Selecting Switches for High-Power Testing Applications
By Kim-Yen Ang and Yee-Ping Teoh, Agilent Technologies

Selecting the right switch technology for your test and measurement systems is vital to ensure optimum test performance in your applications. RF performance (insertion loss and isolation), reliability, switching time, and power handling are some of the key parameters that must be taken into consideration when choosing a switch technology. There are two major types of connectorized RF and microwave switches: electromechanical switches and solid state switches. Typically, mechanical switches have lower insertion losses and higher power handling capability, while the solid state switches have faster switching speed and a longer operating life. This article deals with high-power handling in switches; more information on selecting the right switch technology for other applications can be found in Agilent’s application note “Selecting the Right Switch Technology for Your Application.”

If high-power handling, up to a couple 100 watts, is required, mechanical switches are the correct switches to use, as solid state switches typically only handle up to 0.5 watts of continuous power. This is because the power handling capability of solid state switches is limited by the breakdown voltage of the semiconductor. The ability of a switch to handle power is very dependent on the materials used for the signal carrying components of the switch and on the switch design. Typically, switch components made of hard materials that have a high resistance to heat work better in high-power applications. The switch design should allow for good heat dissipation to ensure long term reliability. An application note from Agilent Technologies, “Power Handling Capability of Electromechanical Switches,” provides useful background on determining the specifications of your switches to handle high power and how certain usage conditions impact the power handling capability of your switches.

Selecting a Switch That Will Handle High Power
Equipment used in high-power applications must be chosen with care to ensure long term reliability and optimum performance. High RF power and/or heat generated by the current will cause the temperature to rise and will affect the performance of most systems. Usually, some form of heat removal, radiation or convection, is used in these systems. In some instances, power limiters and attenuators are used to reduce the power. However, for certain applications such as base-station transmission and mobile-phone testing, there is still the need for high power switching.

Figure 1 shows how the power handling of a typical switch is usually specified in datasheets. Two switching conditions should be considered: “hot” switching and “cold” switching. Hot switching occurs when RF/microwave power is present at the ports of the switch at the time of the switching function. Cold switching occurs when the signal power is removed before activating the switching function. Hot switching causes the most stress on internal contacts, and can lead to premature failure. Cold switching results in lower contact stress and longer life, and is recommended in situations where the signal power can be removed before switching. In this case, the switch specified as in Figure 1 will be able to handle power at an average of 1W hot switching, or an input power of up to 50W (peak) and up to 100 W (average- frequency dependent, refer to Figure 2) when used in cold switching mode.

Usually cold switching is done in pulse applications. It is important that the maximum voltage is not exceeded, as this will destroy or degrade switches. Cold power switching failure is caused by the dielectric breakdown of the switch along the conductors. This occurs when switching pressure is applied between the jumper mechanism and RF connector pins, causing heat buildup from skin effect and resistive losses which then gets transferred to the dielectric material, ultimately causing the breakdown.

Unfortunately, the specifications in datasheets do not really reflect the true power handling capability of your switch. In microwave applications, device self-heating will limit performance, especially when handling high power. Microwave heating is dependent on frequency and power as well as ambient temperature. Hence, you will need to look into the specifications like frequency and temperature as well as the insertion loss at high power. Power handling capability decreases with frequency and temperature and insertion loss increases with input power and temperature, as shown in Figure 2.

As a general guide, prior to choosing a switch for high power applications, review the following operating conditions:

1. Output power and switching condition
2. Frequency of operation
3. Power (CW/ Peak) and duty cycle
4. Effective temperature
5. Insertion loss
6. Termination

Output Power
High power switching degrades a switch faster. The higher the output power, the faster the switches will degrade due to a higher level of contact stress. Exceeding the maximum power capacity of a switch will result in deterioration and possible failure of the switch contacts through arching, burning, metal transfer or plating separation. Ideally, cold switching is used whenever possible. In cold switching, higher power, up to 100W (average), can be used, compared to only 1W (average) in hot switching.

Frequency of Operation
The relationship between frequency and cold switching power at a temperature of
75 °C is shown in Figure 2. Power handling capability decreases as the frequency increases.

Power (CW/ Peak) and Duty Cycle
Average, peak (instantaneous) and the pulse width are used to determine the duty cycle, i.e. how long a pulse can be sustained without damaging a switch.

For example, a switch rated as in Figure 1 will have a duty cycle of 2%. Typically, for this type of switch, the recommended pulse width is less than 10 µs.

Effective Temperature
The ambient temperature of the environment where the switch is in operation affects the power-handling rating. When the ambient temperature gets higher, the heating effect within the switch will increase, reducing the maximum power handling capability of the switch. A room temperature of 25 degrees Celsius might not reflect the realistic operating condition of the switch. When used in an enclosed switch matrix, the typical operating temperature goes up to approximately 40 degrees Celsius. This means that the actual power rating will be lower than the specified power rating at 25 degrees Celsius. At 75 ° C at 4 GHz, the max rating is 30W; at 25 °C, the max rating is 100W at cold switching. Agilent switches are tested at 75 °C for both cold and hot switching instead of room temperature to account for a more realistic operating mode of the switches.

Insertion Loss
The other important consideration is the insertion loss of the switch; typically, the insertion loss of Agilent electromechanical switches is very low, ranging from 0.1 dB at low frequencies to 1.5 dB at high frequencies. Factors that influence insertion losses are: path length, types of material used in signal-carrying surfaces, switch design, transmission line design and impedance matching. Insertion loss can play an important role in both high and low power applications. In high-power systems, this additional loss may require that the source power be increased to compensate for the loss. This can damage switches and other equipment. In other systems, additional power may not be available due to the prohibitive cost of supplying more power.

Termination
The termination of your switch also affects power handling. Typically, the maximum average power that can be handled by a thin film 50 ohm terminated switch is 1 watt. If unterminated, this could double to 2 watts. For a full review of the types of power handling terminology, high power failure mechanism and test setup, see the Agilent application note on power handling. 3

Conclusions
Before setting up any test system for high-power testing, a thorough understanding of the definitions of different types of power handling ratings is needed to ensure the proper selection of switches for the application. To summarize, the condition of switching (hot/cold), the type of power (magnitude, frequency, CW/peak), the available power (insertion loss at specific power level), and the working environment (effective temperature) must be taken into account.

References
1. Agilent Solid State Switches Application Note, Selecting the Right Switch Technology for Your Application, July 2007, 5989-5189EN

2. Application Note, Power Handling Capability of Electromechanical Switches March 2007, 5989-6032EN

3. Agilent L Series Multiport Electromechanical Coaxial Switches Technical Overview, 5989-6030EN

Agilent Technologies
www.agilent.com
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