With the development of modern wireless communication system, mobile communication, radar, satellite communication and other communication systems have higher requirements on the switching speed, power capacity and integration of the transceiver switch, Therefore, it is of great significance to study the bus technology and develop the bus module to meet the special requirements of the military. We will use the idea of virtual instrument to realize the hardware circuit in the way of software The RF switch can be directly controlled by the computer, and can be easily integrated with the bus test system. It can maximize the application of computer and microelectronics technology in the field of testing, and has a broad development prospect.

Design and implementation of VXI bus interface circuit

VXIbus is an extension of VMEbus in the field of instruments. It is a modular automatic instrument system controlled by computer. It relies on effective standardization and modularization to realize serialization, generalization, 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, it is very convenient to set up and use the system, and has been widely used. It has gradually become the preferred bus for high-performance test system integration.

VXI bus is a fully open, modular instrument backplane bus specification suitable for all instrument manufacturers. VXI bus devices are mainly divided into register based devices, message based devices and memory based devices. At present, register based devices account for the largest proportion (about 70%) in applications. VXIbus register based interface circuit includes four parts: Bus Buffer driver, 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 implemented by 74als245 chip, the rest parts are implemented by FPGA. A FLex10K chip epf10k10qc208-3 and an EPROM chip epc1441p8 are used to design and implement the software MAX + PLUS II.

Analysis of design scheme of VXI bus interface circuit

1.1 bus buffer driver

This part completes the buffer receiving or driving of data line, address line and control line in VXI backplane bus to meet the requirements of VXI standard signal. For a16 / D16 devices, only the buffer drive of data bus D00 ~ D15 is realized. According to the requirements of VXI bus specification, this part is implemented by two 74ls245 chips and gated by dben * (generated by data transmission response state machine).

1.2 addressing and decoding circuit

The addressing lines include address lines a01-a31, data strobe lines DS0 * and DS1 *, and long word lines lword *. The control line includes address strobe line as * and read / write signal line write *.

The circuit is designed by MAX + PLUS II. Two pieces of 74688 and one piece of 74138 are used in the design.

The function module decodes address lines a15-a01 and address modification lines am5-am0. When the device is addressed, the address information on the address line and address modification line is received and compared with the logical address la7-la0 set by the hardware address switch on this module. If the logic value on am5-am0 is 29H or 2dh (due to a16 / D16 device), the address lines A15 and A14 are 1, and When the logic value on a13-a06 is equal to the logical address of the module, the device is addressable gated (caddr * is true). Then the results are sent to the next level of decoding control, through decoding the address a01-a05, the register of the module in 16 bit address space is selected.

1.3 data transmission response state machine

Data transmission bus is a group of high-speed asynchronous parallel data transmission bus, which is the main part of VMEbus system information exchange. The signal line of data transmission bus can be divided into address line, data line and control line.

The design of this part adopts the text input design mode of MAX + PLUS II. Because the timing of dtack * is complex, it is designed by AHDL and realized by state machine.

The function module configures the control signals in VXI backplane bus, and provides timing and control signals for standard data transmission cycle (generating data transmission enabling signal dben *, and response signal dtack * required by the bus to complete data transmission). In data transmission, the system controller first addresses the module, and sets the corresponding address strobe line as *, data strobe line DS0 *, DS1 * and write * signal line controlling the data transmission direction as the effective level. When the module detects the address matching and the control lines are valid, it drives the dtack * to low level, so as to confirm to the bus controller that the data has been placed on the data bus (read cycle) or received successfully (write cycle).

1.4 configuration register

Each VXIbus device has a set of “configuration registers”. The main controller of the system obtains some basic configuration information of VXIbus devices by reading the contents of these registers, such as device type, model, manufacturer, address space (a16, A24, A32) and required storage space.

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

This part of the circuit design uses MAX + PLUS II schematic design method, using 74541 chip, its function module.

The ID, DT and St registers are read-only registers, while the control registers are write only registers. In this design, VXI bus is mainly used to control the on-off of these switches. Therefore, as long as the data is written to the channel register, the on-off state of the relay switch can be controlled, and the data can be read from the channel register to query the relay status. According to the design requirements of the module, appropriate contents are written in the corresponding data bits, so that the RF switch of the function module can be effectively controlled.

Design of 2-module functional circuit board

Each VXIbus device has a set of “configuration registers”. The main controller of the system obtains some basic configuration information of VXIbus devices by reading the contents of these registers, such as device type, model, manufacturer, address space (a16, A24, A32) and required storage space.

