Need pointer on designing low pass filter circuit
To design a filter, you must specify the parameters;

attenuation or gain in passband
ripple in passband for &gt;=2nd order filters (&gt;=0dB)
stop band f-3dB = BW
stop band f-60dB defines order of filter needed or similar bandstop attenuation spec
Source impedance
Load impedance

In this example, a better design replaces 20 Ohm with 47uH and C = ~4.7nF . Thus attenuation in PB f = 6dB and breakpoint is at -9dB, while impedance remains at 50 Ohms to load in the passband.
Thought experiment
What happens when \$Z_L(f)=2\pi f L= R_S+R_L\$ ? for R=100 total  L=53 uH and 47uH is a common 10% value.