FYI, here's just a quick comparison of my "MSO4014+" (500MHz) internal vs. external 50 Ohms termination (Chinese BNC-Through terminator with 3*150 Ohms 1210 resistors in parallel), fed with a square wave signal with <50ps rise time (heritage of Leo Bodnar). First I tested without bandwidth limitation and then again with a 200MHz limit since this was the "class" of oscilloscope that the OP was refering to. The screenshots with the feed-through terminator have the corresponding trace of the internal terminator as a reference in the background.
Unfortunately, my VNA currently is borrowed to a friend so I cannot check the performance of the external terminator, yet a quick test with my SSA3000X also reveals part of the (sad) truth: The SSA had been normalized with the return loss bridge prior to the tests. Trace A resembles a measurement with a Minicircuits SMA termniator directly at the test port of the RLB. Trace B is the BNC-through terminator, directly attached via a SMA/BNC adapter (no cable in between). Up to about 200MHz, it at least stays in the ballpark of the Minicircuits terminator. Trace C shows the same configuration as in trace B, but now connected to input A of my MSO4000 (1MOhm configuration -- well
). Trace D resembles the input of the MSO configured to 50 Ohms and the BNC-through terminator removed. Depending on the range switch of the scope input, return loss may vary. The trace shown is one of the better results...
It seems that the feed-through terminator is fairly usable up to 100Mhz and with some compromise, to 200MHz. Above this, ...meh...
Depending on the quality of the feed-through terminator, you may get better results. But the input capacity of the (average) scope will mess up the return loss of even the most accurate terminator. I guess for proper upper VHF performance, you need a scope with a front-end particularly made for that.
Hope this makes sense....Cheers,
Thomas