You need to test, we're here to help.

You need to test, we're here to help.

20 June 2022

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.

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."

No comments:

Post a Comment