What Is a Switch Assembly?

The purpose of the switch matrix is to control the opening and closing of the circuit. The signal switching system in the automatic test equipment usually consists of two or more matrix switches, which are connected to each other according to various interface standards to form a flexible switch from test resources to UUT.

The switch network is an important part of the automatic test system. It is responsible for controlling the signal flow and is the key to the interface design of the automatic test. In automatic test equipment, switches are generally divided into power switches, signal switches (matrix switches),

Switch design should follow these basic principles:

(1) Use switch system modules with open business standards. Products with open standards have diversified supply sources, serialization of varieties, convenient maintenance and upgrades, and are conducive to the selection of switch systems and future technical support.

(2) Adopt modular and expandable switch system structure. The modular and extensible switch system structure can not only conveniently expand the scale of the switch system, but also make the switch system compatible upwards, which is helpful to realize the transplantation and interoperation of the test system TPS.

(3) On the premise of meeting the requirements of transplantation and convenient configuration, reduce the number of spare expansion switch terminals. In high-frequency signal transmission, open switches are coupled to adjacent signal channels through stray capacitance

The main types of switches that make up a switch assembly / system are the following.

(1) Reed Relay. The reed switch has a fast switching speed (compared with an electromagnetic relay), a small on-resistance, and the switch is in a sealed structure, but its ability to carry large currents and high voltages is poor. When a faster switching speed is required, a reed switch can be selected.

(2) Mercury relay. It has a long service life, very low on-resistance, and no contact jitter. However, the installation position of mercury relays is sensitive and must be installed correctly to work properly. At the same time, mercury is greatly affected by environmental factors, which limits the application scope of such relays.

(3) Electromagnetic relay. At present, the most widely used are electromechanical electromagnetic relay switches. This type of relay has both power switches suitable for high-power signal switching, high-frequency switches suitable for microwave and RF signal switching, and optical fiber switches for optical signal switching. This type of relay switch has the advantages of large open-circuit isolation resistance, small on-resistance, and large operating current. However, it is generally large in size, slow in switching speed, short in service life, and requires large driving current.

(4) Field-effect transistor (FET) switch. It is a non-contact electronic switch. It has the advantages of small size, low driving current, high reliability, strong anti-interference ability, long service life and fast switching speed (up to microsecond level). It is suitable for high density and high power. In applications where signals are frequently switched, the on-resistance of the FET switch is large, and there is leakage current when it is turned off. Generally it does not have bidirectional conduction capability, and the cost is high. ^[2]

The design principle of the switch matrix is modular division and configuration according to functions, and corresponds to the definition of the signal port of the automatic test system, which is conducive to the expansion of the interface and the formation of a modular test system structure. In the actual switch system design, multiple switch topologies are often used to form a hybrid switch system, and various switch resources with modularity are flexibly configured and cascaded to form an efficient structure that meets the needs of testing.

For example, a 4 × 40 hybrid switch system structure composed of a 4 × 4 matrix switch and four 10-to-1 multiple switches can effectively expand the number of input / output channels of the matrix switch. The disadvantage is that this structure cannot achieve a complete 4 × 40 channels can be switched arbitrarily. For example, when channel A is connected to channel 0, channels B, C, and D cannot be connected to channels 1 to 9 in this multi-channel switch module. The hybrid switch structure is an economically considered expansion scheme of the switching channel, which can be grouped according to the different timing requirements of the detection / excitation signal to achieve the channel switching between the UUT measuring point group and the testing instrument. ^[2]