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Introduction : With the development of wireless communication technology in recent years, the requirements for the RF front-end circuit part are getting higher and higher. The research on wireless LAN technology in the 5GHz frequency band is also becoming more and more extensive. In order to use a set of antennas for transmission and reception, a single-pole double-throw switch is required for transmission and reception switching. At present, the control elements in communication mainly use microwave semiconductor devices with small size, light weight, low control power and fast control speed. For example, PIN tube, FET tube, varactor tube and Schottky tube. The characteristic of PIN tube is that it can control high-power RF signals with low DC level. This paper introduces the design theory and method of single-pole double-throw switch. It first gives a brief introduction to PIN tubes and the design of switches using PIN tubes, then uses software to simulate the switch circuit and optimize the circuit. Working principle of PIN tube The PIN diode is a PIN junction device with a very small doped I region between the P-type and N-type contact regions, which obtains the characteristics that are very necessary for certain device applications. When reverse biased, the I region will lead to an extremely high diode breakdown voltage, and the device capacitance is reduced by increasing the distance between the P region and the N region. When forward biased, the conductivity of the I region is controlled by the charge injected into the end region. This diode is a low-distortion bias current controlled resistor with good linear performance. PIN tubes are widely used in RF microwave circuits to complete functions such as amplitude modulation, attenuation, and level calibration, and can be made into excellent switches and attenuators. 
When the PIN tube is reverse biased, it has a very high impedance to RF microwaves. When an appropriate forward current is passed through it, it shows a very low impedance and can be used as a switch. In applications such as RF microwave switches or attenuators, the circuit parameters require that the device have smaller series resistance and total capacitance distributed parameters. At the same time, according to the specific needs of the switching circuit, the device is required to have a very fast switching speed. During design, the device structure and parameters must be selected based on the use requirements. Design of PIN switch The different impedance characteristics of the PIN tube under positive and negative bias can be used to control the on and off of the circuit and form a switch circuit. There are two commonly used switch circuits according to their functions. One is the on-off switch, such as a single-pole single-throw switch, which controls the on and off of the signal in the transmission system. The other is a switching switch, a single-pole double-throw or multi-throw switch, which controls the signal path. In essence, they are all switches, but the difference is single-channel and multi-channel, or it can be said that it is a way to control the size of the signal and a way to control the path. An ideal switch has infinite attenuation when disconnected and zero attenuation when turned on. Since the impedance of the PIN tube can neither be reduced to zero nor increased to infinity, the attenuation of the actual switch is not infinite when disconnected and not zero when turned on. Generally, we can only require the ratio of the two to be as large as possible. The attenuation of the switch when turned on is called insertion loss, and the attenuation when turned off is called isolation. Insertion loss and isolation are the basic indicators for measuring the quality of switches.  The structure of a single-pole double-throw switch : According to the different connection methods between PIN tubes and transmission lines, the simplest switches can be divided into two types: parallel type and series type. For parallel type switches, when the tube is high impedance, the transmission power is slightly affected, and the insertion attenuation is very small, which is equivalent to the on state of the switch. When the tube is low impedance, most of the transmission power is reflected back, and the insertion attenuation is very large, which is equivalent to the off state of the switch. For series type switches, the situation is just the opposite. Series structure switches are often used in situations where the insertion loss is small within a wide bandwidth. The circuit implementation of this design is relatively simple, and there is no need to punch holes on the printed circuit board. Parallel structure switches are often used in situations where large isolation is required within a wide bandwidth. This type of switch has good heat dissipation, so the power capacity is also large. Since the tube in the parallel type switch is easy to connect with the waveguide transmission line, etc., and has advantages such as good heat dissipation conditions, parallel type switches are more commonly used in practice.  Insertion loss: Isolation: When the parameters of the PIN tube used are known, the isolation and insertion loss of the switch can be calculated from the above formula. Optimization of circuit parameters 1. Optimization of microstrip gap: The microstrip gap width for installing PIN tubes needs to be optimized. If the gap is too large, the PIN tube lead is too long, and the additional inductance of the PIN is large when it is forward biased, which increases the insertion loss (series tube) and reduces the isolation (parallel tube) of the circuit; if the gap is too small, the gap capacitance will increase, which increases the insertion loss (parallel tube) and reduces the isolation (series tube). Therefore, a compromise should be made between the two to meet the needs during design. 2. Optimization of the length and width of the microstrip line. The length and width of each section of the microstrip line have a great influence on circuit matching. 3. Optimization of DC blocking capacitors: In order to prevent the DC static operating points of the switch front and rear circuits from affecting each other, a DC blocking capacitor is connected in series on the switch microstrip line. A small series capacitance can increase the isolation of the switch, but at the same time increase the insertion loss of the switch; correspondingly, a large series capacitance value is conducive to improving the flatness of the curve and reducing the insertion loss of the switch, but this will deteriorate the isolation of the switch. Therefore, it is necessary to choose an appropriate capacitance value. SPDT Switch Simulation and Results The design uses Skyworks' SMP1320-007 (SOT-23 package) with low internal lead inductance. The switch structure is a parallel switch and is simulated using software. Figure 4 is a circuit model of a SOT-23 with low internal lead inductance: It can be seen that the inductance of the lower lead L2 is only 0.4nH, while the inductance of a general SC-79 package is 0.7nH. The circuit board uses standard FR4 material with a thickness of 0.78mm. The simulation results can be obtained (@5-6GHz): Isolation (Min): 19.75dB Insertion loss (Max): 1.25dB Conclusion and Analysis : The simulation results are somewhat different from the expected results, and the matching circuit needs to be further optimized. However, the performance is still improved compared with the PIN switch made of other packaged diodes. In the actual layout, the parasitic inductance and capacitance of the vias must also be considered, and the circuit performance will be reduced.
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提升卡
变色卡
千斤顶