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Douglas C. Smith

 Address:  P. O. Box 1457, Los Gatos, CA 95031
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Technical Tidbit - April 2009
Construction of a Series 50 Ohm Termination

Three pieces of the termination

Figure 1.
Components Used to Construct 50 Ohm Series Termination

Abstract: A shielded series 50 Ohm termination can be a useful addition to a troubleshooting kit. A simple way to construct such a termination is presented.

Discussion: There are many uses for a series 50 Ohm termination (not to be confused with the parallel 50 Ohm termination used with some less expensive oscilloscope inputs). One of these uses involves modifying the frequency response of current probes. Another use is to match impedances. For instance, a 25 Ohm resistor can be used to match a 50 Ohm load to a 75 Ohm source. Another interesting use will be covered in the May 2009 Technical Tidbit next month.

Figure 1 shows the three components used for this project, two BNC barrel adapters and a two Watt, 51 Ohm carbon composition resistor (47 Ohms will also work). In Figure 2, the three components have been pushed together to form one unit. The leads of the resistor have been trimmed to a length so as to properly seat in the BNC connectors. The resistor leads should cleaned to expose shiny metal and be tinned with solder. The tinned leads just fit into the BNC center pin holes of the barrel adapters.

Three pieces joined

Figure 2. Components Joined to Form One Unit

Next, four pieces of 1/16th inch brass rod from a hardware store are soldered to the BNC barrel adapters as shown in Figure 3 to complete the ground connection between the BNC adapters and provide stiffness for the unit. Before soldering, use fine sand paper to remove the shiny coating on the BNC adapters to make soldering easier.

Four ground wires added

Figure 3. Four Wires Added to Connect the Grounds and for Stiffness

In Figure 4, the assembly has been wrapped in copper EMI tape to improve the ground connection between the BNC adapters. An inexpensive source of copper tape is described in the September 2000 Technical Tidbit on this site. The copper tape should be soldered to the BNC adapters to insure a good connection
Copper tape added

Figure 4.Copper Tape Added Over Assembly to Improve Ground Connection

Finally, the assembly is covered with two layers of heat shrink tubing to produce the final unit in Figure 5.

Finished termination

Figure 5. Final Assembly of Series 50 Ohm Termination with Two Layers of Heat Shrink Tubing
Summary: The series 50 Ohm termination described here can be constructed quickly and will be a useful addition to a troubleshooting kit. Uses include impedance matching and current probe frequency response modification. One unusual use for this termination will be covered in the next Technical Tidbit for May 2009.

Additional articles on this website related to this topic are:
  1. Current Probes, More Useful Than You Think (~170K)
    • (1998 IEEE EMC Symposium paper)
  2. September 1999: Measuring Voltages Using Current Probes
  3. March 2000, Improved Construction Technique for a 50 Ohm Termination
  4. July 2000, A Resistive Current Probe
  5. September 2000, Copper Foil Tape, Anyone?
Equipment related to this Technical Tidbit:
  1. Fischer Custom Communications TG-EFT high voltage pulse generator (more on this next month)
If you like the information in this article and others on this website, much more information is available in my courses. Click here to see a listing of upcoming courses on design, measurement, and troubleshooting of chips, circuits, and systems. Click here to see upcoming seminars in Newport Beach, CA.

Click here for a description of my latest seminar titled (now also available online as a WebEx seminar):

EMC Lab Techniques for Designers
(How to find EMC problems and have some confidence your system will pass EMC testing while it is still in your lab).

D-104 mike
My new website for engineers and technicians,, is coming! The site will contain technews and analysis programs, cartoons, multimedia tutorials and more.The site will be open around May 1st.
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Copyright © 2009 Douglas C. Smith