Download Design and Construction

Georgia Institute of Technology
School of Electrical and Computer Engineering
ECE 4006 Senior Design
Bringing Gigabit Ethernet to the Masses
Design Paper
Talal M. Jaafar
Ibrahima B. Sow
Mohammad F. Zaaman
Supervisor: Dr. Martin Brooke
Design and Construction
At this point we assume that the reader has read the background paper of this
project and is familiar with the objectives of the project.
The design portion of this project is the most important part because it will
determine the success of the project. Therefore, it is paramount that enough attention is
given to the design of the module that is being put together. As stated in the previous
portion of this report, the goal of the project is to detach the fiber-optic transceiver from
the Ethernet board and mount it on a separate circuit board that is outside the computer.
The first step involved in the design is the identification of the parts of the Ethernet board
that have to be moved with the HFBR- 53D5 fiber-optic transceiver to form the new
opto-electronic module that will be located on a circuit board that will be outside of the
computer case. The circuit diagram figure 1 shows the complete system with the driving
circuitry.
Figure 1. Intel Pro/1000 Gigabit Ethernet circuit diagram
The task at hand involved the following challenges. Proper coupling of the power
lines, proper decoupling of the ac lines, proper grounding of the external board and
properly soldering all the surface mount components, which is a serious challenge due to
the size of the components.
Once the components that will be moved to the experimental board were
identified, they were erased from the circuit diagram and the diagram in figure 2 was
obtained. Essentially, the experimental module will contain the HFBR-53D5 Fiber-Optic
Transceiver, power supply connectors, signal transmission SMA jacks and the circuitry
associated with the transmission lines and the power supply.
Figure 2. Circuit that will be designed for the experimental board .
On the Intel Pro/1000 Ethernet Card, the transceiver was powered by a special
power supply system that was essentially a single power supply that used a filtering
circuit to keep the Transmitter’s power supply separate from the Receiver’s power
supply, and to keep the two supplies from interfering with each other and noise from
propagating from one side to the other through the power lines. Since the addition of the
filtering circuitry would increase the complexity of the experimental board, it was
decided to simply use two 5V DC power supplies, which can be found at Radio Shack.
The only drawback to this approach is the addition of the power supply jacks to the
experimental board and the fact that every time the board is used, two power supplies
have to be plugged to the wall outlet. The connection between the experimental module
and the board will be done through coaxial cables. The challenge here is the selection of
coaxial cables that can support the speeds involved. Essentially, the system is sending
digital signals at one Gbits/s using unipolar NRZ (non return to zero) signaling. This
translates to a square wave that can be estimated to have a fundamental frequency of
500MHz, and to get a proper signal it is advisable to pass the first 5 harmonics; therefore,
the coaxial cables that are to be used should handle at least 2.5GHz. For this reason, RG316 coaxial cables were chosen. These cables can handle speeds up to 3 GHz or more,
and they offer another advantage, which is that they are small so they will not be too
cumbersome. On the Ethernet Card, the coaxial cables will be directly soldered to the
wholes where the pins of the transceiver were located, and on the experimental board, the
coaxial cables will be attached using SMA connectors so that the board can be detached
from the Ethernet Card at any time. The choice of the SMA connectors was mainly due to
the size of the cables, the speeds involved, and the amount of space available on the
experimental board. SMA connectors can handle speeds up to 18 GHz, while regular
BNC connectors at best will handle only 3 GHz. Providing ample space on the
experimental board is very important because later groups will have to work on the board
and probably modify some parts of it; therefore, considering the size of the components,
it is paramount that they have enough space to be able to work comfortably on the board.
Among the components of the circuit shown in figure 2 are the transmission line
termination resistors, DC de-coupling capacitors and AC coupling capacitors. The
transmission line terminations are required because the board is big enough for the traces
on it to be considered transmission line. Therefore, impedance matching is absolutely
necessary to avoid harmful reflections in the lines. The role of the AC coupling
capacitors is to keep harmful DC components that might slip into the signals from ever
reaching the transceiver on the Tx side, or the Ethernet card on the Rx side. DC signals in
the high speed AC lines have the potential to cause some signal distortions and
attenuation, and thus detection problems. Essentially AC coupling consists of simply
passing the signals through a high pass filter formed by a capacitor and usually the
impedance matching resistor of the line. These can be clearly seen in the circuit diagram
of Figure 3. The DC de-coupling capacitors have to counter act the inductance introduced
by the power supply lines and the grounds. A conductor has the undesirable behavior of
acting like an inductance as its length increases, the effect of this behavior as frequencies
increase is that the impedance of the line also increases drastically. To deal with this
problem, DC de-coupling capacitors are used to counteract the inductance of the lines.
The DC de-coupling capacitors also protect the circuit from the transient current pikes
that results from the very fast switching and they also help reduce the noise levels on the
power supply lines.
The other notable feature of the design is the maintenance of the differential
signaling used on the original Ethernet card. Differential signaling has some benefits that
cannot be ignored in such a design. In essence, using differential signaling is like using
two copies of the signal in the detection phase. One wire is the positive leg of the path
and the other is the negative leg. There are two big differences. First, the two wires carry
inverted copies of the voltage waveforms, meaning when one wire has a rising edge on it,
the second has a falling edge. The second is that the receiver compares the two signals,
producing a 1 whenever the signal on the positive leg is greater (more positive) than that
on the negative leg. It produces a 0 when the opposite is true. Another way of saying this
(which justifies the word ‘differential’) is that the two Voltage signals are subtracted
(positive – negative) and a positive result is a 1 and a negative result is a 0. Differential
signals have much better noise immunity. This can be seen by thinking mathematically
about what is happening. Let’s assume the positive leg has a value of P and the negative l
leg has a value of N. Then the sign of the subtracted signals P – N is the result of the
signal. Now let’s add a noise error E. This gives us P+E and N+E, because most noise
sources cause common mode noise, or noise that adds (nearly) equally into both legs of
the signal. Now when the receiver subtracts the noisy signals, it gets (P+E) – (N+E) = P –
N, as before. What is very interesting about this is that E may be large (even larger that P
or N) and this cancellation still occurs.
The circuit diagram of the completed board looks like the following.
Figure 3. Experimental board circuit diagram
Figure 3 shows the circuit diagram of the board and figure 4 below shows actual
photographs of the board that was built. From the circuit diagram above, a PCB was
machined and was used for the experimental board. It is important to not here that the
signal detect lines was established between the experimental board and the Ethernet card
using a simple wire that was thin enough. And the ground on the experimental board was
connected to the ground on the Ethernet card to provide a common reference voltage.
Figure 4. Experimental Board