科技论文MicrocontrollerBasedOpticalTransceiverDesign

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1、科技论文题 目 Microcontroller-Based Optical Transceiver Design 系 别 尚德光伏学院 专 业 微电子技术 班 级 微电0901 学生姓名 学 号 1指导教师 2012年 4 月Microcontroller-Based Optical Transceiver Design1 IntroductionUnder the impetus of the “Three Net Combined”, Fiber access programme such as FTTH (fiber to the home) is widely used. Under

2、the voice of the light for copper, the optical network is developing rapidly. Optical transceiver plays a role of electro - optical, electro - optical conversion in optical communications, and is an essential device for optical communication. As it relates to high-speed circuit design, precision mac

3、hining and optical design, the cost of optical transceivers occupies an important part in fiber optical communication systems and while the high prices of optical transceivers become the bottleneck of restricting fiber access promotion. Further reduce the cost of optical transceivers will benefit to

4、 promote the application of optical access and speed up the pace of light for copper. Optical transceiver mainly consists of circuits, optical transmitter components and optical receiver components. And the Part of the circuit which also includes laser driver, optical receiver signal amplifier and c

5、ontrol section. In current ,The circuit part of optical transceivers on the market is using three specific chip. The company has always been in the study of laser driver and receiving signal amplifier circuit integration in a device, with the controller using the common embedded processor solutions.

6、 Due to only use a specific chip and a common chip, we can significantly low the cost of the circuit part. The PHY1076 chip developed by PHYWORKS company is such a chip. It is intended mainly for 1.25Gbps to 2.5Gbps optical transceiver with a simple external circuit, and it requires only an ordinary

7、 8-bit microcontroller to realize the control circuit. This paper studies the performance of the PHY1076 which controled by the selected ATMEL companys ATMEGA88 microcontroller, designed the optical transceiver samples, and conducted the performance test, with the ultimate success of the design of t

8、he 1.25G optical transceiver.2 Discussion About The Design And Working Principle Of Optical TransceiverIn the development process of optical transceiver, there are many different outline package. SFP (Miniaturization hot-swappable optical transceiver module) is currently one of the most advanced pac

9、kage in 5Gbps rate, with a small, hot-swappable, low power consumption, high system integration and the ability to digitally Diagnostics and so on.This design uses laser driver circuit and optical receiver amplifier circuit ntegrated PHY1076 as a special chip , using ATMELs AVR ATMEGA88 microcontrol

10、ler to control and implement DDM functionality, coupled with the corresponding TOSA (transmitter optical components), ROSA (receiver optical components) and structural parts to designe a SFP optical transceiver which can work in 1.25Gbps 10km transmission distance. System block diagram shown in Figu

11、re 1:Figure 1 the internal structure of fiber optic transceivers map1) Transmitter works: Serial data signal from the TX + / - side of the system input to the PHY1706 laser drive section in the form of differential signal terminal. After amplificationd, drive circuit converted to differential modula

12、tion current signal loading to the TOSA (transmitter optical components) to control the laser TOSA to emit light pulses, and couple into the fiber to sent to the remote.2) Receive part work principle : Optical pulse signal inputs to the ROSA (receiver optical components), and ROSA converts optical p

13、ulse signal into the differential voltage signal output to the PHY1076 the limiting amplifier section. After limiting amplified the signals ,it outputs differential voltage serial digital signals from the RX + / - side of PHY1076.3) Control and DDM parts: PHY1076 is a mixed analog-digital chip, its

14、internal includes multiple analog-digital (A / D) and digital-analog converter (D / A) devices. Parameters on the transmit and receive paths are converted into digital stored in the State register for monitoring through ADC, and converts register value into analog for control by a DAC. These registe

15、rs can be read and set by an external controlle. DDM (Digital Diagnostic Monitor) means that the fiber-optical transceivers is capable of monitoring parameters such as ransmitting power (Tx_power), received power (Rx_power), laser bias current (Ibias), operating voltage (Vcc) and the module internal

16、 temperature (Temperature ) in real-time and set alarm flag when the parameters exceed the setted-value. The ADC integrated within PHY1076 can monitor the transmit power, receive power and bias current. However the operating voltage and temperature sensing is required for another ADC conversion. The

17、 realization of all these alarms relays on external controller.ATMEGA88 AVR microcontroller from ATMEL Corporation is an 8-bit microcontroller, with FLASH, RAM, EEPROM, internal clock and the ADC integrated inside. Without any external circuit to constitute a system ,and supporting on-line programmi

