Gigabit Ethernet Optical Link Design Project
Group 5:
Emily Lyon
Spiro Sarris
Carlo Espinosa
Winston Czakon
Eric Woods
David Pérez
December 3, 2002
School of Electrical and Computer Engineering
Georgia Institute of Technology
TABLE OF CONTENTS
Section
Page
PROJECT SPECIFICATION
1
GROUP ORGANIZATION
1
BACKGROUND
2
Ethernet
2
Gigabit Ethernet Signals
2
IEEE 802.3z Standard
5
VCSELs and PDs
7
System Level Design Topology
8
PROCEDURE
9
Receiver Test Circuit
9
Optical Link Power Budget
9
VCSEL and Photodetector Specifications
10
Optical Transmitter Design
12
Optical Receiver Design
14
Printed Circuit Board Layout
15
Optical Transmitter and Receiver Implementation
15
Testing of Optical Transmitter and Receiver
16
Modified Transmitter Design
17
Unconnectorized Receiver Alignment Testing
17
Unconnectorized Transmitter Alignment Testing
19
Comparative Analysis of Connectorized Transmitter Boards
19
RESULTS
19
Electrical Receiver Resutls
19
Connectorized Optical Transmitter and Receiver Results
20
Unconnectorized Receiver Alignment Results
20
Comparative Connectorized Transmitter Results
22
ISSUES / DIFFICULTIES
22
REFERENCES
24
ii
PROJECT SPECIFICATION
The purpose of this project is to design and implement an optical transmitter and receiver
that comply with the optical Gigabit Ethernet standard IEEE 802.3z. During the course of the
project, the group members will follow a task and time management plan created by the group.
The design must be carried out on a $350 budget allowance provided by the Georgia Tech ECE
department. A system level design topology is provided by the instructors and recommended as
guidance for the system design. As an initial step, a receiver board is provided without optical
photodetector circuitry. After the electrical-only receiver board is assembled and tested, the
group must design photo detector circuitry to integrate with the provided receiver design. Next,
the group will design an optical transmitter circuit intended to interface with the optical receiver.
Finally, the transmitter and receiver designs will be implemented using unconnectorized
optoelectronic parts and fiber were manual alignment is required. Parts provided include the
Maxim 3266 trans-impedance amplifier (TIA), Maxim 3264 limiting amplifier, and the Maxim
3287 laser driver. The signal source used as a pattern generator for testing is the Tektronix
GTS1250 GBIC test system. The oscilloscope used for measurements is the Tektronix TDS7154
Digital phosphor oscilloscope.
GROUP ORGANIZATION
To accomplish the goals of this project, a Gantt chart was created as a timeline of how the
work should be divided up and completed. It follows from the chart, which can be seen at
http://www.ece.gatech.edu/academic/courses/ece4006/fall2002/group5/ganttchart.html, that
research and gathering information is to continue through the duration of this project. Emily
conducted most of the Gigabit Ethernet research with other small contributions from the rest of
the group. Emily is also responsible for the most of the group management and creation of the
Gantt chart. After enough information was gathered on the IEEE 802.3z standards and the
specifications of the different components, we were able to start looking for the vendors that
would send us the parts in a timely and cost effective manor. The ferrite bead inductors and the
1k and 100k potentiometers ordered by Spiro, the printed circuit boards and VCSELs ordered by
Winston and Carlo have arrived, and the photo detector ordered by Eric have arrived. Winston
completed most of the assembly of the prefab boards, as he was the most experienced with
soldering. Winston also designed the original layout for the transmitter and receiver with Spiro
and Carlo also working on the finalization of the design. Winston, Spiro, and David completed
1
the testing of the prefab board. Other critical elements to the design were the optical link budget
worked on by Eric, Carlo, Spiro, and David. Winston and Carlo did all of the soldering. The
connectorized transmitter and receiver were tested by Spiro, Carlo, and Winston. The second,
more aggressive transmitter board was designed by Spiro and Carlo. The testing and
comparative analysis was completed by Spiro, Dave and Carlo. The final alignment testing of
the unconnectorized receiver board was done by Emily with the help of Eric and Carlo.
BACKGROUND
Ethernet
The Ethernet is a networking protocol that was developed in the early 1970s by Xerox
Corporation, initially running at a data rate of 10Mbps. Eventually, Fast Ethernet was
developed, increasing the data rate to 100Mbps. With the increasing need of greater speed and
more bandwidth, Gigabit Ethernet was considered for development in mid-1995. A Gigabit
Ethernet Task Force was created in 1996 that included up to 200 people and 50 companies to
develop the necessary components and appropriate standards [1]. The Gigabit Ethernet
increased the data rate up to 1000Mbps or 1Gpbs, and was built on top of the existing Ethernet
topologies. The Gigabit Ethernet is the same as the original Ethernet from the data layer
upwards; below, it combines the IEEE 802.3 Ethernet standard with the ANSIX3T11
FibreChannel standard [1]. This combined the high-speed physical interface with the frame
format of the Ethernet to produce a fast, reliable, cost-effective, and scalable networking protocol
(NN, Cisco).
Gigabit Ethernet Signals
The various signals involved in the operation of a Gigabit Ethernet transmitter and
receiver can be classified as high or low frequency and high or low sensitivity. The different
combinations of these classifications determine amount of care and accuracy required to design
an operational circuit. Table 1 indicates the classifications of several of the signals involved in
the Gigabit Ethernet link.
