element14 Community
element14 Community
    Register Log In
  • Site
  • Search
  • Log In Register
  • Community Hub
    Community Hub
    • What's New on element14
    • Feedback and Support
    • Benefits of Membership
    • Personal Blogs
    • Members Area
    • Achievement Levels
  • Learn
    Learn
    • Ask an Expert
    • eBooks
    • element14 presents
    • Learning Center
    • Tech Spotlight
    • STEM Academy
    • Webinars, Training and Events
    • Learning Groups
  • Technologies
    Technologies
    • 3D Printing
    • FPGA
    • Industrial Automation
    • Internet of Things
    • Power & Energy
    • Sensors
    • Technology Groups
  • Challenges & Projects
    Challenges & Projects
    • Design Challenges
    • element14 presents Projects
    • Project14
    • Arduino Projects
    • Raspberry Pi Projects
    • Project Groups
  • Products
    Products
    • Arduino
    • Avnet Boards Community
    • Dev Tools
    • Manufacturers
    • Multicomp Pro
    • Product Groups
    • Raspberry Pi
    • RoadTests & Reviews
  • Store
    Store
    • Visit Your Store
    • Choose another store...
      • Europe
      •  Austria (German)
      •  Belgium (Dutch, French)
      •  Bulgaria (Bulgarian)
      •  Czech Republic (Czech)
      •  Denmark (Danish)
      •  Estonia (Estonian)
      •  Finland (Finnish)
      •  France (French)
      •  Germany (German)
      •  Hungary (Hungarian)
      •  Ireland
      •  Israel
      •  Italy (Italian)
      •  Latvia (Latvian)
      •  
      •  Lithuania (Lithuanian)
      •  Netherlands (Dutch)
      •  Norway (Norwegian)
      •  Poland (Polish)
      •  Portugal (Portuguese)
      •  Romania (Romanian)
      •  Russia (Russian)
      •  Slovakia (Slovak)
      •  Slovenia (Slovenian)
      •  Spain (Spanish)
      •  Sweden (Swedish)
      •  Switzerland(German, French)
      •  Turkey (Turkish)
      •  United Kingdom
      • Asia Pacific
      •  Australia
      •  China
      •  Hong Kong
      •  India
      •  Korea (Korean)
      •  Malaysia
      •  New Zealand
      •  Philippines
      •  Singapore
      •  Taiwan
      •  Thailand (Thai)
      • Americas
      •  Brazil (Portuguese)
      •  Canada
      •  Mexico (Spanish)
      •  United States
      Can't find the country/region you're looking for? Visit our export site or find a local distributor.
  • Translate
  • Profile
  • Settings
Personal Blogs
  • Community Hub
  • More
Personal Blogs
Legacy Personal Blogs Using 50-Ohm Coax From DUT to Oscilloscope
  • Blog
  • Documents
  • Mentions
  • Sub-Groups
  • Tags
  • More
  • Cancel
  • New
  • Share
  • More
  • Cancel
Group Actions
  • Group RSS
  • More
  • Cancel
Engagement
  • Author Author: caroline_teledynelecroy
  • Date Created: 26 Mar 2019 10:20 AM Date Created
  • Views 4195 views
  • Likes 5 likes
  • Comments 0 comments
  • digital oscilloscope
  • bandwidth
  • dut
  • probes
  • oscilloscope
  • teledynelecroy
  • lecroy;
  • teledyne
Related
Recommended

Using 50-Ohm Coax From DUT to Oscilloscope

caroline_teledynelecroy
caroline_teledynelecroy
26 Mar 2019

In our recent exploration of 10x passive probes, we've determined that while these types of probes are great general-purpose tools, they're not necessarily going to do the job in specialized measurement circumstances. They're relatively low-bandwidth, low-SNR probes that impose some limitations and, in some scenarios, can deliver potentially misleading or erroneous measurement results if used without clear understanding of their capabilities.

 

But what if we remove the probes from the equation by connecting our DUT's output directly to the oscilloscope's analog input using a 3-ft. 50-Ω coaxial cable? What if we also use a 1-MΩ input impedance on our oscilloscope when doing so?

