The GN2255 clock-data recovery chip from Semtech cleans signals in 50 Gbps PAM4 data streams and integrates an optical laser driver.
As 5G, computing, video, gaming, and other applications push ever more data across networks, optical connections must keep up, lest they become the bottleneck. Today, optical networks using PAM4 modulation can push 50 Gbps per lane with higher speeds coming soon. A 400 Gbps Ethernet link, therefore, uses 8×50 Gbps lanes.
Optical modules at the end of fiber-optic cables convert baseband, RF demodulated electrical bit streams to optical and vice versa. Receivers also need to extract clocks from the data, a process known as clock-data recovery (CDR). The GN2255 CDR from Semtech combines CDR with driver circuits to excite the optical lasers in a module.
Dubbed “Tri-Edge” technology, the SiGe-based GN2255 lets engineers use the same optics in 25 Gbps modules but with double the data rate. That’s because PAM4, with its four amplitude levels, sends two bits per symbol as opposed to one for non-return-to-zero (NRZ, also called PAM2) modulation. See Figure 1. When transmitting to the optics, the GN2255 uses analog signal processing to “clean” the electrical signals before sending them to the optics. It does the same on the receive side before sending the module’s electrical signal to the host computer or network element.
Figure 1. NRZ signals send one bit per symbol as where PAM signals send two bits per symbol.
According to Semtech’s Raza Khan, who spoke to 5G Technology World on February 28, using PAM4 also reduces power consumption by 30% as opposed to DSP for CDR functions. Semtech also claims that the Tri-Edge technology adds just 1 ns of latency to the optical module as opposed to adding as much as 100 ns to latency per optical module.
The GN2255 also integrates optical drivers with the CDR circuits (Figure 2). Optical drivers convert signal levels to those needed to drive the module’s laser. That reduces module complexity by eliminating a separate driver IC.
Figure 2, The GN2255 from Semtech integrates clock-data recovers and optical drivers into one device.
Currently wireless radio units demodulate RF signals into NRZ data streams and modulate NRZ streams up to RF transmission. That may change, according to Khan, to where PAM4 signals to and from the optical cables remain as PAM4 to the radio. The GN2255 supports PAM4 signal in both directions. It also supports NRZ in today’s cellular radios.
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