单片机发发光器设计中英文对照.doc

Microcontroller-Based Optical Transceiver Design

1 Introduction
Under the impetus of the “Three Net Combined”, Fiber access programme such as FTTH (fiber to the home) is widely used. Under 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 machining 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 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 control 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. 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 8-bit microcontroller to realize the control circuit. This paper studies the performance of the PHY1076 which controled by the selected ATMEL company's ATMEGA88 microcontroller, designed the optical transceiver samples, and conducted the performance test, with the ultimate success of the design of the 1.25G optical transceiver.

2 Discussion About The Design And Working Principle Of Optical Transceiver
In 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 package 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 ATMEL's AVR ATMEGA88 microcontroller 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 Figure 1:
Figure 1 the internal structure of fiber optic transceivers map
1) 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 modulation 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 pulse 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 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 registers 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 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 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 programming 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 control 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 Parameters
In optical fiber communication system, the average optical power of emitted light pulse and 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 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 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 obtain 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 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 laser diode and a photoelectric sensor diode. Laser light power is proportional to and the current, and the laser's cathode connected with the PHY1076’s Laser_bias pin. PHY1076’s data in internal power set register is input directly to the DAC, then DAC produces an analog voltage output to control output 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 laser’s output optical power. The reverse leakage current of photoelectric sensor diode is proportional to the laser’s 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 role 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 laser’s light-emitting efficient. At this moment we need to adjust the value of the power setting 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 APC
2) The Realize of Extinction Ratio Control:
PHY1076 internal 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 control 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 laser’s modulation current is set by a special register, then output control voltage 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 cann’t introduce the feedback 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 control
3) ATMEGA88 Control 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 PHY1076’s internal register to control. While the PHY1076 provides I2C interface for accessing.We use ATMEGA88 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 interface. MCU’s 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. ATMEGE88 connected with the PHY1076 as shown in Figure 4.
Figure 4 ATMEGA88 connection diagram with the PHY1076
Laser’s 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 the threshold current will increase. In order to achieve the stability of the output optical power and extinction ratio, we must adjust the laser’s 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. 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 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 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 laser’s bias current and modulation current, which adjust the output optical power and extinction ratio.
Values of numerical data table are obtained from test. During the process of transceiver debugging, test the the sample’s each output of eye pattern under different temperature and modify the corresponding temperature data, so as to make the output optical eye pattern, optical 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 the 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 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 designed 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 temperature environment. The results is shown in Table 1.
Table 1 the parameters of optical transceiver test results
Parameters
Units
Design requirements
Test Results
-40ºC
25ºC
80ºC
Average transmitted
dBm
-9～-3
-6.54
-6.10
-6.01
Extinction ratio
dB
>9
12.77
10.95
9.7
Emission eye socket
802.3z-requirements
-
-
-
Receiver Sensitivity
dBm
<-23
-28.2
-29.6
-29.4

As it can be seen from the table. The laser’s 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 all good. As the temperature goes high ,it need to provide greater modulation current. So the signal’s 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 correctness of the design.

5 Summary
After discussing the program, software design ,hardware design and sample’s debuging and testing. Ultimately,we successfully realized the design of the single-chip design 1.2