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P/N: C-13-155(C)-FDFB-SLC8A-G5

155 Mbps Single Mode SFF LC Transceiver

DS1960 Oct1508-Rev A.0

Printed Circuit Board Layout Consideration

A fiber-optic receiver employs a very high gain, wide bandwidth transimpedance amplifier. This amplifier

detects and amplifies signals that are only tens of nA in amplitude when the receiver is operating near it’s limit.

Any unwanted signal current that couples into the receiver circuitry causes a decrease in the receiver’s sensitivity and

can also degrade the performance of the receiver’s signal detect (SD) circuit. To minimize the coupling of unwanted

noise into the receiver, careful attention must be given to the printed circuit board.

At a minimum, a double-sided printed circuit board(PCB) with a large component side ground plane beneath

the transceiver must be used. In applications that include many other high speed devices, a multi-layer PCB is highly

recommended. This permits the placement of power and ground on separate layers, wich allows them to be isolated

from the signal lines. Multilayer construction also permits the routing of signal traces away from high level, high

speed sinal lines. To minimize the possibility of coupling noise into the receiver section, high level, high speed

signals such as transmitter inputs and clock lines should be routed as far away as possible from the receiver pins.

Noise that couples into the receiver through the power supply pins can also degrade performance. It is

recommended that a pi filter be used in both transmitter and receiver power supplies.

EMI and ESD Consideration

Source Photonics’ transceivers offer a metalized plastic case and a special chassis grounding clip. As shown

in the drawing, this clip connects the module case to chassis ground then installs flush through the panel cutout.

This way, the grounding clip brushes the edge of the cutout in order to make a proper contact. The use of a

grounding clip also provides increased electrostatic protection and helps reduce radiated emission from the module

or the host circuit board through the chassis faceplate. The attaching posts are at case potential and may be

connected to chassis ground. They should not be connected to circuit ground.

Plastic optical subassemblies are used to further reduce the possibility of radiated emission by eliminating the

metal from the transmitter and receiver diode housings, which extend into connector space. By providing a

non-metal receptacle for the optical cable ferrule, the gigabit speed RF electrical signal is isolated from the

connector area thus preventing radiated energy leakage from these surfaces to the outside of the panel.