Frequency
High
Low
Sensitivity
High
TIA input
DC power at
receiver input
Low
VCSEL driver outputs
DC power at
receiver output
Table 1. Classification of signals.
2
In order to preserve the integrity of high frequency signals, the lines on which they travel should
be isolated to avoid coupling with other high frequency signals. This can be achieved by
keeping the lines spread far apart compared to the height of the dielectric on which they are
printed. Transmission line issues must also be taken into consideration to avoid any unwanted
resonance or reflections. The simplest solution to the transmission line issue is to make all signal
line lengths /10 or less. If the line lengths are this short, then transmission line effects such as
phase shifts, reflections, and resonance, are negligible. Various methods of transmission line
optimization exist when a line must exceed /10.
High sensitivity signals are especially subject to electromagnetic interference (EMI),
attenuation, and undesirable coupling. Providing a good return path for the signal can minimize
these effects. One way to realize a clear return path is to place connections from the signal and
return paths to a ground plane in close proximity in order to prevent crosstalk. However, care
must be taken such that coplanar waveguide (CPW) behavior does not couple signals into the
substrate.
One important performance measurement of digital communications equipment is the bit
error rate, or BER. The bit error rate is defined as
BER 
No. _ of _ errors
No. _ of _ bits _ sent
(Eq. 1)
An error is misinterpretation of a logical 1 or 0 value. These errors are a result of numerous
problems such as inadequate signal levels, interference, and poorly designed hardware only to
name a few. A typical bit error rate for decent Gigabit Ethernet equipment is 10-9 errors/bit or 1
error
/Gbit.
To determine the maximum lengths of signal lines to use for Gigabit Ethernet hardware,
the frequencies of the propagating signals must be determined. The IEEE 802.3z standard for
Gigabit Ethernet specifies “Not Return to Zero”, or NRZ, data signals. NRZ means that if a
logical 1 is sent immediately following another 1, the signal does not drop to zero between those
two instances in time.
An example of a NRZ Gigabit Ethernet signal is 1010101, a 500MHz square wave.
Using r = 3.0, the wavelength, , is 20cm. So, reasonably, a 2cm line could be treated as a wire
in the hardware design. Depending on signal amplitudes and the desired quality of the output
3
signals, higher order harmonics of the square wave dominant frequency signal must be
considered.
Harmonic No.
Frequency
(f)
500 MHz
1.5 GHz
2.5 GHz
1
3
5
Relative
Amplitude
1
0.424
0.254
Wavelength
()
20 cm
6.7 cm
4 cm
Maximum Line
Length (L)
2 cm
6.7 mm
4 mm
Table 2. Harmonic analysis of 500 MHz square wave
To obtain acceptable results without addressing transmission line effects, the fifth harmonic of
the data waveform should be considered, and the maximum microstrip line length should be
4mm.
A graphical method of measuring the performance of digital hardware is the use of a
digital storage oscilloscope to produce an eye diagram. The following setup is implemented to
obtain such a diagram.
Clock
Trigger
Pattern Generator
Oscilloscope
Test Circuit
Data
In
Out
CH1
Figure 1. Setup for producing eye diagrams
The clock of the signal source or pattern generator triggers the oscilloscope. The signal or
pattern is processed by the test circuit, and then enters the oscilloscope on a data input channel.
The oscilloscope is set up to acquire a single point, the input level at CH.1, on each clock pulse
and display it. After a specified number of points have been displayed, new data points are
acquired and the older points are removed from the display. Two examples of eye diagrams are
displayed in Figure 2.
4
(a)
(b)
Figure 2. Example of a good and bad eye diagram.
The diagram in 2(a) is very clear because the digital signal levels that the oscilloscope acquires
are clearly high or low. The diagram in 2(b) indicates a very poor digital signal. The signal
levels that are acquired by the oscilloscope in 2(b) vary between the logical 0 level and logical 1
level. It must be noted that a clear or “open” eye diagram is not necessarily indicative of a low
bit error rate. Although the signal levels clearly correspond to a logical 0 or 1 value, an eye
diagram shows nothing of whether the value it displays is the correct logical value.
IEEE 802.3z Standard
The IEEE 802.3z standard was finished and ratified in July 1998. Clause 38 defines the
“PMD sublayer and baseband medium, type 1000BaseLX and 1000BaseSX”, which is longwave and short-wave optical transmission for the Gigabit Ethernet respectively. The design
project that this group has worked on depends on this standard. In order to begin this task,
knowledge and some understanding of the appropriate background and standard were needed.
The transmit characteristics can be seen at.
The first section in the IEEE 802.3z Standard, Clause 38, that is needed for the project is Section
38.3 “PMD to MDI optical specifications for 1000BASE-SX”. It first defines the operating
range and modal bandwidth for both multi-mode (MMF) and single-mode fiber. This project
will utilize 62.5m MMF. The standard provides 2 ranges that are specified in Table 38-2,
which can be seen at
http://www.ece.gatech.edu/academic/courses/ece4006/fall2002/group5/standard_TX_char.html;
for 160MHz, the range is 2m-220m, and for 200MHz, and the range is 2m-275m. For this
project, testing will be done with a 2m patch cable and a 100m-cable link. There are 3
subsections in 38.3 that specify the transmitter optical specifications, receiver optical
5
specifications, and worst-case power link power budget and penalties. The parts of the tables that
were considered in the design process are highlighted.
The wavelength range is defined as 770nm to 860nm; VCSELs that operate at 850nm
were chosen for the transmitter board. The other important characteristic is the launch power.
