CONTENT

 

The best way to transmit high-speed signals in today's networks, telecommunications and even industrial systems is the use of fiber optic modules and fiber optic cables. Design engineers are developing copper interconnect cables and backplane solutions that can transmit data at high speeds. When the speed is more than Gbps, or the transmission distance is more than five meters, the design engineer must solve the design problems faced by the optical fiber module engineer. In this paper, we will discuss the circuit design of optical fiber module, and discuss how to apply it to the design of high speed cable interconnect and backplane.

 

Figure 1 is a typical small size pluggable (SFP) optical fiber module. In this module, the signal is transmitted by sending an optical sub device (TOSA). The TOSA is driven by a laser diode driver chip, which is required to maintain a bias current on the TOSA, and to drive the laser diode to transmit light pulses that represent the data. The receiving end is a receiving optical device (ROSA), which consists of a receiving PIN diode and a transimpedance amplifier (TIA). TIA converts light energy into electrical signals.

 

When the output power of the optical link or longer laser is low, the ROSA TIA output will appear small signal swing after TIA amplification by limiting post amplifier based on TIA signal can be predicted when needed, without considering the input amplitude.

 

 

Figure 1: (a) typical SFP module internal circuit; (b) optical module circuit structure diagram.

 

The main function of the post amplifier is to amplify the signal with the lowest noise and provide the standard logic level to the output. The post amplifier can transmit peak to peak voltage (the voltage referred to below is the peak value) of the differential signal as low as 5mV, and will be amplified to a standard CML or LVPECL logic level. The high speed serial chip behind the optical module can reliably decode the ROSA input signal.

 

When two or more frames need to be connected by a copper cable, the signal amplitude will decay. Attenuation depends on the copper wire, signal speed and cable length. For example, RG-174 coaxial cable attenuation of 1.3dB/ m @1.5GHz. Therefore, the 10 meter cable will produce 13dB attenuation. If the differential signal of the 400mV is sent by the 10 meter RG178 cable at 1.5MHz frequency, the output is 90mV.

 

LVDS, CML and LVPECL devices are difficult to decode signals below 100mV. For example, a 10 meter RG-174 cable will be difficult to decode (even if possible). When the signal frequency is increased by the cable, the cable length is shortened. Therefore, with the increase of signal frequency, the differential signal of 400m before transmission will decrease rapidly to 100mV.

 

The limiting post-amplifier amplifies the input signal to the appropriate CML or LVPECL level predictably, even if the input level is only 5mV. The disadvantage of using a post-amplifier is to use signal detection (SD) or signal loss (LOS) pins. This pin warns when the signal is lost or a valid signal is received. This pin can be adjusted to set the peak-to-peak level, giving an SD or LOS indication to increase the diagnostic function to the high-speed system. Figure 2 shows the structure of the rear amplifier block diagram and the implementation of the function.

 

 

Figure 2: the structure and function of the limiting post amplifier.

The use of a limiting amplifier for the transmission distance of up to 1 meters backplane solutions (including multiple connectors and expansion cards) are good. The post amplifier on a distant location, the system design engineers can effectively remove the eye is fault data or clock. However, sometimes the fault eye greatly attenuated, the only way to reproduce the original signal is a low noise amplifier. This method has become more and more important with the increasing speed of backplane.

When the signal speed reaches 4Gbps~6Gbps, to reproduce the attenuation of the signal may not be sufficient to solve the signal integrity problem by using a post amplifier, so the market appeared new with pre emphasis function device to drive back the longer distance line. When the driver sends a signal to the backplane, the device can amplify the rising edge of the signal in a short time to increase the slope. Figure 3 shows the working principle of a driver with pre emphasis. Devices with adjustable amplitude on the rising edge and controllable duration of amplification have the greatest flexibility.

Another solution for dealing with the challenges of transmitting high speed signals or long distance transmission signals is to increase the equalization of the location of the signal (EQ). It is not a new concept to increase the balance function of the receiver, which has been used for many years in the video and high speed communication system. The advantage of equalization is that it can reduce the reflection and compensate the loss of the transmission medium, so as to get the best effect of the input data.

 

 

Figure 3: schematic diagram of the transmitter with pre emphasis.

When the speed of more than 4Gbps in the back board and the use of pre emphasis and balance when the effect is very significant. Figure 4 is the 6.4Gbps 223-1 eye PRBS mode data transmission in FR4 cable transmission in the 1 meters. The eye diagram obtained in the 1 meters at the end of the two differential cable connector, a signal at the start, one in the end signal.

 

 

Figure 4: the eye through a 1 meter FR4 cable 6.4Gbps 223-1 PRSB data model.

At 1 m, no pre emphasis or equalization signal may be caused by scattering, reflection and impedance mismatch. Increase the pre emphasis, signals began to appear some characteristics of eye, but the signal quality is not good enough. Once the receiver was balanced, eye diagram of 6.4Gbps signal is a very open. These experiments show that receiver equalization is the best solution for design engineers to realize high speed signal transmission using FR4 cable. In addition, when the pre emphasis and equalization are used, the best signal with the lowest bit error rate is obtained.

Another test was performed on a 5 m Amphenol SkewClear cable. Figure 5 illustrates the balance available to help rebuild the eye. 223-1 PRSB mode data is transmitted along the cable and monitored at the far end. When the equilibrium is added, the signal is reconstructed.

 

 

Figure 5:6.4Gbps 223-1 PRSB eye model data into the 5 meter Amphenol SkewClear cable.

 

The devices used to provide pre emphasis and equalization in these tests are SY58626 and SY58627 of Micrel. Today, design engineers can use these IC to extend the distance of high speed serial connections. The development of a driver with a pre emphasis function and a balanced receiver will help create the next generation of high speed interconnect solutions. In addition, the design engineer can now be directly applied to fiber optic transceiver cable and backplane. The post amplifier is very useful for the generation of minimal signal capabilities for the available data, which should be paid attention to by the design engineer in preparation for the design of the next generation of networks, telecommunications, and even industrial systems.