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
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.
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.
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
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.
Figure 5. Final Assembly of Series 50 Ohm Termination with Two Layers of Heat Shrink Tubing