The optical link budget, explained in a later section, was verified to meet this specification. The
important characteristic for the receiver is the vertical eye-closure penalty, which indicates the
quality of data is coming into the receiver. This determines whether or not a suitable circuit is
used to transmit and receive the data. Receiver characteristics can be seen at
http://www.ece.gatech.edu/academic/courses/ece4006/fall2002/group5/standard_RX_char.html
One of the most important specifications in the project is the link power budget, defined at
http://www.ece.gatech.edu/academic/courses/ece4006/fall2002/group5/standard_link_power.htm
l. It is the basis for the optical link budget calculations that determine the final design of the
transmit and receive boards for this project.
The next section that is used for this project is Section 38.5, “Jitter Specifications for
1000BASE-SX and 1000BASE-LX”. These values will be necessary for the testing of the final
transmitter and receiver boards, but at this stage in the design process they do not mean much, so
the table is not included in this paper.
Section 38.6 “Optical measurement requirements” specifies what measurements should
be made in testing the equipment, and how they should be made. The measurements that are
necessary for this design project include 38.6.5 “Transmitter optical waveform (transmit eye)”,
and 38.6.11 “Conformance test signal at TP3 for receiver testing”. The first defines what the
transmit data should conform to, i.e. the pulse shape characteristics. The transmitter eye mask
definition can be seen at
http://www.ece.gatech.edu/academic/courses/ece4006/fall2002/group5/standard_TX_eyemask.ht
ml.
The second part of Section 38.6 is the conformance testing. This defines the receive data
mask, and how to measure the vertical and horizontal eye-closure, and can be seen at
http://www.ece.gatech.edu/academic/courses/ece4006/fall2002/group5/standard_eyeclosure.html
. These measurements determine whether or not the system being tested is sending acceptable
data through the optical link. This acceptable receive data eye mask is used to measure the data
6
received when testing the transmitter and receiver boards to determine the success and/or failure
of the designs.
In Section 38.7.2 “Laser Safety,” the required safety standards for the lasers in use are
defined. The problem with this section is that it only refers to the IEC 60825-1 eye safety
standard, which to this point in time cannot be found. However, the US government does
classify laser devices by output power18. Finally, the last relevant section of the IEEE 802.3z
standard that is used for this project is Section 38.11, “Characteristics of the fiber optic cabling.”
This defines the characteristics of the optical fiber to be used. This includes the modal
bandwidth, attenuation, and connection insertion losses associated with the cabling. These
characteristics are used in determining optical link budgeting, along with the understanding of
the system and how it works.
VCSELs and PDs
Vertical Cavity Surface Emitting Lasers (VCSELs) and photodiodes (PDs)
constitute the optoelectronic physical elements of the optical transceiver system. These
components are made from III-V semiconductors – usually gallium arsenide (GaAs) or a ternary
combination such as aluminum or indium gallium arsenide. These more advanced materials are
required for operation at speeds exceeding 1GHz because the significantly higher electron
mobility – gallium arsenide has approximately five times that of silicon [16]. This characteristic
allows for efficient operation of the devices in extremely high-speed regimes.
These lasers have certain metrics that define their performance in any application, such as
slope efficiency (η) – the increase in output optical power versus input current above the
threshold current. The threshold current (Ith) is defined as the minimum current to create a
carrier population inversion and to initiate lasing. ITH is linearly temperature dependent with a
positive slope, but typically does not shift more than 1mA from the value at the IEEE standard
noise temperature of 20˚C, generally 1 – 10mA. These lasers are semiconductors and as such are
inherently sensitive to temperature, so there is a maximum safe operation temperature, which is
reflected in the maximum modulation current for the device. Design specifications state that for
operation of an 850nm laser, the output power cannot exceed 1mW, the limit for a class II laser
device as set by the FDA [17]. The heating issues shows how important the series resistance –
almost always greater than 20Ω – is to predicting device behavior, and cannot be neglected when
7
modeling the device. This stands in contrast to the standard diode, which generally has
negligible series resistance.
Photodiodes fundamentally operate as photomultipliers; that is, they amplify incoming
photons and create a cascade of carriers that then cause a change in the output device current.
There are two primary types of photodiodes: junction (p-i-n) and avalanche. Junction
photodiodes – specifically p-i-n diodes – have much faster response times and higher
sensitivities. The devices have a layer of p-type material, then intrinsic (undoped) material, and
finally a layer of n-type material on the bottom. In practice this region is in constant depletion
and creates the device’s parasitic capacitance. In 1GHz optical systems, typical ranges are 1-2pF
– anything larger would increase the time constant, depress the device’s operating frequency and
bandwidth, and render the device unusable for the desired purpose. In practice, most PDs are
made with small active areas so that the capacitance can be minimized, since it is a function of
both intrinsic region width and exposed device area. PDs are characterized by their internal
quantum efficiency -- the number of carriers generated for each photon absorbed. Simply,
photodiodes are modeled as capacitors in parallel with diodes for high-speed systems.
System Level Design Topology
Figure 3 shows the top-level design topology suggested by the instructors for this project.