18、ng downloads and single-step debugging. System design and software debugging is very convenient. Integrated hardware I2C module can directly provide the external I2C interface in line with SFP-MSA specifications. And this is a single chip widely used in the field of home appliances and industrial co

19、ntrol with large use, stable and reliable performance, low prices. The design uses the chip PHY1076 to control the operating parameters and implement DDM functions.3 Control And Implementation Of Key ParametersIn optical fiber communication system, the average optical power of emitted light pulse an

20、d extinction ratio are two very important parameters. According to the different transmission distance, we need to set a different value. On one specific optical transceiver is hoped that the light emission power and the extinction ratio can be maintained within a certain range. To maintain a stable

21、 light power you need to use APC (automatic power control) circuit. At the same time, because of temperature characteristics and aging characteristics of the laser, the luminous efficiency will change, it is also the need for temperature compensation. The automatic control of extinction rat requires

22、 adjustment the size of modulation current according to temperature changes in real time. Optical transceiver in the early, mainly uses a dedicated analog devices, so ia is difficultachieve the power of APC, temperature compensation and automatic extinction ratio control ,or it is difficult to obtai

23、n satisfactory results. PHY1076 is a modular mixing device, using the analog part of its high-speed channel design, while its power control and modulation current is carried out by the register. So long as the external controller can monitor the temperature, you can adjust the register values based

24、on the temperature in order to automatically change the output power and modulation current, with the power and extinction ratio maintained at a certain range of purposes.1) Control And Achieve Of The Average Optical Power: PHY1076 internal APC circuit is shown in Figure 2. TOSA is integrated with a

25、 laser diode and a photoelectric sensor diode. Laser light power is proportional to and the current, and the lasers cathode connected with the PHY1076s Laser_bias pin. PHY1076s data in internal power set register is input directly to the DAC, then DAC produces an analog voltage output to control out

26、put current in voltage-controlled current source.And the output current of current source supply the laser with DC bias current through the inductive coupling. Therefore, to modify the value of the power setting register is to modify the lasers output optical power. The reverse leakage current of ph

27、otoelectric sensor diode is proportional to the lasers emission power. The current accesses PHY1076 from the MPD pin, after amplified and converted into a voltage signal,it is introduced into the control side of voltage-controlled current source as a negative feedback signal in order to play the rol

28、e of automatic power control. But the power range controled by APC is limited. When the temperature becomes larger, APC will not provide enough current to maintain power to be stable due to the reduce of lasers light-emitting efficient. At this moment we need to adjust the value of the power setting

29、 register to gain greater bias current to maintain power stable. This design is setting the value of register based on temperature by an external microcontroller to achieve temperature compensation purposes.Figure 2 Working Principle of APC2) The Realize of Extinction Ratio Control: PHY1076 internal

30、 modulation current control circuit is shown in Figure 3. Extinction ratio is defined as the ratio of optical power when sending data to 1 and optical power when sending data 0 in optical fiber communication system. Its value will affect the bit error rate of communication systems,hence we need to c

31、ontrol within a certain range. In AC coupled circuits of modulated lasers, the average transmit power is affected by the DC bias current with the extinction ratio affected by modulation current. Within the PHY1076, when lasers modulation current is set by a special register, then output control volt

32、age after digital-to-analog conversion so as to control the size of the output modulation current. Therefore, set the value of the register properly can obtain the ideal extinction ratio. Since there is no way to detect the size of the extinction ratio during working,we cannt introduce the feedback

33、circuit to achieve the automatic extinction ratio control. It is more feasible to look for the statistical law in extinction ratio and temperature change, and conduct temperature compensation by an external controller accordance with law. Figure 3 Schematic modulation current control3) ATMEGA88 Cont

34、rol Algorithm Analysis And DDM Implementation: From the above analysis we can see that the key parameters of optical transceivers like the average power and extinction ratio relay on settings PHY1076s internal register to control. While the PHY1076 provides I2C interface for accessing.We use ATMEGA8

35、8 microcontroller to control in this design. ATMEGA88 microcontroller hardware inside has a standard I2C interface to provide system equipment with external I2C required as SFP-MSA (SFP Multi Source Agreement). In order to communicate with the PHY1076, the design using software to simulate a I2C int

36、erface. MCUs main tasks include providing temperature compensation function for the power control, providing extinction ratio automatic control function,setting parameters of enlarge and receiver part in PHY1076,initializing the PHY1076, providing DDM functionality and record product information. AT

37、MEGE88 connected with the PHY1076 as shown in Figure 4.Figure 4 ATMEGA88 connection diagram with the PHY1076 Lasers luminous efficiency and threshold current is inversely proportional to the ambient temperature, that is, when the ambient temperature rises, the luminous efficiency will reduce with th

