Re: mixed signal
Here the simple exemple of a MASH Delta-Sigma
1) High-level simulation, with matlab for eg.
In this step you simulate both the analog and digital parts of your design. It also means that u need to model the analog part. This is one of the hardest part. A precise model means a long simulation time, and on the opposite simple model means non accurate results. I would say: experience.
You decide at this level the analog architecture, the spec of the circuits, and the digital algorythm
2) Analog Circuit design
Good old analog design....
3) Digital Circuit design
VHDL or Verilog
4) Mixed-signal simu
If u can get the environment to do so it's nice. But it usually take time.
5) Layout
The tricky part. Digital is generally automatically designed. But u need to do the analog part, and interface. Also take care for noise isolation and clock routing.
As for mixers, I'm not sure it's the best mixed-signal exemple. Not so much logic circuit. It's closer to RF, which is another topic in my opinion.
Good luck