Transmitter (TX)
Receiver (RX)
Optical Fiber
Laser Driver
VCSEL
Photodetector
Trans-impedance
Amplifier
Limiting
Amplifier
Figure 3. Top-Level Design Topology for Gigabit Ethernet Transmitter and Receiver
The VCSEL converts an electrical signal from a laser driver into an optical signal (infrared
light), and then radiates that signal through an optical fiber to the PD. The PD takes the optical
signal and converts it into an electrical current signal. Connected to the VCSEL is a laser driver
chip, which powers the VCSEL and provides it with the signal. The output signal from the PD
goes through the TIA, which converts the current to a voltage, and then into the limiting
amplifier. The limiting amplifier “cleans up” the digital signal by amplifying all of the input
8
voltages above 4mV to the amplifier saturation voltage of 1.3V at the output [5]. These are the
main components used in the transmitter and receiver circuits, and the specifications associated
with these determine the passive networks used to control the components.
PROCEDURE
Receiver Test Circuit
Before designing any circuits, the group was provided with a printed circuit board and all
of the parts necessary to assemble a receiver circuit with an electrical signal input. The purpose
of this exercise was to familiarize the group members with how the board worked, and more
importantly with the assembly of the board. The circuit was assembled and tested with the setup
in Figure 4.
Input Attenuator
WW
Pattern
Generator
RX Board
Oscilloscope
Figure 4. Receiver Test Setup
The patterns used in this test were PRBS7 and K28.7. For each input pattern, the input
attenuation was swept in 10dB steps from 0dB to 60dB. Eye diagrams of the receiver output
were recorded for each attenuator step. These diagrams are shown in the Results section.
Optical Link Power Budget
The objective of the optical link budget is to minimize the power dissipation of the signal
from the laser driver to the TIA. The desired output of the VCSEL is to be at the highest
possible emitted power that still remains below the Gigabit Ethernet compliant eye safety limit of
1mW. As the current through the VCSEL varies above the VCSEL threshold current, the light
intensity increases linearly. The range of current to drive the VCSEL is to be determined.
Figure 5 shows a block diagram of the signal flow through the optical components.
9
Iin
VCSEL
Ieff = Iin - Ith

Pout = Ieff*


PRX = Pout* 
PD
TIA
IPD*R
in = total current into the VCSEL
Ith = VCSEL threshold current
Ieff = effective current of the signal
slope efficiency of the VCSEL
Pout = Output power of the VCSEL
PRX = Input power into the PD
 = attenuation through cable and connectors
R = Responsivity of the PD
Figure 5. Block diagram of the logical flow for the optical link budget.
Table 3 shows the optical link budget for the three different types of VCSELs and the PD
that were selected.
Characteristic
Range of Ith (mA)
Range of (mW/mA)
Desired Emitted Power by VCSEL (mW)
Iin (mA) with max, Ith(max)
Iin (mA) with max, Ith(min)
Iin (mA) with min, Ith(max)
Iin (mA) with min, Ith(min)
Range of Total Current, Iin (mA)
Range of Attenuation Loss (dB)
Range of Power Input to PD (mW)
Responsivity of Lasermate PD (mA/mW)
Range of Current Output to TIA (A)
MTR300-N10
4 to 6
.05 to .1
1
16
14
26
24
14 to 26
0 to 7.5
0.178 to 1
0.4
72 to 400
HFE4384-522
1.5 to 6
.06 to .3
1
9.33
4.83
22.67
18.17
4.83 to 22.6
0 to 7.5
0.178 to 1
0.4
72 to 400
HFE4084-322
1.5 to 6
.1 to .4
1
8.5
4
16
11.5
4 to 16
0 to 7.5
0.178 to 1
0.4
72 to 400
Table 3. Optical Link Budget.
The range of total current indicates Iin from Figure 5. The VCSEL’s current should be
within this range to operate properly.
VCSEL and Photodetector Specifications
The Gigabit Ethernet Standard proposes that the VCSEL and the Photo detector operate
at 850nm with a speed of at least 1.25Gbps. In this project, the optical components interface
with a 62.5/125m MMF. The connectorized VCSEL and PD will be shipped pre-aligned via an
SC connector. The unconnectorized components will require manual alignment to determine
their alignment tolerance. The MAX3287 laser driver chip documentation also recommends a
common-cathode VCSEL. Table 4 shows a comparison of five different VCSELs. The most
important parameters to our applications are the slope efficiency and the threshold current.
10
VCSEL
HFE4381-521
MTR300-N10
HFE4384-522
HFE4084-322
OPK210
Manufacturer  (mW/mA) Ith(mA)
Honeywell
0.04
3.5
Metrodyne
0.05
4
Honeywell
0.15
3.5
Honeywell
0.25
3.5
Optek
0.24
3.5
Connectorized Cost/Unit ($)
Yes
19.52
Yes
16.05
Yes
17.65
No
15.75
No
16.82
Note:  and ITH are the typical values.
Table 4. Comparison of Different VCSELs.
The HFE4384-522 was chosen for the connectorized VCSEL because of higher
performance, but due to vendor problems, the MTR300-N10 will be utilized first. The two
unconnectorized VCSELs in the list have very similar properties, but the HFE4084-322 was
chosen considering the cost and availability. Plots of the selected VCSELs emitted power vs.
current are shown in Figure 6.
Figure 6. Plots of emitted power vs. current: a) MTR300-N10 b) HFE4384-522 c) HFE4084-322 (fig. 13a is not to
scale with 13b and 13c)
For the PD, a Lasermate GaAs p-i-n PD RSC-M85A306 was chosen because it was the only SC
connectorized PD available with a unit cost of $50. These devices have high responsivity and a
low capacitance as shown in Table 5, since these characteristics are desirable. The comparable
unconnectorized PD, PDT-A85A30, will also be ordered for our unconnectorized PD.
