I use mosfet to generate a pulse, input the pulse into the transmission line, and after the reflection, test the waveform as shown. Why did the third reflected waveform become like this? I calculated a circuit output impedance of 50 ohms and a transmission line characteristic impedance of 25 ohms. Is it a simulation software problem or a calculation error?
The source impedance isn't exactly 50 ohms. There's a parallel capacitive component caused by C3 in series with the drain-source and drain-gate capacitances of the MOSFET.
So while low frequency components of the signal see very close to 50 ohms, high frequency components of the signal will see a low-impedance termination.
The result is you see multiple reflections of the high frequency components of the signal, but only one reflection (off the ideal open termination at the far end of the transmission line) of the low frequency components.