Going the Distance, Going for Speed

Currently, the ultimate methodology for transmitting fast signals in today’s Networking, Telecom and even Industrial systems is Fiber Optic Modules along with Fiber Optic Cable. Designers are now pushing copper Interconnect cables and backplane solutions that perform at very high speeds. When speeds above several Gbps are reached, or distances exceed more than five meters, designers have to deal with many of the same issues that Fiber Optic Module engineers experience. This article will investigate the circuitry in a Fiber Optic Module and show how some of this circuitry can be applied to high-speed cable interconnects and backplanes.

Figure 1 illustrates the internal circuitry of a typical SFP Module. In this graphic, signals are sent via a Transmit Optical Sub-Assembly (TOSA). The TOSA is then driven by a Laser Diode Driver chip that maintains a bias current on the TOSA and energizes the laser diode quickly to transmit pulses of light that represents data. On the receiving side, is a Receive Optical Sub-Assembly (ROSA). The ROSA consists of a receiving PIN diode and a Transimpedance Amplifier (TIA). The TIA converts optical energy into an electrical signal.

 

 

When optical links are long, or output power from the laser is low, the result is a small signal swing at the output of the TIA on the ROSA. A Limiting Post Amplifier (Post Amp) is then used after the TIA to provide predictable amplification of the TIA signal regardless of input amplitude.

The primary function of a Post Amp is to amplify small signals with minimal noise and to provide standardized logic levels at the output. Post Amps are able to take differential signals as small as 5mVpp and boost them to standard CML or LVPECL logic levels. These “boosted” levels of the input signal from the ROSA are then able to be reliably decoded by the high speed serialization chips down stream from the Optical Module.

 

Copper Chassis Interconnect

When two or more chassis need to be connected using copper cables, there will be attenuation of the signal amplitude. The amount of attenuation depends upon the cable used, signal speed and cable length. As an example, RG-174 coaxial cable will show an attenuation of 1.3dB/meter @ 1.5GHz. Hence, 10 meters of cable presents 13dB of attenuation. If a differential 400mVpp signal is sent over 10 meters of RG178 cabling at 1.5MHz, the resulting output will be 90mVpp.

LVDS, CML and LVPECL devices begin having difficulty decoding signals below 100mVpp. Therefore, for the example of 10 meters of RG-174, the signals at the far end will be difficult, if not impossible, to decode. As the frequency of the signal through the cable is increased, the useable length will continue to shorten. So, what starts out as a differential 400mVpp signal, can quickly become <100mVpp as speeds increase.

 

Limiting Post Amplifier

A Post Amp will predictably regenerate the input signal to the appropriate CML or LVPECL levels even if the input were to go as low as 5mVpp. A side benefit of using a Post Amp is the availability of a Signal Detect (SD) or Loss of Signal (LOS) pin. This pin will give off an alarm when either the signal is lost (LOS) or when a valid signal is being received (SD). This pin can be adjusted to set a p-p level at which the SD or LOS indication is given thereby adding to the diagnostics features of a high speed system. Figure 2 illustrates a block diagram of a Post Amp and also shows the function that the Post Amp performs.

 

 

Backplanes

Backplane solutions that run up to one meter (including multiple connectors and riser cards) can benefit from the use of Limiting Post Amplifiers. By placing a Post Amp at the destination of a long run, the system designer can effectively clean up faulty eyes of transmitted data or clock. Sometimes, however, there is considerable attenuation of the eye such that the only way to recreate the original signal is through use of a low noise Post Amp. This becomes more important as the speed of the backplane increases.

 

Drivers with Pre-Emphasis

As signals approach 4Gbps to 6Gbps, the use of Post Amps for recreating the attenuated signal may no longer be sufficient to meet Signal Integrity issues. To that end, new devices have been introduced on the market with Pre-Emphasis to drive longer runs of etch. These parts amplify the rising edge of the signal for a short time to increase the slew rate as the driver pushes the signal down the backplane. Figure 3 shows how a driver with Pre-Emphasis works. Devices that have adjustable levels of amplification on the rising edge as well as control over the duration of the amplification provide the maximum amount of flexibility.

 

 

Receivers with Equalization

Another solution for dealing with signals that are running at a high speed or traveling a long distance is to add Equalization (EQ) at the destination of the signal. Adding EQ to a receiver is not a new concept; EQ has actually been done either actively or passively on video and high speed communication systems for many years. The benefit of using EQ is that it reduces reflections and compensates the transmission media in order to attain the best possible reception of incoming data.

When Pre-Emphasis and EQ are used in combination on a backplane at speeds above 4Gbps, the outcome can be dramatic. Figure 4 shows data taken with a 6.4Gbps 2²³ -1 PRBS pattern into 1 meter of FR4. These eye patterns where taken at the end of the 1 meter differential trace through two connectors, one at the beginning of the signal, the other at the end of the signal.

 

 

A signal without any Pre-Emphasis or EQ at one meter can be very noisy due to dispersion, reflections and mis-match. After adding Pre-Emphasis, the signal starts to show some signs of an eye. However, the signal is still far from readable. Upon adding the equalization to the receiver, a very solid eye for the 6.4Gbps signal is then realized. These experiments show that receiver EQ is a designer’s best solution to add in high speed signal paths for long runs of etch on FR4. Furthermore, when using Pre-Emphasis and EQ together, the result is the best overall signal with the lowest possible Bit Error Rates.

Additional tests using five meters of Amphenol SkewClear® cable were also performed. The results shown in Figure 5 illustrate how EQ alone can help in reconstructing a useable eye. A 2²³ -1 PRBS pattern was sent down the cable and monitored on the far end. The signal was reconstructed when EQ was added.

 

 

The devices used to provide Pre-Emphasis and Equalization in these experiments were comprised of Micrel’s SY58626 and SY58627 IC solutions. The ability for designers to choose such ICs that extend the reach of their high speed serial connections is now a reality. Continued advances in drivers with Pre-Emphasis and receivers with EQ are clearly going to help build the next generation of high speed interconnect solutions. In addition, designers can now directly apply Fiber Optic Transceivers to cables and backplanes. The Post Amps ability to re-generate very small signal swings to usable data is an invaluable tool that should be given consideration as engineers prepare for their next-generation designs in Networking, Telecom and even Industrial systems.