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.



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.



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
 

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