38、e threshold current will increase. In order to achieve the stability of the output optical power and extinction ratio, we must adjust the lasers bias current and modulation current according to temperature changes. The look-up table method is used to set the bias and modulation current in the design

39、. Specifically, we establish two data tables-the modulation current power setting table and power setting table, each value corresponds to a temperature of the power setting register. As is shown in figure 4. ,modulation current set list is 80 bytes with one byte per two degrees, while power setting

40、 table takes 40 bytes with one byte for every 4 degreess. Both temperature range is -40 to 120 centigrade, which meets the requirements of industrial temperature . When an external temperature sensor added to the microcontroller, microcontroller converts voltage sent by the temperature sensor into a

41、 temperature value through ADC, then look up the temperature look-up table to find the corresponding data. Sent the data to the power set register and modulation current setting register in PHY1076 accordingly, adjust the lasers bias current and modulation current, which adjust the output optical po

42、wer and extinction ratio. Values of numerical data table are obtained from test. During the process of transceiver debugging, test the the samples each output of eye pattern under different temperature and modify the corresponding temperature data, so as to make the output optical eye pattern, optic

43、al power and extinction ratio of optical transceiver to meet the requirements. Finally, save these data to a temperature lookup table and the internal EEPROM in ATMEGA88. In the real application environment, when repowers, the transceiver load the EEPROM data into RAM area, and then you can stable t

44、he average output optical power and extinction ratio in the whole temperature range.4 Analysis Of Results And Test Based on the above discussion of the design program, we select the PHY1076 dedicated chip and ATMEGA88 microcontroller, plus appropriate external circuit design circuit boards with the

45、TOSA, ROSA welded together in custom enclosures to achieve a 1.25Gbps SFP optical transceivers. Debug Internal registers in PHY1076 in order to make parameters of optical transceivers meet requirements about 10km Gigabit Ethernet protocol optical interface in 802.3z agreement. Meantime,using the des

46、igned debug software on the host computer to debug temperature lookup table of the ATMEGA88 to determine the specific values for each temperature. Which completed the design of optical transceivers. And then test all parameters of optical transceivers at low temperature, room temperature and high te

47、mperature environment. The results is shown in Table 1.Table 1 the parameters of optical transceiver test resultsParametersUnitsDesign requirementsTest Results-40C25C80CAverage transmitteddBm-9-3-6.54-6.10-6.01Extinction ratiodB912.7710.959.7Emission eye socket802.3z-requirements-Receiver Sensitivit

48、ydBm-23-28.2-29.6-29.4As it can be seen from the table. The lasers output optical power and extinction ratio are within the required parameter range with small changes. When tested eye pattern under each temperature,we found the performance at low temperatures, room temperature, low temperature are

49、all good. As the temperature goes high ,it need to provide greater modulation current. So the signals undershoot is obvious as a slight eye line appearing in the 0 signals in the eye pattern. But the overall margin on the template test are greater than 40%. Thus validated the feasibility and correct

50、ness of the design.5 SummaryAfter discussing the program, software design ,hardware design and samples debuging and testing. Ultimately,we successfully realized the design of the single-chip design 1.25GSFP SFP optical transceivers. Features of this program is to integrated the laser -driven with re

51、ceiving and amplifing section, to use a common single-chip microcontroller to control programs. Both are able to reduce product costs and increase productivity theoretically. Since this program is a new program, technology maturity is to be improved, system compatibility and market potential problem

52、s are yet to be tested. In addition to the amount, the cost advantage is also reflected no great advantage. But you can still believe that with the acceleration of network speed and the cost pressures when optical access, new technologies will become more perfect and market share will grow.At the ap

53、pointed time ,cost advantage will be reflected.基于单片机控制的光收发器设计1 引 言在“三网合一”的推动下，光纤到户等光纤接入方案的应用日益广泛。在光进铜退的呼声下，光网络迅速发展。光收发器在光通信中起到光电、电光转换的作用，是光通信必不可少的器件。由于涉及到高速电路设计、精密机械加工和光学设计，光收发器的成本占据了光纤通信系统和的重要部分，而较高的光收发器价格成了制约光纤接入推广的瓶颈。进一步降低光收发器的成本将有利于光接入的应用推广，加快光进铜退的步伐。光收发器主要由电路部分、光发送组件和光接收组件组成。其中电路部分又包括激光驱动、光接收信号

