Gigabit
Infrared Optical Communications Physical Layer
Design-1394b
ECE 4006C Design Report (Revised)
Zubair P. Siddiqui,
Mubin Ansari,
Introduction
1394b is the improved version of IEEE – 1394. 1394b is basically designed to
optimize the performance of bit rate transfer while at the same time keeping the cost low.
Our 1394b project is basically divided into three fundamental sections. They are the
transmitter, opto-electroncis, and receiver sections.
This report focuses on design details for implementing the transmitter section of
our 1394b project. The main topic for discussion is the Maxim 3287 evaluation kit. The
specifications of the Evaluation board and the chips, which will be used in our design, are
discussed in detail. The MAX3287 VCSEL evaluation kit that we will be using is fully
assembled. It is basically a surface-mount demonstration board that allows optical and
electrical evaluation of the MAX3287 1.25 Gbps laser drivers in the common-cathode
configuration required by a VCSEL. Basically a common-cathode configuration is when
the cathode of a laser is connected to the ground and the laser itself is driven at its anode.
Although, the Maxim board is fully assembled, changes will be made to match our needs
to build a circuit that is suitable for operating the transmitter.
In addition to the above discussion, this report lays the recommendation for the
development of our own board. This will be done in the final phase of our project. This
section will focus on suggestions for building our own board.
General Specifications & Features
The MAX3287 is a high-speed laser driver for fiber optic LAN transmitters. It is
optimized for gigabit Ethernet applications. It is comprised of a bias generator, a laser
modulator and comprehensive safety features. An additional automatic power control
(APC) circuit adjusts the laser bias current to maintain average optical power at a
constant level, irrespective of temperature or laser properties.
Operating on a supply voltage of 3.0 to 5.5 volts, the MAX3287 is optimized for
operation at 1.25Gbps and can switch 30mA of laser modulation current. Other products
of the same family, such as the MAX3296 are optimized for operations at 2.5Gbps.
The safety features incorporated in the MAX3287 provide for single-point fault
tolerance. They include dual enable inputs, dual shutdown circuits and a laser-power
monitor. These safety features are essential to detect faults that could cause dangerous
light output levels.
Detailed Description
As mentioned before, our laser driver is composed of a bias generator with APC, a
laser modulator, safety circuitry and a power-on reset (POR) circuit. The functional
diagrams of figures 1 and 2 illustrate this layout.
Figure 1. Simplified Laser Driver Functional Diagram.
Figure 2. Laser Driver Functional Diagram
Bias Generator
The bias generator circuit is made up of a power-control amplifier, a controlled
reference voltage, a smooth-start circuit and a window comparator. An outline of this
circuit is shown in figure 3.
Figure 3. Bias Generator Circuitry.
The BIASDRV output of the generator circuit drives an external PNP (for
common-cathode lasers) transistor. This transistor in turn, provides the DC bias for the
laser. Incase of a fault, the BIASDRV output is disabled. This ensures that the transistor
driving the laser is switched off and the DC bias for the laser is removed. If the laser
package is equipped with a monitor diode (MD), the APC circuitry adjusts the laser bias
current to maintain a constant laser power output with respect to changes in temperature
and laser properties. In order to extend the life of the laser and to comply with safety
requirements, the smooth start circuitry prevents current spikes to the laser during powerup or enable stages.
Modulation Circuitry
As shown in figure 4, the modulation circuitry is made up of an input buffer,
current generator and a high speed current switch. Together, these components can drive
a current of up to 30mA into a 25 ohm load.
Figure 4. Laser Modulator Circuitry.
The resistors at the MODSET and the temperature coefficient (TC) pins set the
amplitude of the modulation current. Rmod, the resistor at MODSET programs the
temperature-stable portion of the modulation current; Rtc, the resistor at TC programs the
temperature increasing portion of the modulation current.
Safety Circuitry
The safety circuitry on the MAX3287 implements two popular safety systems. A
simplified version of the circuitry is shown in figure 5. The APC maintains laser safety
using local feedback - the safety features monitor laser driver operation and force a
shutdown if a fault is detected. The shutdown is maintained until a reset is encountered
(either by one of the enable pins or by power on/off).
Figure 5. Simplified Safety Circuit Schematic.
The other safety mechanism implemented by the circuitry is the Open Fiber
Control (OFC). This mechanism uses interlocks to prevent eye hazards. The dual enable
and fault inputs help in implementing the OFC standard. The different modules that make
up the safety circuitry are as follows.
