Overview on Noise in Ring Topology Mixers
Technical Overviews
For a classical double-balanced ring-diode mixer, it is expected that the conversion loss (4–5 dB) should match the noise figure. This document explores the formal definition of noise figure and shows the ADS noise figure definition. It is also shown that the noise figure for a mixer does not have to exceed its conversion loss, and that loss in the mixer baluns is an important source of noise that must be considered.
Throughout this discussion we assume a low-side injection, down-converting mixer, although the issues discussed are equally applicable to any mixer. RF is the input signal, LO is the local oscillator, and IF is the output signal. In terms of frequencies, RF=LO+IF and there exists an input image IM=LO–IF that can also mix down to the same IF as the RF. We are not concerned with the output image signal LO+RF.
Throughout this paper we will shift nomenclature back and forth between noise voltage ( vn ), noise power (N) and equivalent noise temperature (T). They are all interrelated by the equation:
(1)
Noise temperature is used preferentially as it has reasonably sized numbers: 447 pV of noise into a 50Ω load is the same as 
W is the same as 290K.
We try to use the rigorous definitions of noise factor and noise figure. The noise factor F is an algebraic quantity greater than or equal to one. The noise figure NF is the noise factor expressed in decibels and is greater than or equal to zero.