Design Paper

Gigabit Opto-electronic Module

ECE 4006

Group Members:

Harsh Sopory

Kaushik Narayanan

Nafeez Bin Taher

Brief Description of Module

The module that will be used in this project is the Agilent HFBR 53D5. This basically consists of a transmitter section and a receiver section. The transmitter section consists of an 850 nm Vertical Cavity Surface Emitting Laser (VCSEL) in an Optical Sub Assembly (OSA). This is driven by an IC that converts differential signals into a signal capable of driving the laser. The receiver consists of a silicon PIN diode that is mounted on the OSA with a transimpedance preamplifier IC. The OSA is connected to a bipolar circuit that provides post amplification and quantization of the signal. A signal detect circuit is also present in the post-amplification part of the circuit, and is used to indicate when the input is valid data and when it is not.

Project Steps

The initial aim of our project is to set up a test bed for the testing of any future modules. The first step in this direction will be to set up an evaluation board to test the HFBR 53D5 to ensure that it can transmit data at Gigabit rates. Agilent provides an evaluation board for the testing of this module and the schematic of what the Agilent board would look like with all the parts and the module on it is given on the next page in Figure 1.

Figure 1. Schematic of Agilent Evaluation Board for HFBR 53D5 module.

We decided that for testing purposes, it would be cheaper to use an evaluation board manufactured here at Georgia Tech itself. To this end, the board specifications were obtained from the Agilent website and the board shown in Figure 2 was made.

Figure 2. Georgia Tech Evaluation Board.

After going over the circuit layout of the Agilent evaluation board, it was also decided that all the parts would not be required. The circuit layout that will be used is given below in Figure 3.


Figure 3. Circuit Layout to be used on Georgia Tech Evaluation Board.

In the construction of this board, there were certain factors that had to be kept in mind. The first one was regarding the power supply filtering circuit and its location, and associated concerns such as the accompanying ground and supply planes used for this power supply. The second one was regarding the transmission lines and their terminations on the board.

Regarding the first concern about the power supply filtering circuit, the problem was dealt with by removing the need for the circuit. The circuit was required on the board because just one power source was supposed to be supplying power to both the transmitter and the receiver in the transceiver. The reason for this setup was to save space when this transceiver was actually being used. By having just one power supply for both the parts, space would be saved on the Gigabit Ethernet card where this transceiver was actually going to be used. By having the same power supply though, an additional consideration was introduced, namely that the power supply of one part would now be sensitive to any changes that might be caused in the power supply due to the operation of the other part of the transceiver. To account for this the power supply filtering circuit was required. By having this, it would be possible to keep the two parts of the transceiver quite isolated and independent of each other’s power supply considerations.

This problem was dealt with on the board by using two separate +5V power supplies for the receiver and the transmitter stages in the module. This eliminated the need to construct a power supply filtering circuit on the board. Also, it does not seem that the absence of this circuit will cause any problems in the future when the evaluation board with the transceiver will be tested with another Gigabit Ethernet card.

The second concern was regarding transmission lines and their terminations on the evaluation board. This problem was solved by eliminating transmission lines from the board completely. This was possible because since the module was to carry data at a Gigabit per second, the maximum frequency of the signal would be 500 Mhz. If the maximum frequency were 500 Mhz, in order to avoid the wires looking like transmission lines, their length would have to be at least less than 1/4th the wavelength of the signal. 1/4th of this signal wavelength can be found to be 15 cm. It can be seen from the board given in Figure 2 that all of the trace lengths are less than 15 cm. Also by avoiding any sharp angles on the board, the chance of the wires behaving like transmission lines towards the signal was decreased.

Finally, the board was also designed to use differential inputs and outputs because this leads to the signal being more robust and resistant to noise.

The rest of the parts that will be placed on the evaluation boards consist mainly of resistors and capacitors. These parts fulfill various purposes. The 0.01 F capacitors that will be used will mainly be used as decoupling capacitors. Decoupling capacitors play an extremely important role in high-speed circuits such as this. The reason for this is that they are used to make sure that the power supply is kept separate (i.e. decoupled) from the circuit. This is important because of the presence of various inductances that are inbuilt into the circuit. On our evaluation board, these inductances are associated with the power supply wire, the evaluation board itself and the solder bond used for the transceiver pins and the elements on the board. The presence of these inductances means that if any high-speed transients are generated during the switching of the circuit, they will be stopped by the inductances. The decoupling capacitors are then used to handle this excess charge that is present in the circuit. By preventing this excess charge from reaching the circuit, the power supply to the circuit can be maintained within the recommended limits and the circuit will be protected from damage. It can be seen from the circuit layout that the capacitors are present only on the power supply lines. These factors are extremely important when considering circuits where the minimization of noise is important. Another way in which the function of these capacitors is made evident is by the fact that they are placed as close to the pins of the circuit as possible. This is to ensure that the inductance between the capacitor and the pin, due to the wire, is as low as possible.

The resistors that will be used will fulfill two main purposes; firstly to terminate transmission lines and secondly, to bias the amplifiers. The network of 68 and 191 resistors that can be seen in Figure 3 can be seen to be placed on the lines leading to the transmitter. They will be used to terminate the transmission lines that are present on the Gigabit Ethernet card and will run from the card to the evaluation board. Also, the resistors will be used to bias the amplifiers present in the transmitter and in the receiver.

A final consideration that should be kept in mind is that all connections to the ground plane should be as close to each other as possible, if not almost at the same point. This will avoid the problem of ground inductances that can come up if the parts are not grounded properly. This problem comes up because at certain frequencies, even the ground begins to exhibit some inductance. Hence by grounding the circuit elements at different point, ground inductances come into effect between the different points and this can cause problems when excess charge is being conducted to ground.

Final Steps

Finally, once the evaluation board has been setup completely with the module on it, and it has been tested for its handling of data at Gigabit rates, it will be connected to the original Gigabit Ethernet card that the module was on. This will be done by soldering the pin positions on the Gigabit card to the corresponding traces on the evaluation board using 50 coaxial cable. For the pins that correspond to the traces that run to the transmitter, the termination network that is present on the evaluation board will terminate the cable. On the Gigabit card though, there is another similar network that is also meant to terminate the transmission lines till that point. This network will have to be taken off the card to prevent the transmitted signals from seeing two impedances before leaving the card. A circuit diagram of the whole setup is given in Figure 4, with the boxed elements representing the elements that need to be removed.

Figure 4. Diagram of whole setup and of the elements to be removed.

Once the evaluation board and the card are connected, a fiber optic link will be set up between the transceiver on the board and another Gigabit Ethernet card. This link will then be tested using a diagnostic program that is provided along with the Gigabit card software. If it is found that communication takes place between the cards as expected, then our purpose will have been achieved because then it will make it easier for any future design of the module to be tested. This will be the case because then the only variable in the setup will be the redesigned module, so it will be easier to find any faults in the design.