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The design of the PCB board for the functional circuit of the radio frequency switch module is crucial in the context of advancing modern wireless communication systems, including mobile communication, radar, satellite communication, and other related technologies. These systems impose stringent requirements on switching speed, power capacity, and transceiver switch integration. Therefore, bus technology has been researched and developed to meet these demands, especially for military-grade bus modules, which hold significant importance.
We employ the concept of virtual instrumentation to implement hardware circuits in software. The radio frequency switch detailed below can be directly controlled by a computer and seamlessly integrated into the bus test system. The application of computer and microelectronics technology in today’s testing field shows promising avenues for development.
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 extensive, making it the preferred bus for integrating high-performance test systems. The VXI bus is a completely open modular instrument backplane bus specification suitable for various instrument manufacturers.
VXI bus devices are mainly categorized into register-based, message-based, and memory-based types. Register-based devices currently dominate applications (about 70%). The VXIbus register base interface circuit comprises four key components: bus buffer drive, addressing and decoding circuit, data transmission response state machine, and configuration and operation register group. Except for the bus buffer driver, which utilizes the 74ALS245 chip, the remaining components are implemented using FPGA technology. Specifically, the design incorporates a FLEX10K chip EPF10K10QC208-3 and an EPROM core EPC1441P8, with design and implementation facilitated by MAX+PLUS2 software.
1.1 Bus buffer driver
This section manages data, address, and control line buffering in the VXI backplane bus, ensuring compliance with VXI specification signals. For A16/D16 devices, the backplane data bus D00～D15 buffering is crucial. As per VXI bus specifications, this task employs two 74LS245 chips strobed by DBEN* generated by the data transmission response state machine.
1.2 Addressing and decoding circuit
The addressing lines encompass A01 to A31, along with data strobe lines DS0* and DS1*, and long word line LWORD*. Control lines include the address strobe line AS* and the read/write signal line WRITE*. This circuit is designed using MAX+PLUS2 schematic design, leveraging components such as two 74688 and one 74138. This module decodes A15～A01 address lines and AM5～AM0 address modification lines. Upon device addressing, it compares received address information with the logical address LA7～LA0 set by the hardware address switch. When addressing criteria are met (e.g., A15 and A14 are both 1), and logic values on A13～A06 match module logic address, device addressing and strobing (CADDR* assertion) occur. Subsequently, lower decoding control selects the module’s 16-bit address space register via A01～A05 address decoding.
1.3 Data transmission response state machine
The data transmission bus serves as a high-speed asynchronous parallel data exchange backbone within the VMEbus system. It encompasses addressing, data, and control lines. Designed using MAX+PLUS2 textual input method, this module employs AHDL language for state machine implementation, crucial for complex DTACK* timing. It configures VXI backplane bus control signals, providing essential timing and control signals for standard data transmission cycles (including generating data transmission enable signal DBEN* and DTACK* response signal necessary for data transmission completion). During data transmission, the system controller first addresses the module and sets valid address strobe lines AS*, data strobe lines DS0*, DS1*, and write control signal lines. Upon successful address and control validation detection, DTACK* drives low, confirming data presence on the bus (read cycle) or successful data reception (write cycle).
1.4 Configuration Register
Each VXI bus device integrates a set of configuration registers, enabling the main controller to retrieve fundamental device details such as type, model, manufacturer, address space (A16, A24, A32), and required memory space. These registers encompass identification, device type, status, and control registers. The design adopts MAX+PLUS2 schematic design, utilizing 74541 chip and associated functional modules. ID, DT, and ST registers are read-only, while control registers are write-only. This design primarily uses the VXI bus to control a batch of switches, enabling switch state control via channel register data writes and status queries via channel register reads.
2. The design of the module function circuit board
Each VXI bus device incorporates a set of configuration registers, enabling the main controller to retrieve fundamental device details such as type, model, manufacturer, address space (A16, A24, A32), and required memory space. The radio frequency circuit board’s frequency range spans 10kHz to 300GHz. As frequency increases, RF circuits exhibit distinct characteristics from low-frequency and DC circuits. Therefore, special attention must be paid to RF signal effects when designing the RF circuit board. The VXI bus controls the RF switch circuit to mitigate interference, utilizing a flat cable connection between the bus interface circuit and RF switch function circuit.
2.1 Component Layout
Electromagnetic compatibility (EMC) ensures electronic system functionality in specified electromagnetic environments. RF circuit PCB design requires minimal electromagnetic radiation and adequate anti-interference capabilities. Component layout directly impacts circuit interference and performance. Components should align directionally where possible, optimizing PCB entry to minimize poor soldering. Components must maintain at least 0.5mm spacing for soldering integrity. Component spacing should maximize PCB board space utilization. Relay placement near signal input/output minimizes RF signal path length, facilitating efficient subsequent wiring.
2.2 Wiring
Post-component layout, wiring is essential. Low-density designs are preferable when assembly density permits, benefiting signal impedance matching. RF circuits necessitate careful signal line direction, width, and spacing considerations to avoid cross interference. System power supply introduces noise, necessitating comprehensive RF circuit PCB design considerations for effective wiring. Traces should maintain distance from PCB edges (≈2mm) to prevent production disconnects. Power lines should be wide to minimize loop resistance, aligned with data transmission direction to enhance interference resistance. Signal lines should minimize length and vias, segregating incompatible signals to avoid parallel wiring; front/back signal lines should intersect at perpendicular angles. This PCB utilizes a four-layer design, isolating RF signal lines in middle layers and shielding via tape for RF signal lines.
2.3 Power and Grounding
RF circuit PCB design emphasizes correct power and ground wiring. Optimal power and ground line choice ensures instrument reliability. PCB current influences noise, necessitating short, thick power lines and consistent alignment with data transmission direction. Multilayer boards reduce interference, with this PCB’s four-layer design positioning top and bottom layers as ground planes. Extensive copper grounding enhances EMC shielding, aids PCB process integrity, ensures signal integrity, and facilitates heat dissipation.
3. Conclusion
The VXI bus system is a globally open modular instrument bus system, accommodating diverse vendors and serving as the current world standard for instrument buses. This article primarily details the VXI bus-based radio frequency switch module development, including bus interface and PCB functional circuit board design. VXI bus control enhances switch operation flexibility, facilitating user-friendly operation.