54、放大和控制部分。目前市场上的光收发器的电路部分使用的是三个专用芯片。一直有公司在研究把激光驱动和接收信号放大电路集成在一个器件上，控制器使用普通的嵌入式处理器的方案。由于只使用一个专用芯片和一个通用芯片，这样就可以大幅降低电路部分的成本。PHYWORKS公司研制的PHY1076芯片就是一款这样的芯片。它主要针对1.25Gbps 到2.5Gbps的光收发器，具有外围电路简单，控制电路只需要普通的8位单片机就可以实现的特点。本文主要研究了PHY1076 的性能，选择了ATMEL 公司的ATMEGA88 单片机进行控制，设计出光收发器样品，并进行了性能测试，最终成功设计了1.25G 光收发器。2 光

55、收发器设计方案及工作原理讨论光收发器在发展的过程中，有许多种不同的外形封装。SFP（小型化可热插拔光收发一体模块）是目前在5Gbps以下速率中最先进的一种封装形式，具有小型化、可热插拔、功耗小、系统可集成度高以及能够进行数字诊断功能等特点。本设计中使用激光驱动电路和光接收放大电路集成的PHY1076 作为专用芯片，使用ATMEL 的AVR 单片机ATMEGA88 进行控制和实现DDM 功能，加上相应的TOSA（光发射组件）,ROSA（光接收组件）和结构件，设计了一款工作在1.25Gbps 传输距离为10km 的SFP 光收发器。系统方框图如图1 所示：图 1 光纤收发器的内部结构图1) 发射部

56、分工作原理：系统的串行数据信号从TX+/-端以差分信号形式输入到PHY1706 的激光器驱动部分。驱动电路进行放大处理后，转换成差分调制电流信号加载到TOSA（光发射组件）上，控制TOSA 中的激光器发出光脉冲，耦合入光纤发送到远端。2) 接收部分工作原理：光脉冲信号输入到ROSA（光接收组件），ROSA 将光脉冲信号转换成差分电压信号输出到PHY1076 的限幅放大部分。该信号经过限幅放大处理后，从PHY1076 的RX+/-端输出差分电压串行数字信号。3) 控制及DDM 部分：PHY1076 是一款模拟数字混合芯片，其内部包括多个模数(A/D)、数模转换(D/A)器。发射和接收通路上的参数

57、都是通过ADC 转换成数字量存入状态寄存器进行监视，通过DAC 把设置寄存器的值转换成模拟量来进行控制的。这些寄存器都可以由外部控制器进行读取和设置。DDM(Digital Diagnostic Monitor)，数字诊断和监控是指的光纤收发器能够对发射功率（Tx_power），接收功率（Rx_power），激光器偏置电流（Ibias），工作电压（Vcc），模块内部温度（Temperature）这些参数进行实时监视，并能够在各项参数超过设定值时设置报警标志位的功能。PHY1076 内部集成的ADC 能够对发射功率、接收功率和偏置电流进行监测。工作电压和温度传感则需要另外ADC 进行转换。而所有

58、这些报警的实现则需要外部控制器来实现。ATMEL 公司的AVR 单片机ATMEGA88 是一款8 位单片机，内部集成FLASH、RAM、EEPROM、内部时钟和ADC。无需任何外围电路即可构成系统，支持在线编程下载和单步调试。系统设计和软件调试都很方便。集成硬件I2C模块，可直接对外提供符合SFP-MSA 规范要求的外部I2C接口。而且此单片机是一款在家电和工业控制领域使用广泛的芯片，用量大，性能稳定可靠，价格低。本设计选用此芯片控制PHY1076 的工作参数和实现DDM 功能。3 关键参数控制和实现在光纤通信系统中，发射光脉冲的平均光功率和消光比是两个非常重要的参数。根据传输距离不同，需要设

59、定不同的值。对具体某一个光收发器则希望其发光功率和消光比能够长期维持在一定范围内。要维持稳定的光功率则需要使用APC(自动功率控制)电路。又由于激光器的温度特性和老化特性，其发光效率会变化，所以又需要进行温度补偿。消光比的自动控制则需要根据温度变化而实时的调整调制电流的大小。早期的光收发器中，大都使用专用模拟器件，所以要实现功率APC、温度补偿和消光比自动控制都非常困难，或者很难得到满意的效果。PHY1076 是一个模数混合器件，其高速通道部分使用模拟设计，而其功率控制、调制电流则是使用寄存器进行的。从而只要外部控制器能够监测温度，就能根据温度调整寄存器的值，达到自动更改输出功率和调制电流，使

