Why Do I Need to Do BNC Probe Compensation?

Ever wonder why oscilloscopes require bulky BNC cables with those fancy switches, while multimeters seem content with plain wires? The key lies in how these instruments handle electronic measurements. One of our Digilent engineers helps answer these questions and lead you into the world of BNC probes, explaining their role in capturing high-speed signals and the purpose behind the 1x and 10x settings.

 

Q: What is the BNC Probe for anyways? Why don’t oscilloscopes just use the regular cables like multimeters do?

A: DMMs measure and update slowly (often only around five times a second) and are intended to give you a measurement at that instant. They are also often designed to withstand high voltage ranges as well as being able to directly measure current; Digilent’s Analog Discovery Pro 5000 series has a DMM integrated directly into the design.
Oscilloscopes are designed to capture changing analog data over time and allow you to see the historical changes that have occurred. To keep up with the fast-changing electronics, oscilloscopes need more bandwidth (a continuous range of frequencies) to be able to properly capture those high-speed changes. Coaxial cables (and the corresponding BNC connectors and probes) are better at preserving the integrity of the high frequency signals than DMM probes or MTE cables.

 

Q: Okay, but BNC probes I’m looking at have a switch to slide between 1x and 10x whereas other cables don’t have any such adjustments. What’s up with that? 

A: Switchable BNC probes allow you to switch the attenuation of the probe, effectively scaling the amplitude of the signal you are measuring. The 10x setting decreases the voltage by a factor of 10 (seems like a slight misnomer but bear with me) by putting a 9 MΩ resistor in series with the 1 MΩ resistor that is commonly present on oscilloscope inputs for a combined 10 MΩ input. On the oscilloscope itself, you can easily account for this downscaling by multiplying the received signal by 10 in software. 1x mode simply bypasses the extra 9 MΩ resistor in the BNC Probe offering no scaling.
Astute readers will note that nuances in measurements in the 10x mode will be inherently distorted because they were decreased by a factor of 10 in hardware before being scaled back up in software, which is certainly true. There are two benefits to the 10x mode though; one is inverse of the above statement in which that you can now also measure voltages that are outside of the specified range (like the –25 V to +25 V range on Digilent’s Test and Measurement devices) because you have scaled down incoming voltage by a factor of 10. The second benefit is that 10x mode gives you access to the advertised bandwidth rating on the BNC probe to use in conjunction with your high bandwidth oscilloscope.

 

Q: Sorry, you just said that the 10x mode lets you use higher frequencies which kind of implies that the 1x mode does not? But wouldn’t 1x mode, as it is presumably just coaxial cable with no attenuation circuitry also be “better at preserving the integrity of high frequency signals”? 

    A: Heh. That question is a large part of what inspired this post to begin with. The short answer is no. All types of connections and junctions between two points, whether BNC probes or traces on a PCB, will add capacitance and affect the electrical signals flowing through them increasing the rise and fall times, the effect of which is, of course, more noticeable on higher frequency signals. With the extra resistance offered in 10x mode, changes in the signal do not have to be driven quite so strongly to correctly propagate through the cable. The physical coaxial cable of a BNC probe is also optimized for 10x mode to offer the best bandwidth, which correspondingly leaves 1x mode with the short end of the stick. 
    Dave Jones on EEVblog has a good video that goes into more depth on this topic: https://www.youtube.com/watch?v=OiAmER1OJh4. 

     

    Q: Let’s say that I have a circuit that needs the higher bandwidth capabilities of the Analog Discovery Pro devices. Can I just connect a BNC probe, switch both it and the oscilloscope to 10x and be on my way?  

      A: Not quite. You will need to compensate your BNC probe at the 10x mode to help eliminate any reflections. A guide on how to compensate a BNC probe is available here: https://digilent.com/reference/test-and-measurement/guides/probe-compensation.
      If you want to know more about the transmission line theory that Dave Jones mentioned which causes these reflections or are just curious about what physically happens when you do not compensate your probe and attenuation in general, then it is my pleasure to direct you to this 1959 video lecture from the inventor of the phototransistor, Dr. Shive: https://www.youtube.com/watch?v=DovunOxlY1k. 

       

      Q: Is there any other material where I can learn a bit more about BNC Oscilloscope Probes

      A: Yes! Here are several resources that I personally found helpful on this topic: 

      1. Teledyne Lecroy’s Application Note AN_016 Probes & Probing circa 1999, https://www.teledynelecroy.com/doc/probes-probing This gives a good overview on the differences, intended applications, and the effect on signals between high impedance probes, low impedance probes, and active probes. 
      2. Electronics Stack Exchange explaining the significance of -3 dB, https://www.tek.com/en/documents/application-note/how-oscilloscope-probes-affect-your-measurement.  
      3. Tektronix Application note How Oscilloscope Probes Affect Your Measurement, https://www.tek.com/en/documents/application-note/how-oscilloscope-probes-affect-your-measurement, that reiterates some of the material from the Teledyne Lecroy Application Note in a slightly different way.  
      4. Electronics Stack Exchange question on why a 1 MHz square wave looks different on a 100 MHz probe vs a 200 MHz probe on the same system, https://electronics.stackexchange.com/questions/706501/100-mhz-probe-and-200-mhz-probe-show-very-different-waveform-on-1-mhz-signal.  

       

      Author

      • James Colvin

        A local Digilent employee who is sometimes tricked into making other content besides documentation and supporting customers on the Digilent Forum, but then I get to write a little more informally so that's a plus. A sassy engineer, lover of puns and dad jokes, father and husband. I know both way too much and simultaneously almost nothing about a number of nerdy topics. If you want to hear me rant, ask me what data rate USB C operates at.

      About James Colvin

      A local Digilent employee who is sometimes tricked into making other content besides documentation and supporting customers on the Digilent Forum, but then I get to write a little more informally so that's a plus. A sassy engineer, lover of puns and dad jokes, father and husband. I know both way too much and simultaneously almost nothing about a number of nerdy topics. If you want to hear me rant, ask me what data rate USB C operates at.

      View all posts by James Colvin →

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