Specifications

Reference Design HFRD-25.2 (Rev.8; 01/09) Maxim Integrated Products
Page 3 of 42
3 Reference Design Device
Details
HFRD-25.2 was engineered to meet the
requirements of GEPON ONT/ONU transceiver
applications (Figure 1) operating at 1.25Gbps
burst upstream and 1.25Gbps continuous
downstream. The design can also be easily
adapted to other PON applications.
The burst-mode upstream transmitter is
implemented using the MAX3643 laser driver and
the Atmel ATTiny24 microcontroller (μC). The
downstream receiver consists of the MAX3747
limiting amplifier. The laser, photodiode, and TIA
are housed in an optical subassembly provided by
Xponent Photonics, completing the ONT/ONU
transceiver.
3.1 MAX3643 - Laser Driver
The MAX3643 burst-mode laser driver provides
bias and modulation current drive for PON burst-
mode ONT applications. When BEN is inactive,
typical modulation and bias currents are 5µA
each.
Laser modulation current can be set from 10mA to
85mA and bias current can be set from 1mA to
70mA using the MODSET and BIASSET inputs.
A sample-and-hold circuit is provided to capture
the monitor diode output during short PON bursts.
If needed, the BEN high-speed signal is mirrored
on an LVCMOS output (BENOUT). For
additional information see the MAX3643 data
sheet available on the web at www.maxim-
ic.com/MAX3643.
3.2 MAX3747 - Limiting Amplifier
The MAX3747 multirate limiting amplifier
functions as a data quantizer for OC-3 through
OC-48 synchronous optical network (SONET),
Fibre Channel, and Gigabit Ethernet optical
receivers. The amplifier accepts a wide range of
input voltages and provides a constant-level,
current-mode logic (CML) output voltage level.
The MAX3747 limiting amplifier features a
programmable loss-of-signal detect (LOS) and an
optional disable function (DISABLE). Output
disable can be used to implement squelch. For
additional information see the MAX3747 data
sheet available on the web at www.maxim-
ic.com/MAX3747.
3.3 Atmel ATTiny24 - Microcontroller
The ATTiny24 is a low-power, CMOS, 8-bit
microcontroller based on the AVR enhanced
RISC architecture that achieves throughput
approaching 1 MIPS per MHz. This allows the
system designer to optimize power consumption
versus processing speed.
The ATTiny24 features a multiple-input 10-bit
ADC, internal temperature sensor, an 8-bit and
16-bit timer with two PWM outputs each, an
internal 8MHz oscillator, a 2-wire interface (I
2
C
compatible), a POR, and brown-out circuit. The
ATTiny24 provides 2k flash, 128Bytes of
EEPROM and 128Bytes of SRAM. For additional
information see the ATTiny24 data sheet available
on the web at http://www.atmel.com/
.
3.4 Xponent Photonics XP3-0063
Diplexer
The XP3-0063 Diplexer from Xponent Photonics
is a single fiber-optical component which uses a
Surface Mount Photonics (SMP) PLC platform to
integrate laser, photodiode, filter, and fiber in a
high-performance and cost-effective solution. The
Diplexer includes an upstream 1310nm laser, a
downstream 1490nm PIN/TIA 1R digital receiver,
and the appropriate optical filtering. The package
is designed to enable a single-step solder process
for simple interface to PCB to support high-
volume assembly. For more information regarding
the Diplexer, please call Xponent at:
626.599.8660, email to [e-mail], or
visit their web page
(http://www.hoyaxponent.com/
).