Effect of source output impedance on reflections in a transmission line

For preventing reflections when the frequencies are higher or close to the transmission line length, for a 50 Ohm coaxial cable a 50 Ohm termination resistor is recommended.

simulate this circuit – Schematic created using CircuitLab

I also used to think/know that source output impedance Rs also needs to be 50 Ohm. But recently I have read in a forum or a text(I'm sorry that I cannot find it at the moment where), someone was mentioning that the source output impedance Rs doesn't have to be 50 Ohm and it is enough only coax characteristic impedance and the Rin are equal/matched.

Is that correct? Can Rs be anything other than 50 Ohm safely?

• You can terminate at either end. But load termination prevents reflections. Commented Dec 14, 2018 at 12:31
• Related: LVPECL termination impedance, and the On Semi app note I linked to there. Commented Dec 14, 2018 at 17:48

You can terminate at either end.

Source termination does terminate reflections, if the load is mismatched.

Load termination causes a continual DC draw of current.

Source termination, with the load open circuited, imposes a 50% initial drop in the voltage on the transmission line; thus a 5 volt output (before the Source termination) appears as only 2.5 volts on the line, UNTIL. Until the transition reached end of the (non-loaded) line, where mother nature uses the energy flows and wave-disruptions to sustain the same current while inverting and generating a reverse (reflected) transition, which causes a doubling of the line voltage that sends a 5 volt wave back toward the source. For a single-user at-end-of-line, this is fine. For a multi-use line with users scatters all along the line, some initially see the 0v, then the 2.5v, then finally the 5volt.

• Im not asking where to terminate. Your answer is not addressing my question. Thanks anyway for the other details. Commented Dec 14, 2018 at 13:05
• A 2.5 volt reflection is sent back to the source. Commented Dec 14, 2018 at 13:55

Is that correct? Can Rs be anything other than 50 Ohm safely?

You can use a zero ohm source and, providing your termination resistor matches the t-line characteristic impedance then all the power sent is dissipated in the load resistor at the end of the line. You only need to match at one end (or the other).

Just consider a 5 volt step from the zero impedance source on a 50 ohm line. Instantly a current of 100 mA is taken and the 5 volt and 100 mA travel down the t-line to the far end. If the load impedance matches the V and I ratio (a ratio measured in ohms of course) then all that power is absorbed by the load and none is reflected back. Job done.

Can Rs be something that is neither 50 ohms or zero ohms?

Yes it can providing the load matches the characteristic impedance of the cable.