Two port scheme is used for modulating signals, which is divided into high port and low port. Can somebody explain how this works?

 

https://www.qsl.net/kb9mwr/projects/dv/dstar/two_point_modulation_r1.pdf

The purpose of the technique is to extend the frequency response down to DC to accommodate long stings of 1's and 0' in a codeword data stream. This is done by sending a portion of the filter output to the modulation input of the VCO and the other portion goes to the PLL Master Oscillator.