 

image

Figure 1: A coaxial cable presents high impedance at low frequencies but acts as a transmission line at higher frequencies

 

As Figure 1 reveals, a 50-Ω coaxial cable exhibits high impedance at low frequencies, but there's the input impedance imposed by the cable's capacitance to consider. At low frequency, the cable looks like a capacitor of roughly 100 pF (30 pF/foot for RG-58 coax). That's about 10x the capacitive load of the 10x passive probe.

 

If you're using the oscilloscope's 1-MΩ input, at low frequencies this will look like a 100-pF load. As frequency increases to, say, the 10-MHz range, we'll see some low impedances in the range of 10-100 Ω. That can load down your DUT considerably. It's like adding a 100-pF capacitor to your circuit.

 

At higher frequencies, the coax will look more like a transmission line, and the signal will bounce back and forth with reflections. With the oscilloscope input set for 1-MΩ impedance and depending on the impedance of the DUT, ringing is a possibility.

 

This is a scenario where situational awareness is critical. We have to know the source impedance of the DUT to understand the impact of reflections. If the source impedance driving the cable is 50 Ω, there's no problem. We'll get half the voltage into the transmission line, it travels down the line, sees the high impedance at the oscilloscope input, bounces back, and is terminated by the source impedance.

 

image

Figure 2: A low-impedance source will result in multiple reflections and ringing

 

But rarely will our DUT present a 50-Ω source impedance to the transmission line. If it's a very low-impedance source, such as a power rail, we'll see a lot of reflection between the source and the 1-MΩ impedance of the oscilloscope input (Figure 2).

 

image

Figure 3: A high-impedance source will look like an RC filter charging up with a long, slow rise time

 

If it's a high-impedance source, we won't get much voltage launched into the line. The signal will reflect back and forth and gradually build up with a long rise time, looking like an RC filter charging up (Figure 3), even though it isn't one.

 

If we're working with frequencies in the range of 20-50 MHz and signals at volt levels, we're just as well served, if not better, by simply using our 10x passive probe, which won't load down the DUT nearly as much as the coaxial cable will. At, say, 10-20 MHz, the cable is usable as long as we bear in mind that it presents a 100-pF load as noted above.

 

If we want high bandwidth and we want to use a 50-Ω cable, then we need to terminate the cable with a 50-Ω impedance at the oscilloscope input. We can achieve very high bandwidths this way. However, we cannot use AC coupling in the oscilloscope. For high DC impedance, we'll need to use an external coaxial DC-blocking capacitor. But, if we use that blocking capacitor and we're using the 50-Ω input to eliminate reflections, we've made a high-pass filter and we will not be able to see the low-frequency components of the signal.

 

image

Figure 4: The addition of a tiny amount of tip inductance severely impacts the transfer function of a coaxial connection

 

Our bandwidth is still limited by the tip inductance and cable attenuation. One cannot overemphasize how important the tip inductance is. For example, in Figure 4, we see the attenuation of RG-174 coaxial cable alone with good coaxial connections (in red), and we also see the response that we get if we add a small breakout region at the tip (in blue), as if we'd added a coax connector soldered to the board. By simply pulling the signal and return apart and adding perhaps 9 or 10 nH or loop inductance, we reap a dramatic drop in the transfer function.

 

At the end of the day, if we want to connect our DUT to an oscilloscope using only coaxial cable and coaxial connections and also maximize bandwidth, we want the cleanest interface possible with the least amount of impedance discontinuity.

 

Previous posts in this series:

Putting Probes in Perspective

Secrets of the 10x Passive Probe

How Equalization Works in 10x Passive Probes
How Tip Inductance Impacts a Probing System's Bandwidth

10x Passive Probes and Cable Reflections

Squeezing More Bandwidth From a 10x Passive Probe

  • Sign in to reply
element14 Community

element14 is the first online community specifically for engineers. Connect with your peers and get expert answers to your questions.

  • Members
  • Learn
  • Technologies
  • Challenges & Projects
  • Products
  • Store
  • About Us
  • Feedback & Support
  • FAQs
  • Terms of Use
  • Privacy Policy
  • Legal and Copyright Notices
  • Sitemap
  • Cookies

An Avnet Company © 2025 Premier Farnell Limited. All Rights Reserved.

Premier Farnell Ltd, registered in England and Wales (no 00876412), registered office: Farnell House, Forge Lane, Leeds LS12 2NE.

ICP 备案号 10220084.

Follow element14

  • X
  • Facebook
  • linkedin
  • YouTube