11
Table 5. Lasermate RSC M85A306 (top) and PDT-A85A3
Optical Transmitter Design
Figure 7. Schematic of Optical Transmitter.
The optical transmitter circuit was designed according to the specifications, examples,
and design equations provided by Maxim in the documentation for the laser driver IC and the
specifications of the Honeywell HFE4384-522 VCSEL.
Typical Value
Slope Efficiency, 
.15 mW/mA
Threshold Current, Ith
3.5 mA
Table 6. Specifications of Honeywell HFE4384-422 VCSEL
12
Forward Voltage, Vf
1.8 V
First, the total input current to the VSCEL was established to provide maximum light output
power within the limitations of the IEC60825-1 eye safety standard. Using a maximum power,
Pmax, of 0dBm or 1mW, and  of .15 mW/mA, the total input current was calculated from equation
2.
Pmax
I total  I th 
I total  3.5mA 

(Eq. 2)
1mW
 10.17mA
.15mW / mA
6.67mA of the total input current is used to produce light, while the other 3.5mA is used to
supply the threshold current. The total current Itotal is composed of a bias current, Ibias, which is
equal to Ith, and a modulation (signal) current, Imod. The bias current and modulation current
must fit the relationship below in order keep the VCSEL turned on.
I bias  I th 
I mod
2
(Eq. 3)
While maintaining the input current above the threshold current, Imod was chosen to be 4.2mA
peak-to-peak (p-p) to maximize the modulation current. Using this value in equation 3 results in
I bias  5.6mA
In order to design for Ith possibly being higher than the typical value of 3.5mA, Ibias was chosen
to be 5.9mA. The resulting power output of the VCSEL is given by equation 4.
Pout   ( I bias  I mod  I th )
(Eq. 4)
Pout  (.15mW / mA)(5.9mA  4.2mA  3.5mA)  0.99mW
Once the bias current and modulation current of the VCSEL were established, the passive
component values to be used with the Maxim 3287 Laser Driver were determined. All of the
capacitor and inductor values shown in the schematic in Figure 14 were provided by the Maxim
3287 datasheet [7]. The resistor Rvmatch was chosen as 25 to balance the return path (OUT-)
with the signal path (OUT+). This is necessary because the signal path includes the 25 series
resistance of the VCSEL. The bias current control resistor, Rbias, was determined by Ohm’s law
as follows in equation 5.
Rbias 
Vcc  V f
I bias
13
(Eq. 5)
Rbias 
5V  1.8V
 542
5.9mA
The value of the modulation control resistor, Rmod, was calculated according to design equations
[7]. Assuming that the load of the VCSEL on the laser driver is 25


 1.06 
1.15
  (1  4.0  10 3 (T  25C ) . (Eq. 6)
I mod  51  
 
 Rmod  250  RTC  250 

When the effects of temperature are not taken into account, RTC is an open circuit and equation 6
reduces to


1.15
I mod  51  
.
R

250

 mod

(Eq. 7)
Using Imod = 4.2mA,
Rmod 
51  1.15
 250  13.71k .
4.2mA
A 25surface mount resistor was used for Rvmatch, a 1k potentiometer for Rbias, and a 100k
potentiometer for Rmod.
Optical Receiver Design
400pF
Figure 8. Schematic of Optical Receiver
The optical receiver circuit was designed according to the material given in the lecture
slides by professors Brooke and Jokerst, along in the MAX3266 datasheet [6]. The only change
that was made from the class-provided receiver circuit was to replace the electrical signal input
with an optical signal input. This was achieved by connecting the anode of a PD to pin 3 and the
14
cathode to pin 4 of the MAX3266 TIA. A 1000pF capacitor was placed from the cathode of the
PD to ground to suppress noise in the input signal.
Printed Circuit Board Layout
The next step in the design project was to take the transmitter and receiver circuits and to
create the layout of the Printed circuit board (PCB). ExpressPCB, the company that was used for
PCB fabrication, provides free layout software to the customers. After the layout was completed
utilizing a specific design procedure that is discussed next, the design was approved by Prof.
Brooke, and then submitted to Express PCB for fabrication.
The PCB is made of the dielectric FR-4 (flame retarded heat class 4) epoxy glass with 1/2
copper cladding. The size is 2.5 * 3.8 inches, and it uses plated through holes (vias) to connect
both sides of the board together. The exposed copper is covered with a tin-lead reflow. The
board design includes 0.02in. minimum diameter holes and .012in. line widths. The dielectric
constant for FR-4 is relatively high from 4.2 – 5. A problem when using FR-4 at 1GHz is the
attenuation, which is 7dB/m (89% loss per meter). This is a concern for high-speed data transfer
circuits such as the transmitter and receiver designed in this project, especially at higher 3rd and
5th harmonic frequencies. The attenuation in the lines effectively reduces the input signal range,
which can be seen in an eye diagram. Possible improvement to the design could be achieved by
decreasing the dielectric constant, thus reducing the loss, by using a ceramic substrate such as
LCP instead of an organic substrate like FR-4 .
The ExpressPCB software has the capability to rotate parts, add vias (automatically add top and
bottom pads for vias), cut, paste, create new components, and place traces. The software does not
have the capability to analyze the circuit, to easily copy and paste parts on different metal layer,
or to rotate parts by a specified angle (other than 90 degrees). However, for the price (free) it is
acceptable.
The PCBs arrived approximately 3 days after the order. A picture of the front and back
of the circuit board before any assembly took place can be seen at
http://www.ece.gatech.edu/academic/courses/ece4006/fall2002/group5/nakedboards.html.
