PCB Technical - Design of PCB board based on RF switch module function circuit

With the development of modern wireless communication systems, mobile communication, radar, satellite communication and other communication systems have higher requirements for the switching speed, power capacity, and integration of the transceiver switch. Therefore, the VXI bus technology is researched and developed to meet the needs of the military. The VXI bus module specially required by the party has very important significance. We will use the idea of virtual instrument to realize the hardware circuit in software. The RF switch designed below can be directly controlled by the computer and can be easily connected with the VXI bus test system. Integration, to maximize the application of computer and microelectronics technology in today's testing field, has broad development prospects.

1 Design and implementation of VXI bus interface circuit

VXIbus is an extension of VMEbus in the field of instrumentation, and is a modular automatic instrument system operated by a computer. It relies on effective standardization and adopts a modular approach to achieve serialization, generalization, and interchangeability and interoperability of VXIbus instruments. Its open architecture and PlugPlay mode fully meet the requirements of information products. It has the advantages of high-speed data transmission, compact structure, flexible configuration, and good electromagnetic compatibility. Therefore, the system is very convenient to set up and use, and its applications are becoming more and more extensive. It has gradually become the preferred bus for high-performance test system integration.

The VXI bus is a completely open modular instrument backplane bus specification suitable for various instrument manufacturers. VXI bus devices are mainly divided into: register-based devices, message-based devices, and memory-based devices. Register-based devices currently account for the largest proportion of applications (about 70%). The VXIbus register base interface circuit mainly includes four parts: bus buffer drive, addressing and decoding circuit, data transmission response state machine, configuration and operation register group. In the four parts, except that the bus buffer driver is realized by 74ALS245 chip, the rest are realized by FPGA. A piece of FLEX10K chip EPF10K10QC208-3 and a piece of EPROM chip EPC1441P8 are used, and the corresponding software MAX+PLUS2 is used for design and implementation.

1.1 Bus buffer driver

This part completes the buffer receiving or driving of the data lines, address lines and control lines in the VXI backplane bus to meet the requirements of VXI standard signals. For A16/D16 devices, as long as the backplane data bus D00～D15 are buffered and driven. According to the requirements of the VXI bus specification, this part is implemented with two 74LS245s, which are strobed by DBEN* (generated by the data transmission response state machine).

1.2 Addressing and decoding circuit

Addressing lines include address lines A01 to A31, data strobe lines DS0* and DS1*, and long word line LWORD*. The control lines include the address strobe line AS* and the read/write signal line WRITE*.

The design of this circuit adopts the schematic design method of MAX+PLUS2. Design using the existing components in the component library, using two 74688 and one 74138.

This functional module decodes address lines A15～A01 and address modification lines AM5～AM0. When the device is addressed, it receives the address information on the address line and the address modification line, and compares it with the logical address LA7～LA0 set by the hardware address switch on this module, if the logical value on AM5～AM0 is 29H or 2DH (Because it is an A16/D16 device), when the address lines A15 and A14 are both 1, and the logic value on A13～A06 is equal to the logic address of the module, the device is address strobe (CADDR* is true). Then the result is sent to the next level of decoding control, and the register of the module in the 16-bit address space is selected by decoding the addresses A01～A05.

1.3 Data transmission response state machine

The data transmission bus is a group of high-speed asynchronous parallel data transmission buses, and is the main component of the information exchange of the VMEbus system. The signal lines of the data transmission bus can be divided into three groups: addressing lines, data lines, and control lines.

The design of this part adopts MAX+PLUS2 text input design method. Due to the complicated timing of DTACK*, AHDL language is used to design and realize through state machine.

This functional module configures the control signals in the VXI backplane bus, and provides timing and control signals for the standard data transmission cycle (generating the data transmission enable signal DBEN*, the response signal DTACK* required by the bus to complete the data transmission, etc.). During data transmission, the system controller first addresses the module and sets the corresponding address strobe lines AS*, data strobe lines DS0*, DS1*, and WRITE* signal lines that control the direction of data transmission to be valid Level. When the module detects that the address matches and the control lines are valid, drive DTACK* to low level to confirm to the bus controller that the data has been placed on the data bus (read cycle) or data has been successfully received (write cycle) ).

1.4 Configuration register

Each VXI bus device has a set of "configuration registers". The system main controller obtains some basic configuration information of the VXI bus device by reading the contents of these registers, such as device type, model, manufacturer, address space (A16, A24)., A32) and the required storage space, etc.

The basic configuration registers of VXI bus devices include: identification registers, device type registers, status registers, and control registers.

The design of this part of the circuit adopts the MAX+PLUS2 schematic design method, using the 74541 chip and the functional modules created by it.

The ID, DT, and ST registers are all read-only registers, and the control registers are write-only registers. In this design, the VXI bus is mainly used to control the on and off of this batch of switches, so as long as you write data to the channel register, you can control the suction or disconnection state of the relay switch, and query the relay status is also read from the channel register The data is fine. According to the design requirements of the module, write appropriate content in the corresponding data bits, so as to effectively control the radio frequency switch of the functional module.

2 The design of the module functional circuit PCB board

Each VXI bus device has a set of "configuration registers". The system main controller obtains some basic configuration information of the VXI bus device by reading the contents of these registers, such as device type, model, manufacturer, address space (A16, A24)., A32) and the required storage space, etc.

