Is It OK to Use an External 50 Ohm Terminator with an Oscilloscope?

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.

Transmission Lines for Oscilloscope Users, Part 4

Figure 1: Characteristic waveform when the source
impedance is lower than the cable impedance.

In Part 3, we saw the pattern of reflections that occur when both the source impedance and the oscilloscope input impedance are higher than that of the interconnect, and how those reflections affected the rise time measurement. Now let’s briefly consider what happens when the source impedance is lower than the impedance of the connecting cable. 

For this example, the source voltage is a 3.3 V square wave and the source impedance is 9 Ω. As before, our transmission line is a 50 Ω coaxial cable connecting the source to the oscilloscope. If the oscilloscope input termination is set to 1 MΩ, we see the interesting waveform shown in Figure 1.

How to Choose Between the Oscilloscope's 50 Ohm Input and 1 MOhm Input

Dr. Eric Bogatin

Figure 1. The same signal from a fast-buffer driver
measured with a 1-meter, 50 ohm cable with
1 megaohm input to the scope, and
same cable with 50 ohm input to the scope.

When every oscilloscope has both a 50 Ohm input and a 1 MOhm input, how do you choose which one to use? Here are my recommendations for when each input should be used. 

For additional information on this topic, check out my webinar on What Every Oscilloscope User Needs to Know About Transmission Lines.  

What Every Oscilloscope User Needs to Know About Transmission Lines

Eric Bogatin, Signal Integrity Evangelist, Teledyne LeCroy

Figure 1. Measured voltage at the oscilloscope from a
fast edge, low impedance DUT, with the oscilloscope at
1 megaohms (left) and 50 ohms (right).

It is easy to take a measurement with an oscilloscope and see a voltage waveform on the screen. It is sometimes hard to take a measurement without artifacts and interpret all the details of the measurement. 

Whenever you measure a signal with a rise time shorter than about 20 ns, assuming a 1 m long coax cable, transmission line effects should pop to the top of your list of potential artifacts to consider and avoid.