Optical Transmitter and Receiver Implementation
All of the components of the transmitter-receiver system were soldered onto the boards
that were fabricated by ExpressPCB. However, some complications arose in the construction of
the boards due to design and PCB layout errors. On the receiver board, the holes for the
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photodiode leads were oriented the wrong way, so the photodiode was installed on the back of
the board to accommodate. A 400pF capacitor was not available to connect from the cathode of
the PD to ground, so this was replaced with a 1000pF capacitor. On the transmitter, the pad for
pin 2 on the MAX3287 laser driver chip was left unconnected, when it should have been
connected to a ground point (GND). This error was easily corrected by soldering pins 1 and 2
together. The last change in the design was made because the Honeywell HFE4382-422
connectorized VCSEL could not be shipped in a reasonable amount of time. A Metrodyne
MTR300-N10 connectorized VCSEL with similar specifications was used instead.
The two potentiometers in the transmitter, Rmod and Rbias, were set to the design values
calculated for the Honeywell VCSEL since the replacement VCSEL was very similar. In testing
the boards, as described in the next section, the resistance of each of these potentiometers was
optimized to yield the most open eye diagram with the least amount of jitter possible. Rmod was

changed from 13.71k
bias
was changed from 542 to 530 . Substitution
of these resistor values back into equations 7, 5, and 4, and assuming typical values of Ith, Vf, and
Rseries of the VCSEL, results in the following values.
I bias  6.0mA
I mod  5.66mA
Pout  .383mW
Testing of Connectorized Optical Transmitter and Receiver
The transmitter and receiver were tested both independently and together, as a system,
using the following setups.
GTS1250
Optical OUT
PD
RX Board
Electrical OUT
Optical IN
GTS1250
RX OUT
Electrical OUT
GTS1250
Electrical out
Electrical IN
TX Board
TX Board
PD
RX Board
VCSEL
VCSEL
Electrical out
Oscilloscope
Oscilloscope
Oscilloscope
Setup A
Setup B
Setup C
Figure 9. Test Setups for Connectorized Transmitter and Receiver.
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In Setup A, the independent receiver test, the Tektronix GTS1250 optical transmitter
output is connected to the PD on the receiver with a fiber-optic cable. The electrical outputs of
the receiver are connected to the oscilloscope, and eye diagrams of the received data were
recorded.
In Setup B, the independent transmitter test, the GTS1250 electrical signal output is
connected to the electrical input of the transmitter. The optical output of the transmitter
(connectorized VCSEL) connects to the optical receiver input of the GTS1250 with a fiber-optic
cable. The electrical RXout port on the GTS1250 is then connected to the oscilloscope, and eye
diagrams of the received data were recorded.
In Setup C, the TX-RX system test, the GTS1250 electrical signal output is connected to
the electrical input of the transmitter. The optical output of the transmitter (connectorized
VCSEL) connects to the optical receiver input (connectorized PD) with a fiber-optic cable. The
electrical outputs of the receiver are connected to the oscilloscope, and eye diagrams of the
received data were recorded
Each test was done using a short fiber-optic patch cable as well as a 100m section of the
same type of cable. All of the eye diagrams were recorded using a persistence time of 2.0s.
Photos were taken of the circuits and of the transmitter test setup (setup B), and they are
available online at
http://www.ece.gatech.edu/academic/courses/ece4006/fall2002/group5/TXRXconn.html.
Modified Transmitter Design
A second layout of the optical transmitter was created to examine the effects of trade-offs
in design objectives. On the first board layout, all of the signal path lines were created so that all
bends in the lines were at 45o angles, and so that sufficient space was allowed among the signal
lines. On the second layout, the total size of the circuit and the number of microstrip lines were
minimized without restricting the use of angles to 45o. The length of the signal path through the
VCSEL was also minimized. The second optical transmitter (TX2) was tested using the same
procedure described in the previous section.
Unconnectorized Receiver Alignment Testing
The set up that is required to test the alignment tolerance of the receiver includes a micrometer,
which holds the unconnectorized fiber in place and moves it in the x, y, and z direction, a
GTS1250 test system, and a TDS7154 Digital Phosphor Oscilloscope. The completed set up is
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shown at
http://www.ece.gatech.edu/academic/courses/ece4006/fall2002/group5/TXRXunconn.html
The first step is to mount the unconnectorized receiver board on a steady surface. This required
drilling holes in the board using a drill press. Up-close pictures of the board is located at
http://www.ece.gatech.edu/academic/courses/ece4006/fall2002/group5/TXRXunconn.html.
After the board is steadily mounted and connected to the oscilliscope, the next step is to align the
unconnectorized fiber, which is connected to the GTS test system, with the photo detector on the
receiver board. Once a signal is found, the next step is to find the alignment that gives the best
eye diagram. This is achieved by moving the micrometer that holds the fiber in all directions
until the best eye is seen. The alignment of the fiber, and a picture of the best eye can be seen at
http://www.ece.gatech.edu/academic/courses/ece4006/fall2002/group5/TXRXunconnData.html.
The next step in the alignment testing is to record all values of the x, y and z, which will be
referenced to zero. Then with the x direction held steady, the y direction is moved slowly in
either direction until ten dots are seen in the middle of the eye with a persistence of 10sec. An
example of ten dots is seen in Figure 10.
Figure 10. Example of eye diagram with 10 dots inside (Click to enlarge).
