Passive Second Order Low-pass Circuit.

Hi everyone.

I am working on an assignment about passive second order low-pass filters. The circuit is attached.

We are given the placements of the passive components (Inductor(L1), Resistor(R1) and Capacitor(C1)) without their values.

The only given specification is that the cut-off frequency is 78 kHz.

My questions are:

1.How to make the predicted low-pass filter Transfer function for this specific circuit?

2.Secondly how do I calculate or choose the component values for the filter? ... ( A suggestion given to me was to choose a Inductor value and a Capacitor value that is at my disposal ant to calculate the required Resistor value to build the circuit after calculations ).

The components must work out so that:

2.1) the phase angle between the output voltage and input voltage is -90 degrees at the cut-off frequency (78 kHz).


2.2) that the ratio of the amplitudes of the Output Voltage to the amplitude of the Input Voltage is 0.7079 at the cut-off frequency.

PS: I have a 1mH Inductor at my disposal,( thats why it is the only value I entered on the circuit for now :) ) and the Capacitors at my disposal is 1uF, 10uF, 22uF, 100uF or 470uF ).

I can't wait to here from you!

by iammichelle
April 05, 2018

The circuit transfer response should be Xc/(Xc+Xr+Xl). Simply substitute for these and you will see how to calculate the circuit values.

by mikerogerswsm
April 05, 2018

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