The frequency range of the radio frequency circuit is about 10kHz to 300GHz. As the frequency increases, radio frequency circuits show some characteristics different from low frequency circuits and DC circuits. Therefore, when designing the PCB board of the radio frequency circuit, it is necessary to pay special attention to the influence of the radio frequency signal on the PCB board. The radio frequency switch circuit is controlled by the VXI bus. In order to reduce interference in the design, the bus interface circuit part and the radio frequency switch function circuit are connected by a flat cable. The following mainly introduces the PCB board design of the radio frequency switch function circuit part.

2.1 Layout of components

Electromagnetic compatibility (EMC) refers to the ability of an electronic system to work normally in accordance with design requirements in a specified electromagnetic environment. For the design of radio frequency circuit PCB, electromagnetic compatibility requires that each circuit module does not produce electromagnetic radiation as much as possible, and has a certain degree of anti-electromagnetic interference ability. The layout of components directly affects the interference and anti-interference ability of the circuit itself. It also directly affects the performance of the designed circuit.

The general principle of the layout: components should be arranged in the same direction as much as possible, and poor soldering can be reduced or even avoided by selecting the direction of the PCB entering the soldering system; there must be at least 0.5mm spacing between components to meet the soldering requirements of the components If the space of the PCB board allows, the spacing of the components should be as wide as possible.

The reasonable layout of components is also a prerequisite for reasonable wiring, so it should be considered comprehensively. In this design, the relay is used to convert the radio frequency signal, so the relay should be placed as close as possible to the signal input end and output end, so as to minimize the length of the radio frequency signal line, and make a reasonable layout for the next step. consider.

In addition, the radio frequency switch circuit is controlled by the VXI bus, and the influence of the radio frequency signal on the VXI bus control signal is also an issue that must be considered during layout.

2.2 Wiring

After the layout of the components is basically completed, the wiring must be started. The basic principle of wiring is: When the assembly density permits, try to use low-density wiring design, and the signal wiring is as thick as possible, which is conducive to impedance matching.

For radio frequency circuits, the unreasonable design of signal line direction, width, and line spacing may cause cross interference between signal transmission lines; in addition, the system power supply itself also has noise interference, so comprehensive consideration must be taken when designing the radio frequency circuit PCB. Reasonable wiring.

When wiring, all traces should be far away from the border of the PCB board (about 2mm), so as to avoid wire breakage or hidden dangers when the PCB board is made. The power cord should be as wide as possible to reduce the loop resistance. At the same time, make the direction of the power cord and ground wire consistent with the direction of data transmission to improve the anti-interference ability. The signal lines should be as short as possible and the number of vias should be reduced as much as possible; the wiring between the components should be as short as possible to reduce the distribution parameters and mutual electromagnetic interference; the incompatible signal lines should be kept away from each other as much as possible, And try to avoid parallel routing, and the signal lines on the front and back sides should be perpendicular to each other: when routing, the corners should be 135 degrees, avoid turning right angles.

In the above design, the PCB board uses a four-layer board. In order to reduce the influence of the radio frequency signal on the VXI bus control signal, the two signal lines are placed in the middle two layers respectively, and the radio frequency signal line is shielded with a grounding via tape.

2.3 Power cord and ground wire

The wiring in the PCB design of the radio frequency circuit needs to be particularly emphasized is the correct wiring of the power line and the ground line. The reasonable choice of power supply and ground wire is an important guarantee for the reliable operation of the instrument. Quite a lot of interference sources on the PCB board of the radio frequency circuit are generated by the power supply and the ground wire, and the noise interference caused by the ground wire is the largest. According to the size of the PCB board current, the power line and ground line should be designed as thick and short as possible to reduce loop resistance. At the same time, make the direction of the power line and the ground line consistent with the direction of data transmission, which helps to enhance the anti-noise ability. When conditions permit, try to use multi-layer boards, four-layer boards are 20dB lower than double-sided boards, and six-layer boards are 10dB lower than four-layer boards.

In the four-layer PCB board designed in this article, both the top and bottom layers are designed as grounding layers. In this way, no matter which layer of the middle layer is the power layer, the physical relationship between the power layer and the ground layer is close to each other, forming a large decoupling capacitor, reducing the interference caused by the ground wire.

A large area of copper is used for the ground layer. Large-area copper paving mainly has the following functions:

(1) EMC. For large-area ground or power supply copper, it will play a shielding role.

(2) PCB process requirements. Generally, in order to ensure the effect of electroplating or the lamination is not deformed, copper is laid on the PCB layers with less wiring.

(3) The signal integrity is required to provide a complete return path for high-frequency digital signals and reduce the wiring of the DC network.

(4) Heat dissipation, copper plating is required for installation of special devices, etc.

3 Conclusion

The VXI bus system is a modular instrument bus system that is completely open in the world and is suitable for multiple manufacturers. It is the latest instrument bus system in the world. The above mainly introduces the development of radio frequency switch module based on VXI bus. Introduced the design of the bus interface and the design of the PCB board of the functional circuit part of the radio frequency switch module. The radio frequency switch is controlled by the VXI bus, which increases the flexibility of the switch operation and is convenient to use.