This distance is recorded, and the y-direction is returned back to the original position. Then the
same thing is done in the other direction. Next, the x-direction is moved while the y-direction
remains constant. After these four distances are recorded, four other positions are found in
between these x and y extremes. After all of these distances are found, all positions are returned
to the original position, and the z-direction is moved a distance away. The procedure is then
repeated for this new z-direction.
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Unconnectorized Transmitter Alignment Testing
The set up required for the unconnectorized transmitter is very similar to the one used for the
receiver with minor changes. The receiver in the GTS test system is not sensitive enough to
receive the signal coming from the transmitter being tested, so the receiver that was tested earlier
is used. The test setup for the transmitter is similar to the receiver set up. Up-close pictures of
the transmitter board can be seen at
http://www.ece.gatech.edu/academic/courses/ece4006/fall2002/group5/TXRXunconn.html.
After the test set up is finished, the unconnectorized fiber is aligned to the VCSEL on the
transmitter board. However, a signal was never received on the oscilloscope, and the board was
tested briefly in an attempt to localize the problem.
Comparative Analysis Of Connectorized Transmitter Boards
In order to perform a comparison of the two boards, the jitter in the output eye diagrams
from both boards at a specified modulation current was measured. This current was lower than
the ideal modulation current; this was achieved by adjustment of the potentiometers. The ideal
value of Rmod was 11.56k; we adjusted it to 78.5k. As a result, Imod decreased to 0.744mA .
The eye diagrams and jitter in the output were recorded for each board.
RESULTS
Electrical Receiver Results
Important eye diagrams for the receiver test board (electrical input signal) with input
pattern PRBS7 are displayed in Figure 11. To see all eye diagrams for various input patterns and
attenuations, click on the diagrams.
10dB
40dB
Figure 11. Select eye diagrams for PRBS7 pattern with varied input attenuations
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The eye is slightly degraded when the input signal is attenuated by 50dB, but it still fits the IEEE
802.3z mask. When the input is attenuated by 60dB, the output signal from the receiver can no
longer be accurately interpreted as a digital signal.
The output eye diagram for the non-attenuated input signal indicates some jitter that
disappears when the input is attenuated by 10dB. The jitter reappears with a 20dB input
attenuation and then worsens for 30dB attenuation. When the input signal is reduced by 40dB, it
loses its integrity as it passes through the receiver circuit, and the logical 0 and 1 values can no
longer be accurately interpreted.
Connectorized Optical Transmitter and Receiver Results
Photos of the connectorized transmitter and receiver board can be seen at
http://www.ece.gatech.edu/academic/courses/ece4006/fall2002/group5/TXRXconn.html. Eye
diagrams from the test results are available at
http://www.ece.gatech.edu/academic/courses/ece4006/fall2002/group5/TXRXconnData.html
The designed receiver board was initially tested with the patch fiber optic cable using the internal
transmitter of the GTS 1250. The eye diagram was acceptable, but it showed a slight jitter. This
is probably due to the staple wire that was used as a metal strip for one of the electrical signal
path from the photodiode to the TIA. The 100 m cable produced a similar eye diagram indicating
a suitable signal quality.
The designed transmitter board tested with the internal receiver of the GTS 1250 also
displayed acceptable eye diagrams. Using the 100m cable also didn’t show noticeable
attenuation through a much longer cable.
The signal quality was enhanced utilizing both the designed transmitter and receiver
boards. The eye diagrams produced with different cable lengths and digital test patterns are
clearly better with thinner lines and much less jitter, indicating less distortion.
The calculated threshold current for the optimize eye diagram with the Metrodyne
VCSEL was 4.52mA, which is very close to the specification value of 4mA. The total current
into the VCSEL of Ibias + Imod + Ith is 16.18mA, which also fits within the calculated range of
acceptable total current of 14mA to 26mA from the optical link budget.
Unconnectorized Receiver Alignment Results
Three different z-direction distances were tested, and the data was collected and put into graph
form in Figure 12.
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Receiver Alignment
150
100
Y
50
Z=0
0
-100
-50
-50 0
50
100
Z=242
Z=442
-100
-150
X
Figure 12. Receiver Alignment Test Results (click for larger chart with plot points).
It was expected that the further up the fiber was, the smaller the circle should have been.
However, it is seen that this is not exactly the case. There is some offsets where the farther
distance is better in one direction than the closer distance. One reason why this could have
happened is that the photo detector could have come from the lower end of the specifications
provided from the vendor. Another reason is that the photo detector was not mounted
completely level, as is shown in Figure 13.
Figure 13. Fiber alignment and mount of photo detector (click to enlarge).
This could have had a significant effect on the results because with the fiber being aligned
directly perpendicular to the board, the signal would have been going in to the photo detector at
an off-angle, causing the alignment to be off in one direction, namely the +x, +y-direction.
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Comparative Connectorized Transmitter Results
The jitter in the output from the first board at the specified current was 120ps, and that of
the second board was 195ps. The eye diagrams can be found at
http://www.ece.gatech.edu/academic/courses/ece4006/fall2002/group5/compare.html
This led to the conclusion that the first board performs better at lower modulation currents.
ISSUES AND DIFFICULTIES
Throughout the course of the project period, the group encountered several issues and
difficulties that resulted in delayed progress. These issues dealt mainly with acquiring
appropriate parts and with group management. The first difficulty encountered with parts was
finding a wide variety of VCSELs and PDs with SC connectors. Once components were found,
the vendors of these parts only wanted to sell in large quantities. One way to overcome this
obstacle was to request free samples. After having placed orders or free sample requests,
shipping/delivery times were longer than expected. One package arrived without the
connectorized VCSEL that was ordered. The vendor offered to send a similar item as a
replacement at no charge due to the initially requested item being out of stock.
