Optical Module Technology Explanation: PAM4 Technology Overview


At present, the optical transmission network generally adopts the non-return-to-zero (NRZ) code transmission method, but when the transmission rate exceeds 28Gbit/s, the system will have strict requirements on the time margin of the transmission link and the transceiver chip, and the high-bandwidth signal will be transmitted in the channel. The losses in the area also significantly increased. This requires us to change the selection of the modulation format of the channel transmission signal. The PAM4 (fourth-order pulse amplitude) modulation method has become the optimal choice as the next-generation access network high-speed signal interconnection technology, because PAM4 uses the fourth-order electrical. In other words, at the same baud rate, the transmission rate of the PAM4 format signal is twice that of the NRZ format signal, and the higher bandwidth efficiency of the PAM4 format will gradually replace the NRZ format. This article is a bit hard-core. If you are interested in the technology of optical module modulation, please read it carefully.


PAM4 pattern


Features and Benefits of PAM4


There are three ways to improve the transmission rate of optical communication, increasing the modulation rate, increasing the number of WDM channels, and using signals with higher modulation formats. PAM4 is a kind of PAM modulation technology, which uses 4 different signal levels for signal transmission, and each symbol period can represent 2 bits of logical information (0, 1, 2, 3), that is, a unit time is four levels. Compared with NRZ (Non-return-to-zero) signal transmission technology, NRZ signal transmission technology is the most used in traditional digital signals, that is, two signal levels, high and low, are used to represent the 1/0 information of the digital logic signal to be transmitted. Each signal symbol period can transmit 1 bit of logic information. Then at the same baud rate, the transmission rate of the PAM4 modulated signal is twice that of the NRZ modulated signal. As a new high-speed serial bus encoding mode, PAM4 exists in almost all standards over 50Gbps. Figure 4.1 below shows the difference between the eye diagrams of NRZ signals and PAM4 signals.


2 Types of PAM4 modulation


Generally, the techniques for realizing PAM4 modulation are divided into two categories, namely, the digital DAC implementation method based on DSP or the Combine method based on analog. The mainstream analog method is based on the MSB+LSB Combiner to realize the PAM4 signal, and there are two NRZ signals for the addition operation. The mainstream digital method is based on high-speed DAC for fast output of 0/1/2/3 level. For PAM4 signal generators, there are currently 3 popular methods for generating PAM4 output signals.


3 ways of PAM4 Signal Generation


Passive coupling of two NRZ pattern generators

The early bit error detectors used two channels of NRZ high-speed pattern generator output signals, and then combined them into one channel of PAM4 signal through passive radio frequency devices (that is, through attenuators and synthesizers). PAM4 signal generators are all extended in this way. The specific implementation of this method is to rely on the NRZ code generator to output two NRZ signals with exactly the same code rate, connect it to an external passive broadband synthesizer through a high-frequency cable, and adjust the phases of the two signals so that the initial phases are exactly the same. Then adjust the attenuator, one of the amplitudes is full amplitude (as the MSB of PAM4), and the other is half amplitude (as the LSB of PAM4). Flat PAM4 signal. From the architectural point of view, it is actually a combination of two NRZ pattern generators, and there are inevitably some limitations. However, when the two NRZ outputs are connected to a wideband power combiner, the performance will inevitably be affected due to signal attenuation, such as jitter, rise time, and additional noise due to poor linearity. At different rates, in order to ensure that the initial phases of the two NRZ outputs are consistent, it is always necessary to connect to an oscilloscope for tedious de-skew calibration each time. The synthesized PAM4 pattern generator index will be seriously deteriorated.


AWG-based PAM4 pattern generator

The arbitrary waveform generator AWG generates the PAM4 output. The principle of PAM4 signal generation is similar to that of the above-mentioned active DAC, except that in general, the AWG is a general-purpose signal source with a built-in high-speed DAC, which can flexibly generate various modulations Signals, such as QAM signals for communication, or PAM4, PAM8 signals. Generally, the settings caused by AWG-based PAM4 signals are more complicated. The biggest obstacle to setting is that it is not intuitive and impossible to directly and conveniently change the speed, rate and pattern. For basic requirements such as transmit equalization in many practical applications, it is very complex and cumbersome to generate through AWG. From a cost point of view, because the PAM4 output generated by AWG requires high bandwidth and high sampling rate, and the software that specifically generates PAM4 patterns is coordinated, the cost per channel is also very expensive compared to other methods. In the early days of PAM4 as a high-speed communication code, AWG-based PAM4 was a compromise because there was no other solution. With the increasing popularity of PAM4 designs, the electrical characteristics of PAM4 signals generated by AWG are different from actual high-speed serial circuits. , and the basic hardware and options are expensive and have been gradually replaced.


Direct PAM4 output pattern generator

The method of directly building the PAM4 signal generator into the BERT. Due to the limitations of performance and function of the above three PAM4 pattern generators, and the complexity and tediousness of building a test environment, the latest PAM4 BERT adopts a simplified architecture with a directly built-in PAM4 signal generator, the core of which is to remove various external cabling and calibration, simplifying the hardware environment, converting clock synthesizers, multi-channel NRZ pattern generators, and high-frequency hardware for complex NRZ to PAM4 conversion, all separate components are directly integrated into one body, and through built-in Calibration enables automatic calibration and compensation for different rates. For the PAM4 signal generator, it can provide excellent signal integrity because there is no external various passive or active equipment and signal degradation caused by cable matching and attenuation. And it can easily achieve 1-channel to multi-channel configuration without any external devices for use.


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