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Pulse Amplitude Modulation (PAM)
The 400 Gigabit Ethernet (GE) standards define 4-level PAM (PAM4) multilevel signaling as a recommended modulation format to implement serial 400GE data center interfaces. This is an evolution from the two state non-return-to-zero (NRZ) modulation used in 100GE. PAM4 effectively doubles the data rate for a link bandwidth at the expense of reduced signal to noise ratio (SNR). You require new measurements to characterize impairments that were not an issue in previous NRZ designs. We can help you ensure accurate and repeatable results to accelerate the development of your PAM4 interface components and systems.
PAM4 Design and Simulation
Conventional signal impairments such as jitter, noise, channel loss, and intersymbol interference (ISI) behave differently in PAM4 designs. You need to perform electro-optical-electrical simulation of your 400GE designs to analyze the trade-offs between PAM4 and NRZ. We can help you save significant design time and minimize the effects of unexpected impairments.
PAM4 Transmitter and Receiver Test
PAM4 designs are far more susceptible to noise than NRZ designs, since four signal levels are packed into an amplitude swing of two. You must perform new transmitter (Tx) test measurements, such as signal to noise and distortion ratio (SNDR) and output jitter, to characterize your PAM4 transmitter devices. Forward error correction (FEC) is used to correct channel errors in PAM4 links and must be considered in PAM4 receiver (Rx) testing. We can help you address these new PAM4 physical layer test challenges to ensure compliance of your 400GE devices to industry standards.
PAM4 Channel Characterization
As the data rate increases using PAM4 multi-level signaling in serial interconnects, the rise time of the data transition from logic zero level to one becomes faster. This faster rise time creates larger reflections at impedance discontinuities and degrades the signal quality at the end of the channel. As a result, you need to account for signal degradation due to physical layer components such as printed circuit board traces, connectors, cables, and integrated circuit (IC) packages when testing the performance of your PAM4 devices. We can help you characterize PAM4 channel effects to ensure the performance of your 400GE devices.
Further Reading
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