I would like to attenuate the signal above 20khz and implement the second order passive low pass filter to filter out the signal effectively which are above 20khz.As you probably realise "Quality factor" is nothing to do with how good it is. It is simply the ratio of the energy stored in the "tank" (the inductor and capacitor) to the energy lost per cycle. It is probably best referred to simply as "Q" to avoid confusion, but it's nothing to do with Q-anon either!
High damping factor = low Q
Low damping factor = high Q
The higher the Q the more it oscillates.
If you want to design a "good" 2nd order passive filter, first, you have to decide what you mean by "good".
1) Do you want the best attenuation of frequencies just above the cutoff frequency?
2) Do you want the flattest frequency response of frequencies below the cutoff frequency?
3) Do you need constant group delay? (That means that all frequencies are delayed by the same amount, and a complex waveform tends to stay the same shape after if has been filtered.
You can't have all three!
If you want best attenuation, you will get a peak in the response around the cutoff frequency, and oscillation if you hit it with a step function or a pulse.
if you want constant group delay, you will not get best attenuation.
If you prefer 1), you need a Chebyshev filter: enter Q>1/√2
if you prefer 2), it's a Butterworth: enter Q=1/√2
and if you prefer 3) it's a Bessel: enter Q = 1/√3
(there are plenty more types, each with different features).
Once you've designed it, don't forget that it only works with the values of load- and source-impedance that you have specified. If you change either, you will change the Q.
If you're serious about passive filter design, get a copy of A. Zverev "Handbook of Filter Systhesis" ISBN 13: 9780471986805.
It's not a thrill-a-minute page-turner!
I would like to chose one of these filters but help me which filter is good to filter out the signal above 20khz and also maintain the flattest frequency response of frequencies below the cutoff frequency.
1) RLC passive low pass filter. 2) pi passive filter. 3) T passive filter.
Actually, please can you provide how to implement these filters and description of these filter such when it will be used and so on...