The frequency range of RF circuit is about 10kHz to 300GHZ. With the increase of frequency, RF circuit shows some characteristics different from low frequency circuit and DC circuit. Therefore, in the design of RF circuit board, we need to pay special attention to the impact of RF signal on the board. The RF switch circuit is controlled by VXI bus. In order to reduce the interference in the design, the bus interface circuit and the RF switch function circuit are connected by flat wire. The following mainly introduces the design of PCB board of RF switch functional circuit.

2.1 layout of components

Electromagnetic compatibility (EMC) refers to the ability of electronic system to work normally according to design requirements in specified electromagnetic environment. For the design of RF circuit PCB, electromagnetic compatibility requires that each circuit module should not produce electromagnetic radiation as far as possible, and has certain 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. General layout principle: components should be arranged in the same direction as far as possible, and poor soldering can be reduced or even avoided by selecting the direction of PCB entering into tin melting system; at least 0.5mm spacing between components is required to meet the tin melting requirements of components; if the space of PCB board is allowed, the spacing of components should be as wide as possible. The reasonable layout of components is also a premise of reasonable wiring, so it should be considered comprehensively. In this design, the relay is used to convert the RF signal channel, so the relay should be close to the signal input end and output end as far as possible, so as to shorten the wiring length of the RF signal line as far as possible, so as to consider the reasonable wiring in the next step. In addition, the RF switch circuit is controlled by VXI bus, and the influence of RF signal on VXI bus control signal is also a problem that must be considered in the layout.

2.2 wiring

After the layout of components is basically completed, the wiring should be started. The basic principle of wiring is: under the permission of assembly density, low-density wiring design should be selected as far as possible, and the thickness of signal wiring should be consistent as far as possible, which is conducive to impedance matching.

For RF circuit, 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 it must be considered comprehensively and reasonably when designing RF circuit PCB.

When wiring, all wiring should be far away from the border of PCB board (about 2mm), so as to avoid wire breaking or hidden danger of wire breaking during PCB manufacturing. The power line should be as wide as possible to reduce the loop resistance. At the same time, the direction of power line and ground wire should be consistent with the direction of data transmission, so as to improve the anti-interference ability. The shorter the wiring between components is, the better, so as to reduce the distribution parameters and electromagnetic interference; The incompatible signal lines should be far away from each other as far as possible, and parallel wiring should be avoided as far as possible, while the signal lines on both sides should be perpendicular to each other; when wiring, it is appropriate to take 135 degree angle at the corner to avoid turning right angle.

In the above design, PCB board adopts four layers. In order to reduce the influence of RF signal on VXI bus control signal, two kinds of signal wiring are placed in the middle two layers respectively, and the RF signal line is shielded with grounding via.

2.3 power and ground wires

In the design of RF circuit PCB, the correct wiring of power line and ground wire should be emphasized. Reasonable selection of power supply and ground wire is an important guarantee for reliable operation of the instrument. A considerable number of interference sources on PCB board of RF circuit are generated by power supply and ground wire, and the noise interference caused by ground wire is the largest. According to the size of PCB 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 power line and ground wire consistent with the direction of data transmission, which helps to enhance the anti noise ability. If conditions permit, multi-layer boards should be used as much as possible. The noise of four-layer boards is 20 dB lower than that of double-sided boards, and that of six-layer boards is 10 dB lower than that of four-layer boards.

In the four layer PCB designed in this paper, the top layer and bottom layer are designed as ground layer. 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, which are close to each other, forms a large decoupling capacitance and reduces the interference caused by the ground wire.

The ground wire layer is covered with copper in large area. Large area copper coating has the following functions:

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

(2) PCB process requirements. Generally, in order to ensure the electroplating effect, or the laminate does not deform, copper is laid on the PCB with less wiring.

(3) Signal integrity requires a complete return path for high-frequency digital signals, and reduces the wiring of DC network.

(4) Heat dissipation, special device installation requirements copper, etc.

3 conclusion

VXI bus system is a kind of modular instrument bus system which is completely open in the world and suitable for many manufacturers. It is the latest instrument bus system in the world. This paper mainly introduces the development of RF switch module based on VXI bus. The design of bus interface and PCB board of RF switch module are introduced. The RF switch is controlled by VXI bus, which increases the flexibility of switch operation and is easy to use.

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