How to select coaxial switches?

How to select coaxial switches?

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Coaxial switch is a passive electromechanical relay used to switch RF signals from one channel to another. These switches are widely used in signal routing situations that require high frequency, high power and high RF performance. It is also often used in RF test systems, such as antennas, satellite communications, telecommunications, base stations, avionics, or other applications that need to switch RF signals from one end to another.

coaxial switches1

Switch port
When we talk about coaxial switches, we often say nPmT, that is, n pole m throw, where n is the number of input ports and m is the number of output ports. For example, the RF switch with one input port and two output ports is called SPDT/1P2T. If the RF switch has one input and 14 outputs, we need to select the RF switch of SP14T.

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Switch parameters and characteristics

If the signal needs to be switched between the two antenna ends, we can immediately know to select SPDT. Although the scope of selection has been narrowed to SPDT, we still need to face many typical parameters provided by manufacturers. We need to carefully read these parameters and characteristics, such as VSWR, Ins.Loss, isolation, frequency, connector type, power capacity, voltage, implementation type, terminal, indication, control circuit and other optional parameters.

Frequency and connector type

We need to determine the frequency range of the system and select the appropriate coaxial switch according to the frequency. The maximum operating frequency of coaxial switches can reach 67GHz, and different series of coaxial switches have different operating frequencies. Generally, we can judge the operating frequency of coaxial switch according to the connector type, or the connector type determines the frequency range of coaxial switch.

For a 40GHz application scenario, we must select a 2.92mm connector. SMA connectors are mostly used in the frequency range within 26.5GHz. Other commonly used connectors, such as N-head and TNC, can operate at 12.4GHz. Finally, the BNC connector can only operate at 4GHz.
DC-6/8/12.4/18/26.5 GHz: SMA connector

DC-40/43.5 GHz: 2.92mm connector

DC-50/53/67 GHz: 1.85mm connector

Power capacity

In our application and device selection, power capacity is usually a key parameter. How much power a switch can withstand is usually determined by the mechanical design of the switch, the materials used, and the type of connector. Other factors also limit the power capacity of the switch, such as frequency, operating temperature and altitude.

Voltage

We have already known most of the key parameters of coaxial switch, and the selection of the following parameters depends entirely on the user's preference.

The coaxial switch consists of an electromagnetic coil and magnet, which need DC voltage to drive the switch to the corresponding RF path. The voltage types used for coaxial switch comparison are as follows:

Coil voltage range

5VDC 4-6VDC

12VDC 13-17VDC

24VDC 20-28VDC

28VDC 24-32VDC

Drive Type

In the switch, the driver is an electromechanical device that switches RF contact points from one position to another. For most RF switches, a solenoid valve is used to act on the mechanical linkage on the RF contact. When we choose a switch, we usually face four different types of drives.

Failsafe

When no external control voltage is applied, one channel is always on. Add external power supply and switch to select corresponding channel; When the external voltage disappears, the switch will automatically switch to the normally conducting channel. Therefore, it is necessary to provide continuous DC power supply to keep the switch switched to other ports.

Latching

If the latching switch needs to maintain its switching state, it needs to continuously inject current until a pulse DC voltage switch is applied to change the current switching state. Therefore, the Place Latching drive can remain in the last state after the power supply disappears.

Latching Self Cut-off

The switch only needs current during the switching process. After the switching is completed, there is an automatic closing current inside the switch. At this time, the switch has no current. That is to say, the switching process requires external voltage. After the operation is stable (at least 50ms), remove the external voltage, and the switch will remain on the specified channel and will not switch to the original channel.

Normally Open

This working mode SPNT is only valid. Without control voltage, all switching channels are not conductive; Add external power supply and switch to select the specified channel; When the external voltage is small, the switch returns to the state that all channels are non conducting.

The difference between Latching and Failsafe

Failsafe control power is removed, and the switch is switched to the normally closed channel; The Latching control voltage is removed and remains on the selected channel.

When an error occurs and the RF power disappears, and the switch needs to be selected in a specific channel, Failsafe switch can be considered. This mode can also be selected if one channel is in common use and the other channel is not in common use, because when selecting a common channel, the switch does not need to provide drive voltage and current, which can improve power efficiency.


Post time: Dec-03-2022