Quasi-elliptical vs pseudo-elliptical filters vs Chebyshev Type 2 filters

The RC low-pass filter (LPF) is the most simple to understand intuitively.

The capacitor reacts slow to the current it gets, voltage will rise slowly. If \$V_{IN}\$ is a fixed voltage the voltage across the capacitor will rise exponentially until it reaches \$V_{IN}\$. DC is not filtered at all.

If you apply an AC signal current goes back and forth through the resistor, charging and discharging the capacitor. If this goes slowly, at low frequency, the capacitor's voltage can more or less follow the charging and discharging, and its voltage will be near the input voltage. But the higher the frequency the faster current direction changes, and the change will take place before the capacitor is fully charged. So the input voltage is no longer reached. For very high frequencies the changes in current direction are so fast that the voltage amplitude across the capacitor is only a fraction of the input.

We have a low pass filter: low frequencies are passed with little attenuation while higher frequencies are attenuated more.

LC filters are far less intuitive.

In this parallel circuit part of the current will circulate between inductor and capacitor, and the net current will decrease the closer you get to the resonance frequency. At the resonance frequency the net current is even completely zero, as if L and C aren't there.

In the same way will a series LC circuit form a zero impedance at the resonance frequency, as if there's only the resistor.

I am confused about the theory: If the device impedance is very high ( say in MΩ region), why putting a relatively small resistor (20KΩ /1MΩ )in series with that should stop return signals ?

Signals approaching a high-impedance connection are akin to waves crashing into a hard surface. They will bounce, and return in inverted form (ringing). In electronics the returned wave can be in negative voltage and large enough undershoot to blow the clamping diodes or even the drivers.

A properly calculated source serial terminating resistor placed close the pin prevents many sorts of problems with 'ringing' and EMI. If your source is 75Ω already, and your termination is Megs'n'Puffs, try this:

• make sure your pcb trace is also 75Ω. You'll likely need change the width and length, and keep in mind it changes with substrate thickness and the layer you are on.
• Place a 66Ω resistor at R2 closest to the pin as possible.
• leave everything else out.

This way, all three pieces are the same impedance, nothing should reflect. Anytimes the line resistance changes, you'll get some reflection. If you do it right, you can from the scope below where there is 1V undershoot:

To this, by adding just one properly sized resistor:

To get the best result, you'll need to load in a simulator like lineSim (Hyperlynx by mentor) and verify.

My feeling would be that in the above examples you've given, you can do without R1 altogether and make R2 66Ω, place your filter closest to one end, and match your trace impedance 75Ω.