With the insatiable need for more compute power comes the need to increase interconnect bandwidth while decreasing costs. PAM4 (Pulse Amplitude Modulation – 4 Level) is one of the more common modulation formats used for data communications. PAM4 encodes two bits of information into four intensity levels (00, 01, 10, or 11), thus doubling the information transferred per clock cycle as compared to NRZ (non-return-to-zero), which carries only 1 bit per symbol (either a 0 or a 1). This enables engineers to use lower-cost components while also transferring twice the amount of information as compared to traditional NRZ modulation.
There are other modulation schemes such as coherent modulation that can carry greater amounts of information over longer distances than NRZ and PAM, but the transmitter and receiver designs become complex and add to the system cost. Since PAM4 is an intensity modulation method, a direct-detection receiver can detect the received PAM4 signal without adding complexity and latency of coherent receivers. As a result, PAM4 has attracted immense amount of interest for high-speed optical interconnect applications such as PCIe6 .
Two of the most common performance metrics in NRZ transmitter design are extinction ratio and the transmitter dispersion penalty (TDP) at maximum dispersion. In PAM4 transmitters, the equivalent quantities are optical modulation amplitude (OMA) and transmitter dispersion eye closure penalty quaternary (TDECQ) . Following is an overview of a PAM4 Photonic IC (PIC) transceiver design and the TDECQ measurement options in the Synopsys OptoCompiler™ design platform .