Considerations in Interfacing HOTLink II to Fiber Optic Module

Question: What are the considerations in Interfacing HOTLink II to Fiber Optic Module? How can I interface a SFP fiber optic module with HOTLink II?

 

Answer:

The considerations for interfacing HOTLink II to a fiber optic module are ac coupling of the link, biasing of the optical module and termination of the transmission line [serial traces].

At the HOTLink II receiver, the serial input has weak internal biasing. Due to the weak internal biasing, if you AC couple the link between the serial driver [Optical module] and the HOTLink II, then you do not have to bias the HOTLink II at its serial differential inputs. This gives flexibility to the board designer as any serial driver can be interfaced to the HOTLink II receiver by AC coupling the link between the serial driver and the HOTLink II serial inputs at the serial driver and terminating with termination resistor between the serial traces equal to the differential impedance of the transmission line [serial traces]. 

You can interface an optical module to HOTLink II receiver, by AC coupling the link  at the optical module, and terminate with a termination resistor between the serial traces equal to the differential impedance of the transmission line [serial traces] as close to HOTLink II serial differential inputs as possible. For example, if the differential impedance of the serial input traces on your board is 100 ohms, then you can terminate with 100 ohms resistor between the differential serial input traces close to the HOTLink II on your board. Please refer to the Optical Module data sheet for other special requirements in the interfacing.

At the HOTLink II transmitter, you can AC couple the serial link between the serial outputs and the optical module. Depending upon the optical module you are using, you might need to bias at the optical module with the biasing circuit so as to meet the input biasing requirements of the optical module as well as terminate the transmission line [serial traces] at the optical module. For example, if you have an LV-PECL optical module with no internal biasing and your serial output traces are 100 ohms differential traces, then you can bias the optical module inputs and terminate the transmission line [serial traces] with a resistor network close to the optical module comprising of  82 ohms resistor to Vcc and 130 ohms resistor to ground on each serial trace. This resistor network will bias to 2V to meet the LV-PECL signal biasing requirement and terminate each transmission line [Optical module serial input] with 50 ohms terminating impedance at the optical module.

SFP fiber optic modules are commonly used fiber optic transceivers. The MSA complient SFP fiber optic modules have internal biasing and internal termination at the transmitter. They may or may not be AC coupled. If the SFP optical module does not have internal AC coupling, you can interface HOTLink II serial outputs to the SFP fiber optic module by AC coupling the serial link between the HOTLink II and the SFP at the output of HOTLink II. At the HOTLink II receiver, the serial input has weak internal biasing, so you can AC couple the link between the SFP optical module and the HOTLink II at the SFP, and terminate with a termination resistor between the serial traces equal to the differential impedance of the transmission line [serial traces] as close to HOTLink II serial inputs as possible.

Please refer to the Optical Module data sheet for other special requirements in the interfacing.