# A 112 Gb/s DAC-Based Duo-Binary PAM4 Transmitter in 28 nm CMOS
## 2 Basic Theory of SerDes  
The high-speed SerDes interface, namely, the high-speed serializer/deserializer (HSS), uses a time-division multiplexing (TDM) and point-to-point communication mode. The SerDes interface and the transmission channel constitute the physical layer devices of the serial data transmission system, as shown in Figure 1. A SerDes transmitter serializes multichannel low-speed parallel data into high-speed serial data and drives the serial signal to the transmission channel. A channel (usually copper wire or optical cable) connects the sender and receiver and acts as a carrier of signal transmission. The receiver samples the signal received from the channel, restores the high-speed serial data to the low-speed parallel data, and then completes the whole process of data transmission.  
When testing a high-speed serial communication system, jitter and bit error rate should be paid more attention to. Jitter refers to the phenomenon that the jump edge of a signal deviates from its ideal or predetermined time. Nonideal circuits, nonideal channels, and noise are the causes of jitter. The total jitter of data can be divided into two  
Figure 1: SerDes system block diagram.  
categories, namely random jitter (RJ) and deterministic jitter (DJ). Random jitter is caused by unpredictable noise in the circuit. Deterministic jitter refers to a repetitive jitter whose behavior is predictable and is caused by certain deterministic factors, such as intersymbol interference, channel reflection, and channel bandwidth; these noises can be corrected by the circuit. Deterministic jitter can be divided into periodic jitter (PJ) and data-dependent jitter (DDJ) according to their generation mechanism. Periodic jitter refers to the jitter that repeats periodically. Because any periodic waveform can be composed of multiple sinusoidal signals, periodic jitter is also called sinusoidal jitter. Periodic jitter has nothing to do with data code type. It is generally caused by external noise (electromagnetic interference, spread spectrum clock interference, etc.) coupled to the system. Data correlation jitter is caused by the nonideal factor of the channel. Because different code types contain different frequency components, and the attenuation and delay of different frequency components of the channel are different, a nonlinear phase shift will occur. Therefore, the nonideal factors of the channel will produce a deterministic jitter related to data. In addition, the impedance discontinuity in the channel will reflect the signal, and the crosstalk of adjacent channels will cause the data jitter. Bit error ratio (BER) refers to the ratio of the number of received error bits to the total number of transmitted bits within a certain period. It is an important indicator to measure the link reliability of transceivers. In the process of signal transmission due to noise, loss, and other nonideal factors, the error code is inevitable, but as long as it is lower than a certain degree, it can be recovered by the algorithm. Different protocols require different bit error rates, but most systems require less than \(10^{-12}\).