Re: Superheterodyne LO stability,power ,Mixer input impedanc
I think it is more for personal taste how to call direct conversion radios. I like this term more than “zero IF”, but I am okay with any other commonly used name. By the way, term “direct conversion” was introduced around 1966 when this technology was reinvented again and quickly become popular. Actually Heaviside in 1906 (if I remember right) connected generator tuned to receiving frequency to the resonant tank of TRF receiver and got big signal amplification. This was the first direct conversion radio about 10 years before Armstrong invented the super heterodyne. It is very interesting that first radios were actually UWB and even direct conversion despite it was well before CW and super heterodynes.
You are right about mathematical expression for quadrature signal, but there is one caveat. I note one very strange thing: practically all electrical engineers do not know what it is Sin(x) or Cos(x) function. They get used to all math formulas and do not afraid them but they cannot answer very simple question: “What it is Sin function". I am not kidding, it is my observation for several decades of teaching, mentoring and working together with electrical engineers. Almost nobody can answer this question right, therefore have no good understanding of these functions. This is why I prefer to explain simple things without formulas with trigonometry functions.