In order to reduce the proportion of data transmission time in the overall system processing time index, the data transmission rate is required to be not less than the E1 (2.048 Mb/s) rate, and the communication link is required to be safe and reliable. Through the analysis of various data communication technologies, we finally chose the fiber-optic keyway and achieved satisfactory results.
1 copper media data link analysis
Since high data transmission rate and long communication distance are required at the same time, it is difficult to meet the index requirements without coding and modulation in the copper medium link, but the high-speed modem is not only expensive, but also the time-consuming handshake process of both parties requires random real-time. The field where the data is transmitted will obviously not be applicable.
Most importantly, traditional copper dielectric links generate both electromagnetic interference signals and are susceptible to electromagnetic interference, and are less susceptible to EMC (electromagnetic compatibility) and EMI (electromagnetic interference) standards.
2 fiber link analysis
In people's traditional impression, it is uneconomical for fiber to be used for short-distance communication, but it has the incomparable advantages of copper media bonding: 1 fiber-optic link neither emits electromagnetic waves nor is it affected, there is no interference between fibers. The bit error rate is greatly reduced. Designers do not have to consider the environmental noise that may be coupled in; 2 fiber provides electrical isolation between the two sides of the communication link, eliminating problems caused by different ground potentials between long-distance devices. At the same time, the designer no longer has to worry about impedance matching; 3 fiber optic transmitter can use digital modulation drive circuit. The digital baseband signal can be directly driven by the fiber optic transmitter as long as it is simply line coded.
Now, the price of fiber optic devices has dropped sharply compared with the past. In the future, as the price of wire ropes rises and the price of fiber optic devices drops further, people will not consider the price factor first when choosing fiber. Therefore, we comprehensively consider the rate and distance indicators, select the appropriate type of fiber, transceiver and its drive circuit, can obtain high-performance price ratio fiber communication key, which is the focus of this paper.
3 fiber optic communication device selection
A basic fiber-optic communication system is very simple: an LED transmitter converts an electrical signal into an optical signal and couples it into a transmission fiber. The optical signal passes through the fiber to the optical receiver, which restores the received optical signal to its original state. Electrical signal output.
• Choice of fiber optic cable: In general, quartz glass fiber is used for long-distance communication links due to its low loss and high bandwidth. For example, Ethernet and FDDI standards specify multimode 62.5/125μm quartz glass fiber. These fine-core fibers require high-precision connectors to reduce coupling losses, and for industrial applications, low-cost cables and connectors are required. Therefore, 1 mm POF (Plymer Optical Fibers) and 200 μm HCS (Hard Clad Silica) fibers are the best choice, and they all belong to step-index multimode fibers.
The typical loss value of 1mm POF is 0.2dB/m at 650nm wavelength, while the 200μm HCS fiber has a typical loss value of only 8dB/km at 650mm wavelength and less at 820nm wavelength. The core of the HCS fiber is quartz glass. The cladding is a patented high-strength polymer that not only increases the strength of the fiber, but also protects against moisture and pollution. The outer sheath is 2.2mm of polyvinyl chloride. HCS fiber can work in a temperature range of 40 ° C ~ +85 ° C, the temperature range of the installation is 20 ° C ~ + 85 ° C, in terms of performance and price to meet the system requirements.
• Selection of working wavelength: The design of optical fiber communication system must consider the impact of fiber loss and dispersion on the system. Since loss and dispersion are related to the operating wavelength of the system, the choice of operating wavelength becomes a major problem in system design.
Taking into account the system's specifications and selected fibers, the choice of 820nm wavelength can reduce the HCS fiber loss to 6dB/km and minimize dispersion.
• Selection of light source: At 820m wavelength, LED is the best choice of light source. Compared with semiconductor laser, LED drive circuit is simple and low cost.
In summary, the optical cable uses 200μm HCS fiber, and the optical transceiver uses HP 820BR wavelength HFBR-0400 series. The HFBR-14X2/HFBR-24X2 in this series is capable of 5MBd at a distance of 1500m and operates over a temperature range of 40°C to +85°C. Available in ST, SMA, SC and FC port models. HFBR-14XZ uses 830nm wavelength AlGaAs type LED. HFBR-24XZ integrates an IC chip including PIN photodetector, DC amplifier and open collector output Schottky transistor. Its output can be directly connected with popular TTL and CMOS integrated circuits. Connected.
4 A practical fiber optic LED drive circuit
In a fiber optic duplex communication system, the function of the driving circuit is to convert electric power into optical power, and modulate the electrical signal to be transmitted to the output of the light source, which simultaneously supplies a bias current and a modulation current that varies with the digital signal.
Designing the LED driver circuit must first consider the current peak value of the LED driving, and if it exceeds the peak drive current, the optical signal will produce an overshoot. As a result, the undershoot of the electrical signal caused by the receiver, coupled with the influence of noise from the amplifier, may exceed the detection threshold of the comparator, causing bit errors. At the same time, the LED has a characteristic that it is more difficult to turn off than to light, and it will cause a tailing phenomenon, which is especially noticeable when a serial driving circuit is used. The parallel drive mode can provide a low-resistance channel for the carriers in the LED, thereby reducing pulse width distortion and slow tailing.
The resistor RSI in the drive circuit is used to adjust the output power of the fiber. Note that the peak drive current of the LED cannot be exceeded, otherwise the optical signal will overshoot. In addition, the SN75451 in the circuit features low impedance and high current rate to avoid long tailing.
Pulse Width Distortion (PWD) is a major factor limiting the fiber link rate due to unequal transmission delays between input and output. Note that PWD is always positive, so we can use the RC circuit to delay the lighting of the LED.
The fiber receiving circuit is an important part of the fiber-optic communication system, and its performance is a comprehensive reflection of the performance of the entire fiber-optic communication system. Its function is to convert the optical signal transmitted from the optical fiber into an electrical signal, and then pass through the amplification, equalization, and decision circuit to recover the original signal at the transmitting end.
On the optical fiber transmission line, the presence or absence of the common optical signal indicates the "1" code and the "0" code. In order to avoid the long connection "0" or the long connection "1" in the code stream, it is advantageous for clock extraction, and coding is required. Circuit. At the same time, the array signal processor sends parallel data, which requires a rate and conversion circuit.
5 PCB board design technology
The performance of fiber optic transceivers is determined in part by the layout and routing techniques of the PCB, so the following basic rules should be observed:
1 When designing a PCB board, it is recommended to use the ground plane to reduce the inductance of the common ground of the power supply. If it is possible to use both a ground plane and a power plane, this will reduce both the ground and the inductance on the power supply pins.
2 The split and openings on the ground and power plane should be minimal, which will reduce the additional inductance and improve the stability of the transmit and receive circuits.
3 The length of the connection between the driver circuit and the LED should be as short as possible to reduce the inductance of the wire.
A 410μF tantalum electrolytic capacitor and a 0.1μF monolithic ceramic capacitor should be placed close to the LED signal, which will reduce the radiated noise of the emitter and increase the light response time of the LED.
A 50.1μF (or 0.01μF) bypass capacitor is required between pins 2 and 7 of the receiver, and its distance from the receiver must not exceed 20mm.
6 Receiver is the most critical part of the fiber connection line. Excessive additional inductance and capacitance in the circuit will reduce the bandwidth and stability of the fiber receiver and reduce the sensitivity of the receiver. It is therefore recommended to use surface mount devices and do not use sockets.
After actual testing, it is confirmed that this circuit has fully met the practical requirements, and the performance index of the whole system is improved by shortening the communication time.
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