Pulse Generator
During the startup stage for the laser, no light is emitted. This causes the APC
loop to be incomplete (open) and a fault is triggered. In order to eliminate this porblem,
an internal fault delay pulse disables the safety system for a programmable period of time.
This allows the driver to begin operation.
Fault Detection
In order to properly implement the safety features, the MAX3287 fault detection
system monitors all critical nodes for safety faults. If any node voltage is significantly
different from its expected value, a fault is triggered and the laser is shutdown.
Shutdown
MAX3287 incorporates dual redundant bias shutdown mechanisms. An optional
external MOSFET semiconductor is derived by the SHDNDRV output. The bias and
modulation drivers have separate, internal disable signals.
Latched Fault Output
As mentioned before, two complementary FAULT outputs are provided in the
circuitry. These outputs latch whenever a fault is detected. They stay in the latched
condition until either the power is reset, or the enable (EN) pins are toggled.
Power-on Reset (POR)
The simple function of the POR circuitry is to shutdown the laser whenever VCC
is stabilizing or is out of the operational range (this can be adjusted by the LV pin). The
circuitry for this section is shown in Figure 6.
Figure 6. Power-on Reset Circuit.
Programming the Bias Current (Common Cathode with Photodiode)
As mentioned before, the BIASDRV output of MAX3287 bias generator drives an
external PNP transistor which in-turn provides the DC bias for the laser. Figure 7 shows
an example of such a setup in which a common cathode with photodiode is used (the
MAX3287 chip is designed to drive a common cathode laser with a photodiode).
Figure 7. Common-cathode Laser with Photodiode.
The external PNP Q1, the laser diode, the monitor diode, Rset and the power
control amplifier form the servo control loop. A capacitor (Cbiasdrv) placed from the
BIASDRV to Vcc ensures low noise operation and completes the APC loop. It also
rejects power supply noise. The time constant determines how quickly the laser bias
current reacts to a change in the average total laser current. A ferrite bead is also
introduced in order to reduce the deterministic jitter.
Self Designed Evaluation Board Proposal
Most of the circuits and features of the evaluation kit explained above are not
required for our project. Since our main concern is minimizing cost and increasing
simplicity, we can easily do away with a lot of things; for example, we do not need to
provide support for different types of VCSELs (with photodiode, without photodiode,
etc.) – only one would be enough. Furthermore, our maxim board provides for biasing a
surface mount laser. We, on the other hand, will be using a low cost VCSEL driven by a
50-ohm transmission line. This VCSEL will be mounted on a board of its own, and will
also have an independent biasing circuit. Hence the biasing features of the maxim board
are also not required.
Figure 8 shows a suggested schematic for the design of our own board.
Essentially, it is the same as the design of our evaluation kit without the unnecessary
parts. The important thing to note is that the outputs are balanced and symmetric; one of
them is terminated by a 50 ohm resistor, while the other is supposed to be connected to
the laser via a 50 ohm transmission line. In the original circuit of the maxim board, one of
the outputs was terminated by a 25 ohm resistor, while the other had a 50 ohm resistor on
it in parallel with the SMA connector. Hence, when connected to a 50 ohm transmission
line, this output would also have an effective resistance of 25 ohms. The problem in this,
however, was that this renders the outputs DC coupled. While it may work with the
biasing circuit on the maxim board, it would drain all the current of an independent laser
bias. Thus we removed one of the resistors and modified the value of the other to have the
capacitor connected directly to the output. This provides for AC coupling at the output.
One point of interest is that the MAX3287 chip is designed to handle common
cathode lasers with photodiodes. Its counterpart, MAX3288 is supposed to be used for
lasers without a photodiode. However, since the laser is going to be biased independently,
it should not be a problem and the MAX3287 will work fine.
According to the Specs, REF
should be connected to MD for
disabling APC. BIASDRV and
SHDNRV can be left open if
biasing is not required. FLTDLY
should be grounded to disable the
safety features. The values for
RTC and RMOD need to be
determined experimentally (for
optimal modulation current)
Figure 8. Proposed schematic for the self designed transmitter board.
Conclusion
The purpose of setting up the transmitter test bed with the MAX3287 evaluation
kit is to verify the workability of the components of our system. Once the maxim board is
tested independently and along with the other modules, we can start working on designing
our own board. Up till now, we have spent a lot of time understanding the circuitry of the
evaluation kits and how the overall system is supposed to work. The next stage is bound
to be more interesting as we will be doing some hands on work