Bootleg soldering was done in order to fix any mistakes that were unforeseen or to correct
overworked part that would become normally unusable. Two examples were the staple, shown in
Figure 14, used in the RX board that became unusable and needed to be replaced. After finding
out the PD was placed incorrectly on the design, and multiple times reheating the traces to
change the PD pin connection, the traces delaminated from the board.
Figure 14. Up-close picture of staple.
The other small problem came from a missing copper patch that was found on the second TX
board and a resistor lead was used to correct the problem shown in Figure 15.
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Figure 15. Up-close picture of use of resistor lead.
This error came around from sending data multiple times to each other threw the web mail
server. This problem probably could have been solved if we used a program with the capability
to create a netlist. A net-list gives you the capability to make sure that only one net is connected
to each other and also that no two net short each other out.
Several unplanned issues with group management also caused delays in the group’s
overall progress. The first was the difficulty in scheduling regular meetings that all of the group
members could attend. A meeting time was established on Wednesday evening when no one had
any other regularly scheduled responsibilities. Group work was also done in short periods
following the class period. With a regular meeting time in place as well as time during and after
class periods, there were still two group members who didn’t attend most meetings. Attempts by
the four active members to communicate with these two members included numerous emails and
several phone calls, most of which were unsuccessful in reaching the goal of maintaining a
coherent group in which all members are active and informed. An unbalanced workload resulted
from varying levels of effort and attendance by each group member.
The burnout of the GTS1250 transceiver module caused a significant delay in our testing
regime. Given that lab and testing time was being scheduled, we lost our time slot and were not
able to get another one for several days. This failure forced the extension of the testing period by
approximately a week. Further, prior results on receiver alignment were also suspect and needed
to be rechecked.
Availability of fibers for testing also caused several delays in our testing procedures.
Since cleaved fibers are fragile – having a diameter of only 62.5μm and made of glass – they
tend to break fairly easily. As such, even with careful handling fibers are likely to break fairly
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often, particularly since several groups were utilizing the lab. However, the lack of available,
cleaved fibers for testing purposes caused a few scheduled lab times to become useless for our
group, as we required them for testing. The procedure for getting the fibers recleaved was
somewhat murky at first – they were reported to one or both of the professors, who then asked
the graduate students to recleave them. The system eventually switched to the groups directly
informing the graduate students, which resulted in much faster resolution of the problem.
However, this served to cause confusion and delays for several weeks. The conclusion that
results is that systems for resolving laboratory issues directly – i.e. connecting the two parties
involved in the resolution (problem reporter and problem solver) without intermediaries.
The last issue deals with the unconnectorized transmitter. Since the unconnectorized
transmitter testing was done at the end of the term, there was not enough time and resources to
fix the board.
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REFERENCES
[1] Introduction to Gigabit Ethernet, Cisco Systems Technology Brief, 1998,
http://www.cisco.com/warp/public/cc/techno/media/lan/gig/tech/gigbt_tc.htm
[2] Why Optical Ethernet? Nortel Networks,
http://www.nortelnetworks.com/corporate/technology/oe/index.html
[3]. IEEE Std 802.3, 2000 Edition
http://standards.ieee.org/reading/ieee/std/lanman/restricted/802.3-2000.pdf
[4] Tektronix GTS1250 Datasheet
http://www.tek.com/Measurement/Products/catalog/gts1250/2GW_13513_1.pdf
[5] Maxim 3264 Limiting Amplifier datasheet
http://pdfserv.maxim-ic.com/arpdf/MAX3264-MAX3765.pdf
[6] Maxim 3266 TIA datasheet
http://pdfserv.maxim-ic.com/arpdf/MAX3266-MAX3267.pdf
[7] Maxim 3287 Laser Driver datasheet
http://pdfserv.maxim-ic.com/arpdf/MAX3286-MAX3299.pdf
[8] Honeywell HFE4384-522 VCSEL datasheet
http://content.honeywell.com:80/vcsel/pdf/HFE4383,4384.pdf
[9] Honeywell HFE4084-322 VCSEL datasheet
http://content.honeywell.com:80/vcsel/pdf/HFE4083,4084.pdf
[10] Honeywell HFE4381-521 VCSEL datasheet
http://content.honeywell.com:80/vcsel/pdf/HFE4380,4381.pdf
[11] Metrodyne MTR3100-N10 VCSEL datasheet
http://www.metrodyne.com
[12] LasermateRSC-M85A306
http://www.lasermate.com/ReGaAs.html
[14] Modulating VCSELS. Application Sheet
http://content.honeywell.com:80/vcsel/technical/006703_1.pdf
[15] “How to Solder”
http://www.aaroncake.net/electronics/solder.htm
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[16] Dissertation: Simulation of Heterojunction Bipolar Transistors. Vassil Palankovski.
http://www.iue.tuwien.ac.at/publications/PhD%20Theses/palankovski/node43.html
[17] Laser Safety Fact Sheet.
http://www.uky.edu/Services/EHS/radiation/laser_fs.html
[18] FDA PERFORMANCE STANDARDS FOR LIGHT-EMITTING PRODUCTS
http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfCFR/CFRSearch.cfm?FR=1040.10
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