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RF & Microwave Terminations

RF & microwave terminations play an important role in many high frequency circuits and systems. They are used to absorb unwanted energy in electrical circuits by converting the RF energy into heat.  In addition, terminations are used to terminate transmission lines that have specific characteristic impedance, typically 50 Ohms.  Standard termination values are 50 Ohms for maximum power transfer; however, 75 Ohm terminations are also used in broadcast applications.  Terminations are also known as dummy loads, when used in high power applications.


Standard Applications

There are multiple uses for terminations in various RF and Microwave circuits and systems.  Typical applications include:

  1. Test Setups and Instrumentation: High power terminations may be used in place of an antenna for testing transmitters, as shown in Fig. 1. The impedance on the terminations can also be changed in order to create a controlled mismatch on the transmission line. This is useful in reflection calibration of reflectometers, or in test setups to reproduce specific characteristics that a system would encounter after installation (transmitter tuning).


    Figure 1.  Transmitter under test using high power termination in place of an antenna.

     
  2. Power Dividers and Combiners: Terminations are placed on the isolation port of hybrid and directional couplers to reduce reflections back to the system that could cause degradation in performance, Fig 2.  In Wilkinson type power dividers, terminations are used to terminate unused ports. The optimum design of these circuits requires proper impedance match on all RF ports.


    Figure 2.  SMT hybrid coupler with chip termination on the isolated port


  3. RF & Microwave Isolators:  These components use ferrites and magnets to direct the flow of RF energy in the circuit.  In isolators, one of the ports is terminated with an RF termination. This termination absorbs the energy being reflected back from the system’s output port as shown in Figure 3.  A commonly used drop-in type isolator with a high power termination is presented in Figure 4.


    Figure 3.  RF isolator used in transmitter application with high power termination on one port.


    Figure 4.  Nova Microwave isolator with Res-Net Microwave high power termination.


Design Considerations for RF & Microwave Terminations

Knowledge of several RF & microwave parameters is required when selecting the best termination for a specific application. Some of these parameters are: 

  1. Voltage Standing Wave Ratio (VWSR): An ideal termination completely absorbs incoming signals over multiple frequencies. This is considered a perfect match (VSWR 1.0:1). In actual applications, inherent physical properties of terminations affect their behavior over frequency. This limitation can be reduced using internal impedance matching.

  2. Capacitance: The operation frequency of a termination depends upon its ability to absorb energy and minimize reflection of signals. Parasitic capacitance and inductance limits high frequency operation.  Some vendors, like Res-Net Microwave, offer terminations that have low parasitic capacitance and excellent impedance match from DC to 40 GHz. 

  3. Power Dissipation: In most RF and Microwave terminations, heat is absorbed by the resistive element, challenging design engineers to maximize heat dissipation. Several dielectric materials are used to dissipate heat, such as Alumina, Aluminum Nitride, and Beryllium Oxide. In addition, Res-Net Microwave offers CVD diamond chip terminations with superior thermal properties and unparalleled high frequency performance. Figure 5 compares commonly used dielectric materials and their power densities.

    Figure 5. Power density for commonly used ceramic materials for high power and high frequency terminations



  4. Temperature Range: The resistive element in a termination limits the operating temperature of a device. This resistive element can be made with a thin-film or thick-film process and can operate from -55°C to +150°C.  During assembly, the temperature is limited by the type of attachment method used, normally under 300°C.
     
  5. Resistance Range:  50 Ohm resistance is standard in most RF and Microwave applications with a standard tolerance from 1% to 5%.  Other resistance values can be manufactured based on the application. Most manufacturers offer resistances values from 0 Ohms to a few Mega Ohms.
     
  6. Physical Size: RF terminations range in size from a few thousandth of an inch to a several inches. There is always a compromise between power, size and frequency that can be can be controlled by selecting the proper materials and applying advanced design techniques.

  7. Weight:  There is a proportional relationship between the power handling capabilities of a termination and its size.   Termination weight is very important in airborne or space applications, but is secondary in telecommunications and broadcast applications.
     
  8. Package Style: The application dictates the termination type used.  Package styles include chip, flange, rod and coaxial.  Chips terminations are mainly use in PC board assemblies with moderate power dissipation, whereas flange terminations can dissipate a large amount of RF power.  Coaxial terminations, one of the most widely used terminations, use different types of connectors, which are dependent on the frequency of operation.  Rod terminations, generally used within coaxial terminations, provide great performance over frequency when installed inside a properly designed cavity.  Several types of RF terminations are presented in Fig.6.


    Figure 6.  Various types of coaxial terminations offered by Res-Net Microwave

    Assembly Recommendations:
     
  9. Flange type terminations:  There are several methods of mounting flanged microwave devices to a heat sink or metal base plate.  It is critical to have very close contact between the bottom of the flange and the heat sink surface area. In addition, there should be no burrs, scratches, or non-flat surface imperfections on the flange or the heat sink surface area.  A thin layer of thermal compound with a maximum "spread" thickness less than 0.002” is highly recommended. The thermal compound improves the contact between the device and the heat sink surface.  Care should be taken to avoid an upward pressure on the termination’s tab to avoid potential damage.  Termination tabs can be soldered with industry standard lead or lead-free solder.  When mounting the device to a heat sink or metal base plate, proper torque is required based on the screw size.
     
  10. Chip terminations: For proper operation, chip terminations need adequate conductive cooling to prevent excessive temperatures that could lead to early device failure. On circuit boards, thermal vias are commonly used to dissipate the heat from the chip termination. In order to lower the thermal resistance between the device and the ground plane, it’s recommend to maximize the amount of conductive vias around and under the chip using heavy copper cladding on the PC board. For best heat dissipation, chip terminations should be soldered on the board. Chip terminations can be soldered by hand with a soldering iron, or by using automated soldering processes like wave or reflow soldering. Specific temperature profiles are required for automated soldering.

Summary

RF & microwave terminations are widely used and perform a critical power management function in many high frequency circuits and systems. This paper provides short overview of some common types of RF & microwave terminations and addresses several critical design parameters for proper selection of RF terminations. In addition, proper mounting techniques for high power terminations for both flange and chip types were discussed.

Authors:

  1. Alen Fejzuli, President, Microwave Group
  2. Gabriel Orozco, Engineering Manager, Res-Net Microwave