Graduate / Motivation Letter for Ph.D Application (what very specific field I am interested in?) [2]

Hi,

I am supposed to write a motivation letter for my Ph.D. Application but I am at a loss.I did some personal exercises before I came to the decision that I really want to make a Ph.D or not.But I dont understand what very specific field I am interested in.In broad sense I am interested in pursuing a Ph.D in Emebedded Systems but into Embedded Sytems there are many specific lines one can opt for like FPGAs,System on chip , cognitive interaction.During my Masters I chose courses in two different fields one was embedded systems and other biomedical image processing(try a new option).Basically my master thesis also focuses on both the fields.The system design is in elctronics where as in software I intend to implement image processing Algorithms.

This Phd program is also multidisciplinary in nature and requires knowledge from the field of digital architecture and their interaction with the humans.Applications in the field of security, education and health-care.

This is my first experience in writing a phd application and I am not really sure where to start from.

Thanks