60、功率和消光比维持在一定得范围内的目的。1) 平均光功率的控制实现：PHY1076 内部APC 电路如图2 所示。TOSA 内部集成了一个激光发射二极管和一个光电感应二极管。激光器的发光功率与电流成正比，激光器的阴极接到PHY1076 的Laser_bias 引脚。PHY1076 内部功率设置寄存器的数据直接输入到DAC，DAC 产生一个模拟电压输出控制压控电流源的输出电流，此电流源输出电流经过电感耦合后给激光器的提供直流偏置电流。因此修改功率设置寄存器的值就可以直接修改激光器的输出光功率。光电感应二极管的反向漏电流与激光器的发射功率成正比。该电流从MPD 引脚接入PHY1076 内部，经过放大

61、和转换成电压信号后，作为负反馈信号引入到压控电流源的控制端，起到自动功率控制的作用。但是APC 能够控制的功率变化范围是有限的。当温度变大时，由于激光器的发光效率降低，APC 将不能提供足够大的电流来保持功率稳定。此时就需要调节功率设置寄存器的值来获得更大的偏置电流以保持功率稳定。本设计中是通过外接单片机来根据温度进行寄存器设置，达到温度补偿的目的。图2 APC 工作原理图2) 消光比控制的实现：PHY1076 内部调制电流控制电路如图3 所示。消光比在光纤通信系统中定义为发送数据为1 时与发送0 时的光功率的比值。其值的大小会影响通信系统的误码率,因此需要控制在一定范围内。在使用交流耦合的调

62、制激光器的电路中，平均发射功率受直流偏置电流影响，消光比的大小受调制电流的影响。在PHY1076 的内部，激光器的调制电流由专门的寄存器进行设置后，经过模数转换器（DAC）输出控制电压，进而控制输出调制电流的大小。因此适当设置该寄存器的值就可以得到理想的消光比。由于没有办法检测工作中消光比的大小，因此无法引入反馈电路实现消光比自动控制。比较可行的办法是寻找消光比与温度变化的统计规律，然后通过外部控制器依规律进行温度补偿。图3 调制电流控制原理图3) ATMEGA88 的控制算法分析和DDM 实现：从上面的分析可以看出，光收发器的关键参数平均发射功率和消光比都是通过对PHY1076 的内部寄存器

63、的设置来控制的,而PHY1076 提供I2C 接口进行访问。本设计中，使用ATMEGA88 单片机进行控制。ATMEGA88 单片机内部有一个标准的硬件I2C 接口，可以用来为系统设备提供SFP-MSA（SFP 多源协议）要求的对外I2C。为了与PHY1076进行通信，本设计中用软件模拟了一个I2C 接口。单片机的主要工作内容包括：为功率控制提供温度补偿功能；为消光比提供自动控制功能；设置PHY1076 的接收放大部分的一些工作参数；对PHY1076 进行初始化；提供DDM 功能和记录产品信息。ATMEGE88 与PHY1076 的连接如图4 所示。图4 ATMEGA88 与PHY1076 连

64、接图激光器的发光效率和阈值电流与环境温度成反比，即当环境温度升高时，激光器的发光效率会降低，阈值电流会升高。为了达到输出光功率和消光比的稳定性，就要根据温度变化来响应调节激光器的偏置电流和调制电流。本设计中采用查表法来设置偏置和调制电流。具体就是建立两个数据表，功率设置表和调制电流设置表，每个值对应一段温度下的功率设置寄存器的值。如图4 所示,调制电流设置表为80 个字节，每两摄氏度占一个字节；功率设置表占用40 个字节，每4 摄氏度占用一个字节；温度范围都是-40120，满足工业温度的要求。给单片机外接一个温度传感器,单片机通过模数转换器把温度传感器送来的电压转换成温度值，然后根据温度查表，

65、找到对应的数据，把数据分别送往PHY1076 的功率设置寄存器和调制电流设置寄存器，调整激光器的偏置电流和调制电流，由此调整输出光功率和消光比。数据表的数值确定则使用测试的方法得出。在收发器的调试过程中，对样品在每个温度下的输出眼图进行测试，并修改对应温度下的数据，使得光收发器的输出光眼图、光功率和消光比满足要求。最后把这些数据保存到温度查找表中，并同时保存到ATMEGA88 内部的EEPROM 中。收发器在实际的应用环境下，重新加电后，就先从EEPROM 中把数据加载到RAM 区，然后就可以在全温度范围内稳定平均输出光功率和消光比。4 设计结果及测试分析本设计根据以上讨论的方案，选择PHY1076 专用芯片和ATMEGA88 单片机，外加适当的外围电路设计电路板，把TOSA、ROSA 焊接到一起装入定制外壳，实现了一款1.25Gbps 的SFP 光收发器。调试PHY1076