite sure what the answer in this case should be. I'd appreciate some insight, please.

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berkeman
Mentor
Hey,
I analyzed the circuit (matched LPF without reflections) hereby attached and produced a simulation of its impedance(freq.). I am now asked to explain why it is important to use a filter with such input impedance at high frequencies. If the filter's impedance at high frequencies were, say, 50ohms I could probably answer - "impedance matching! Thus benefiting from maximum power transfer etc.", but since at high frequencies the impedance approaches 0 I am not quite sure what the answer in this case should be. I'd appreciate some insight, please.
What is the purpose of the input capacitor? It looks misplaced. Does that AC voltage source to the left have a built-in output resistance/impedance?

Baluncore
2019 Award
The circuit has no resistance apart from the load, so high frequency energy must be reflected back into the signal source. The LPF is matched over it's pass-band. At high frequencies the input impedance mismatch will reflect unwanted high frequency energy back towards the generator where it will be absorbed in the output resistance.

I can't see the detail in your image but; you appear to model the signal source as a low impedance voltage. You need to add a series resistor to set the output impedance of the generator. That will match the HF portion of the spectrum and absorb the reflected spectral components.

I did not design this circuit. It was given to me as is by the instructor. I was asked to analyze its impedance wrt frequency and then explain the importance of using a filter with such input impedance at high frequencies. Now what is in fact the importance at high frequencies?

Baluncore