High Frequency Measurements Web Page
Douglas C. Smith

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Improved Construction Technique for a 50 Ohm Termination

Figure 1. Resistors Mounted on a Fiber Washer

In a technical article on this website that describes how to build a simple DC-1GHz probe, it is necessary to construct a 50 Ohm termination on the end of a female BNC connector. The resistors were soldered to the rim of the BNC connector. This can be a difficult and tedious process. This article shows and easier method for constructing the termination.

Fiber washers can be purchased at many electronic supply houses. A popular size of these washers fits neatly into a female BNC connector. Figure 1 above shows the first step in constructing the termination. First, double stick tape is placed on the washer and trimmed to fit. Four 200 Ohm 1206 surface mount resistors are then placed on the tape which holds them in place.

Figure 2 shows the washer with attached resistors placed into a female BNC connector. Position the resistors on the washer so that they just contact or come very close to the walls of the connector.
Figure 2. Washer & Resistors in BNC Connector

To complete the assembly, a soldering iron is held against the outside of the BNC connector and a small amount of solder is applied to the inside wall. Just enough solder is used to insure a good joint between each resistor and the wall of the BNC connectors. Figure 3 shows the resistors soldered into place.
Figure 3. Resistors Soldered to BNC Connector

From this point, a center post of 16 Gauge copper wire is inserted into the hole in the center of the resistors. Copper foil tape should be wound on the wire so that the foil just touches the resistors and solder is then used to connect the resistors to the foil tape and wire as shown in the plans for the DC-1GHZ probe.

The characteristics of the washer and double stick tape are not likely to affect the performance of the termination up to 1 GHz. It you want to be sure, the impedance of the termination can be measured. If used in the DC-1GHz probe, flat probe response indicates the termination is working properly.

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Copyright © 1999 Douglas C. Smith