Serializer
The serializer is the transmit half of a SerDes: it takes a wide parallel word and squeezes it into one fast bit stream, one bit at a time. Picture a freeway with 32 lanes merging into a single lane at a tollbooth that snaps a car through 32× faster — every car still gets through, but now in single file at blistering speed. Practically it is built from a tree of 2:1 multiplexers driven by ever-faster clocks (e.g. 32:16, 16:8, 8:4, 4:2, 2:1), with the final stage handing one bit to the output driver every unit interval.
The serializer's hardest job is the last 2:1 stage and the output driver, which must toggle cleanly at the full line rate (tens of Gb/s) into a lossy channel. Designers add pre-emphasis there — a feed-forward equalizer that deliberately overshoots edges — so the signal arrives at the far chip looking like a clean square wave rather than a smeared blob. The serializer also embeds the clock into the data (via line coding) so no separate clock wire is needed.
A binary tree of 2:1 multiplexers, each stage clocked twice as fast, collapses 32 parallel bits into one serial stream.
The serializer needs a clean, low-jitter multi-phase clock from a PLL; any jitter on that clock directly widens the data jitter the receiver must tolerate.