Recently, a reader posed the question in the Comment field on Dr. Eric Bogatin's blog post, How to Choose Between the Oscilloscope's 50 Ohm Input and 1 MOhm Input: "Is there any difference between using an external 50 Ohm terminator instead of the internal 50 Ohm termination on the oscilloscope--for example, using a RG58/RG174 cable?"
Eric answered:
"In principle, you can use the oscilloscope input set for 1 MOhm termination, then add an external 50 Ohm termination resistor on a BNC Tee connector, for example. This has the advantage that you can actually use any resistor for a load, or terminate signals with an RMS voltage larger than 5 V.
However, there are two problems with using this approach for high-speed signals with rise times shorter than 1 nsec, which require an oscilloscope with bandwidth larger than 1 GHz.