Just a little demonstration of internal/external termination vs. a proper 50 ohms signal path. This seems to be required, as I’ve often seen claims like “who needs a scope with 50 ohms inputs, we can always use a BNC-T with a terminator”.
Initially I should define the goal: a return loss of 14 dB (VSWR 1.5:1) is generally considered acceptable in the industry. This means, a proper scope shouldn’t drop below 14 dB RL (or above 1.5 VSWR) within its bandwidth, in order to be usable, i.e. show reasonable pulse fidelity and measurement accuracy for all the ones who use their oscilloscopes for serious tasks.
First a proper through terminator, a venerable HP10100C. This performs beautifully when just connected at the end of a 50 ohms coax cable, but gets pretty ugly as soon as it’s connected to the scope input:
SDS2354X+_RL_200mV_HP10100C
The limit of 14 dB RL is reached at 64 MHz already. Now the alternative, even cheaper solution, a BNC-T with a terminator connected to one leg and the input signal to the other:
SDS2354X+_RL_200mV_BNC-T
As can be seen, the result is pretty much the same. Of course – at frequencies that low a few centimeters aren’t a big deal. This time I’ve measured the minimum RL within the 500 (actually 600) MHz bandwidth, and it’s only 6.94 dB. Utterly useless.
Finally the real thing – the scope switched to 50 ohms, which alters the internal signal path and eliminates the input capacitance that’s always present in high impedance mode:
SDS2354X+_RL_200mV
We get a return loss of more than 17 dB throughout the entire measured range (up to 1 GHz). At 200 MHz, we still have 30 dB RL, which is equivalent to 1.07:1 VSWR.
And now, what’s the consequence?
Look at the next screenshot:
SDS2504X HD_PR_50
This is with the scope switched to its 50 ohms input path. It shows a nice 10 ns wide pulse with 1 ns rise and fall times – exactly as I’ve programmed it on my pulse generator. Yes, there is a little overshoot – and we certainly want to know about that.
Now let’s try the HP10100C through terminator and use the 1 megohm input channel instead:
SDS2504X HD_PR_BNC-T
Some “experts” will say: “oh, this is much nicer now, without the ugly overshoot and ringing!”
The reality is, that we’ve lost all the fine detail due to linear distortion (frequency dependent amplitude error), so we don’t see the overshoot anymore and we also cannot measure the rise and fall times accurately.
But yes, you can say it’s not really a problem for low bandwidth oscilloscopes, because the 200 MHz bandwidth limit appears to have a far worse effect on that particular pulse measurement:
SDS2504X HD_PR_50_BWL200MHz