6 GBit/s PAM-4 signal generation with variable gain distributed power combiner

In this work, a proof of concept implementation of a variable gain distributed power combiner used for the generation of four level data signals (pulse amplitude modulation, PAM-4) is demonstrated. Different gain settings in the two combiner branches enable the creation of a four level signal at the output of the combiner from two binary input data signals. Compared to lumped current summing topologies often used in digital-to-analog converters (DACs) and PAM driver circuits, the ultra-high bandwidth capability of the combiner due to the distributed structure enables very high data rates. The prototype implemented on a printed circuit board (PCB) shows a small-signal bandwidth of 3.5 GHz in the high-gain branch, which provides a low-frequency gain of 9.1 dB. In time domain measurements the generation of a 3 GBaud/s PAM-4 signal is demonstrated. Also the pre-distortion capability of the combiner is shown, which is realized by changing the gain difference of the two branches. Since the approach is fully scalable to integrated circuit technologies, the proposed circuit topology has the potential to create PAM-4 signals with ultra-high baudrate.