How to connect unused input and output ports such as HMC747, HMC749, HMC746, etc. Is it connected in series with 50 ohms to ground? In other words, how to use a single-ended interface instead of a differential interface?
Unused inputs should be terminated with a similar network with the used inputs. If used ports are AC coupled, then unused ports can be terminated to gnd with DC block cap and 50 ohms. If the used port is DC coupled, then the unused port should be terminated to the high supply rail of the part(GND for parts having -3.3V, Vcc for parts having +3.3V)