Power Handling Capability of Electromechanical Switches
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Introduction
RF/microwave electromechanical switches are used in a wide variety of signal routing applications for test and measurement systems. Typical applications for electromechanical switches include the selection of multiple signal sources to one output, selection of multiple input signals to one measurement instrument, transfer switching to insert or remove a device in a signal path, and matrix switching of multiple inputs and outputs. Keysight Technologies electro-mechanical switches provide broadband performance with low insertion loss, high isolation, and exceptional repeatability throughout a long operating life. One important parameter for electromechanical switches that is often misunderstood is power handling. The ability of a switch to handle power is very dependent on the design and materials used in a switch. In the datasheet, there are different power-handling ratings for the switches such as hot switching, cold switching, average power, and peak power. Understanding these different terms will help you avoid catastrophic failure of the switch or instruments.
Table of Contents
Hot Switching
Cold Switching
Internal terminations: average and peak power
Internal terminations: average and peak power
Power handling test setup
Conclusion
Hot Switching
Hot switching occurs when RF/microwave power is present at the ports of the switch at the time of the switching. It causes the most stress on internal contacts and can lead to premature failure. Table 1 shows an example of a power handling specification. In the table, the hot switching specification is shown as “Switching: 1 W CW” which indicates that the switch can be hotswitched at 1 W CW.
When a circuit carrying power is opened, there is always some arcing between the contacts if the voltage and current are above the minimum spark voltage and current. This is similar to an electrical arc. When two electrodes carrying an electric current are drawn apart, the strong forces will draw electrons from one electrode to the other, resulting in an arc. This arcing causes degradation of the switch contacts by leaving deposits on them and burning them.
The worst-case for arcing is at DC voltages where the voltage is constant and will sustain an arc longer. At AC voltages, the voltage passes through zero twice each cycle, this quick cycling is not constant so the arc will often extinguish. By referring to the specifications in Table 1, you can be sure to select the right power level. The specified 1 W CW causes a minimal arcing effect for hot switching. When the power level goes higher, the stronger electromagnetic field will cause more arcing, and lead to premature failure of the switch contacts.
Cold Switching
Cold switching occurs when the signal power is removed before switching. Cold switching results in lower contact stress and longer life. It is recommended in situations where the signal power can be removed before switching.
Cold switching is the power handling rating that users normally refer to. Figure 2 shows the supplementary characteristic for cold switching at 75 °C. This refers to the ambient temperature of the environment where the switch is in operation. When the ambient temperature gets higher, the heating effect within the switch will increase and will reduce the maximum power handling capability of the switch. The power handling of Keysight’s switches are specified at an operating temperature of 75 °C instead of 25 °C. This is due to the fact that 25 °C 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 °C. This means that the actual power rating will be lower than the specified power rating at 25 °C.
