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20 June 2022

Is It OK to Use an External 50 Ohm Terminator?

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.

Problem #1 is that on high-speed oscilloscopes, there are actually two amplifiers. One uses the 1 MOhm input resistor and the other uses the 50 Ohm resistor. When you change terminations, you are also changing amplifiers. The 1 MOhm amplifier will only go up to 1 GHz bandwidth, whereas the 50 Ohm amplifier will go up to the full bandwidth of the oscilloscope.

Problem #2 is that when you use a BNC tee (or any external adapter), you are adding a stub to the signal. You will see the impact of the stub at frequencies greater than 1 GHz.

But, if you are measuring < 1 GHz signals on an oscilloscope with ≤ 1 GHz bandwidth, using an external 50 Ohm termination is perfectly fine."

For more on this topic, see Eric's webinar: "What Every Oscilloscope User Needs To know About Transmission Lines."

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