Experiement - 2 - Figures of TDR of LPF. Part 2.
Figure 4 c. Time Domain Response of Low Pass Filter for T ~ τ .
The leading or positive Edge of the Square Wave experiences a Rise Time and the lagging or negative Edge of the Square Wave experiences a fall Time. The suppression of the higher frequency components results into rounding off the EDGES of the Square Wave.
Figure 4 d. Time Domain Response of Low Pass Filter for T<<τ .
Since (1/T)Hz fundamental component and the harmonics of square wave lie well beyond the Pass Band of LPF hence the nearly all the fundamental and harmonics are suppressed. What is left is the DC component. Therefore at the output we see the Mean Value of the Square wave with a small poitive ramp for (+)ve half and negative ramp for (-)ve half of the Square Wave.
Figure A. Frequency Spectrum of an Infinite Pulse Train.