Design Report

Design Report

Gigabit Ethernet

Agilent/Intel OE

ECE 4006 C

Group3 Members: Karen Cano, Scott Henderson, and Di Qian.

February 9, 2002

Introduction

In the past 30 years, Ethernet has evolved from 10 Mbps to 10 Gbps. With the help of the IEEE 802.3 standard, the original protocol layers have remained virtually unchanged and progress increases even beyond 10 Gbps. For this project, the goal is to build a functional optoelectronic module. The first step towards this goal is to purchase specific SC connectorized VCSELs and PDs. These opto-components should meet the specifications of the MAX3287 (TX) and the MAX3266 (RX) boards, which they will have connections to. Two VCSELs from Honeywell were studied in comparison (HFE4380-521 & HFE4384-522). After many calculations, it was decided that the HFE4384-522 (with higher slope efficiency) provided more reliable performance. In addition, a PD from Lasermate (RSC-M85A306) was selected to interface with the trans-impendance amplifier on the RX board.

More specifically, the completion of the final project includes ensuring that the VCSEL and PD meet the specifications of the Maxim boards, purchasing these parts, soldering them onto separate boards, connecting these to the respective Maxim board, and finally testing them using an Intel/Agilient test-bed. If this is successful, the group will design and build its own test boards, connect these to the opto-componentst, and test these components again

Design Specifications

Before purchasing the components, the group conducted a detailed study of emitters and detectors. The most important design specifications of the VCSEL include operating frequency, wavelength, connector receptacle sleeve, threshold current (Ith), and slope efficiency. Gigabit Ethernet requires the frequency of the laser emitter to be higher than 1 GHz, therefore the VCSEL frequency has to be greater than 1 GHz. A wavelength of 850 nm was required for the multi-mode fiber. The electrical end of the VCSEL was to be SMA connectorized and SC connectorized at the optical end. SC was chosen above ST ans FC because it has the lowest energy loss, providing a better eye diagram. They were also chosen because of their compatibility with the connectors used on the TX and RX boards.

The VCSEL has to be modulated well above Ith to enable fast switching between spontaneous and stimulated emission. Ith marks the point between slow and fast switching. Therefore, for gigabit speeds it is important that the laser be driven above Ith. The value of Ith has to be such that it provides a considerable range of output power or light into the PD, generating enough current to drive the trans-impedance amplifier. The slope efficiency shows how many watts of power are emitted per amps of current. The higher the slope, the more light that is emitted from the current.

The relevant parameters of the photo-detector include bandwidth, wavelength, connectors, capacitance, dark current, and most importantly responsivity. The bandwidth of the PD has to be at least 3 times larger than the bandwidth of the VCSEL in order to maintain its normal functionality. For this reason, a bandwidth of 3 GHz was desired. The wavelength and connector specifications are the same as the VCSEL’s. The capacitance and dark current are usually considered as factors of noise inside the opto-electronics. They can be tolerated as long as they fall in the range of toleration. In this case, these two specifications will be chosen by their typical values(1.2 pF,1nA). The responsivity of the PD represents the amount of current generated when a certain amount of photon energy hits the PD. Therefore the value of responsivity has to be high enough to convert the light coming from the laser into the current required to drive trans-impedance amp on the RX circuit.

Link Budget

The VCSEL and PD will be connected to TX and RX boards, respectively. The specifications of these boards were analyzed to determine the appropriate values for the design parameters of the VCSEL and PD. The main goal was to make sure that the TX circuit could drive the laser, and that the photo-diode output current could drive the trans-impedance amplifier on the RX. This analysis is known as link budget. As aforementioned, two different Honeywell SC connectorized VCSELs were compared, one with high slope efficiency and one with low slope efficiency. For the PD, a Lasermate SC connectorized model was used. The design specifications that will be discussed in calculating the link budget are shown in the following figures.

To determine whether or not the TX could drive the laser, the DC bias or drive current of the TX was obtained from the MAX3287 specifications. The Maxim board has an adjustable DC bias current, between 0 and 300 mA. Next, the drive current of the VCSEL was obtained. Ith of the VCSEL has to be DC biased, from 1.2 to 1.5 times its value. In other words, if the maximum Ith of the VCSEL is 6mA (as it is for both Honeywell VCSELs), then it needs to be DC biased to 7.2 mA. This is well within the range expected for this project, 4-8 mA. This value of 7.2 mA is also well below the DC current coming into the laser from the TX (max 300mA), meaning that the TX will drive the VCSEL. Only 7.2 mA are needed to drive the VCSEL at fast switching speeds (lasing speeds, rather than LED). Fast speeds are required for gigabit applications.

The modulation current of the TX is adjustable between 2 and 30mA. This means that the range of current that will be modulated lies between 7.2 mA and 30 mA. Ith or the drive current of the laser, multiplied by the slope efficiency of the laser tells us how many watts of light can be outputted from the laser, given 7.2 mA of current. Using a low slope efficiency of 0.04 mA/mW (HFE4380-521), 0.288 mW of power are generated from the VCSEL. Using a laser with a high slope efficiency of .15 mW/mA (HFE4384-522), 1.08 mW of power are generated. The maximum power or range of emitted light from the laser is maximized at 1.2 mW for the low slope efficiency VCSEL, and at 4.5 mW for the high slope efficiency VCSEL. These values were obtained by multiplying the TX's modulation current (30 mA) by the slope efficiency of the VCSEL.

This range of output power is not all incident on the PD. Power is lost at the SC connectors and in the fiber. The exact losses have not been estimated, but it is thought that the maximum loss will be around 3dB. For purposes of computing the link budget, a 3dB loss will be assumed. This factor is converted to attenuation using this equation:

10^-(dB loss/10). Therefore, for 3 dB, the attenuation is 0.5, or half the power. Taking half the range of output power will provide the actual amount of light incident on the PD.

The responsivity of the photo-diode is also very important. It is expressed in A/W. The new range of output power multiplied by the responsivity of the photo-diode produces the current that will go into the trans-impedance amplifier. Based on the specifications of the trans-impedance amplifier on the MAX3266 , this current should be greater than 80 micro Amps. This is the minimum value needed to drive the RX. Below are the calculations taken to generate the link budget and the results, both for the high slope efficiency VCSEL and the low slope efficiency VCSEL.

From this table, it is evident that to drive the RX at 80 micro Amps, the high slope efficiency VCSEL will be much more reliable, having a range of currents beginning at 189 micro Amps as shown in Table 2. If the losses in the fiber and the connectors turn out to be less than 1/2 the output power, then the low slope efficiency VCSEL might still be a good option (and a cheaper one). Another alternative could be to find a PD with higher responsivity. Nonetheless, the HFE4384-522 VCSEL seems like a great option. It can be driven by the TX, and it can drive the RX without a problem, even after considering an attenuation of 1/2. The prices have not been obtained, so a final decision cannot be made until this information is known, as well as an exact estimate of the losses incurred through fiber misalignment, etc.

In addition, the VCSEL has to be common cathode. The MAX3287 that will be used to evaluate the 1.25 Gbps VCSEL, drives common cathode lasers. “Short-wavelength laser diodes (wavelength 980nm) and VCSELs typically require a common-cathode configuration. In the common-cathode configuration, the laser’s cathode connects to ground and the laser is driven at its anode.” The VCSELs considered in this paper are both common-cathode.

The available photo-diodes and VCSELs were only found to be SC connectorized. Since they lack the SMA connector, electrical drive circuitry will be built using surface mount components. A resistor will be needed to make the VCSEL SMA connectorized. For the photo-diode, a resistor and possibly an inductor and capacitor will be needed. The values of the resistors should be around 30 ohms. Components that minimize loss at speeds of 1 GHz, will be chosen for the values of the capacitors and inductors.

Some VCSELs also come with additional features such as a feedback photo-diode, which is used to monitor the VCSEL’s performance. This type of VCSEL will not be considered, because the additional cost added by this functionality is not relevant to this design project.

Construction and Testing

The opto-electronic components will be attached to a separate board than the Maxim test setup itself, there will be a small amount of board layout that will have to be completed followed by some soldering. This soldering will secure the VCSEL and photodiode to individual external boards (bought at radio shack and later etched by Bob House for comparison). This will finally be connected to the main Maxim boards. Since the Maxim setup already has SMA connections attached to it, the next step is to put SMA connectors on the opto-components. After the connections have been prepared, the Maxim boards can be attached to the opto-components. Now the setup is ready to connect to the Intel/Agilent test-bed and be checked for possible problems.

Upon properly attaching the devices to the Maxim boards, extensive testing will be done. These tests will include the construction of eye diagrams using the “persistence” mode on the oscilloscope (Tektronix 7000), and a bit error rate test using the BERT device. A few possible problems might arise at this point. Faulty components could cause a closed eye or no eye at all. There are many reasons why the parts might be non-functional. They might fail very quickly once they are turned on due to manufacturing defects, or there may be a short or open circuit on the board due to the hand-soldered joints. The defects are only manufacturer dependent, and the only way to deduce this type of problem is to do final testing on it. The heat from the soldering may also damage the components in some way. VCSEL and PD specifications called for no more than 10 seconds of exposure to 500oF. Careful soldering is the only way to avoid this. A sub-par eye may also be produced if some of the component values turn out to be inconsistent. For instance, the capacitance value (across the junction) in the PIN photodiode may cause the transit time to be a bottleneck in the system. This should be taken care of by purchasing decent components vs. very inexpensive (and thus lower quality) components. Also, the Maxim board specifications must be in line with the various component values. The SMA and SC connections may also attenuate the signal. The solution to this is to order parts with connectors already in place if possible. Otherwise, the SMA connectors can be carefully put in place, but the SC connectors are very hard to attach. The circuit layouts in Figure 4 and 5 were designed to mount the VCSEL and PD. These designs would be implemented on the TX and RX boards which will in turn be connected to the Maxim boards.

Conclusion

The opto-electronic section of the Intel/Agilent project is about half design and half laboratory technician work. The initial comprehension of the test-bed and the construction of the opto-module require a very small amount of design. Later on, the board will be redesigned in order to possibly improve the performance and lower the price. This schedule of events reflects how some (not all) companies start when they design products. First, they check the market to see what is currently out there. Next, they purchase products that they think they want their product to function like. Finally, using a bit of reverse engineering, they come up with their own ideas to improve the existing product, and they move on to the construction and marketing process. This particular project gives some insight as to what it is like in the workplace, and gives the student a chance to get some hands-on experience. After thorough research, our group decided to purchase the RSC-M85A306 PIN PD from Lasermate, at a cost of 45.00 $, with a minimum responsivity of 0.35A/W. We have not yet decided on the VCSEL since the prices are still unavailable. However, the HFE4384-522, with high slope efficiency of 0.15 mW/mA, appears to be the most reliable. Thus far, it is a prime candidate for our project.