  Parametric amplifiers have become a workhorse in superconducting quantum
computing, however research and development of these devices has been hampered
by inconsistent, and sometimes misleading noise performance characterization
methodologies. The concepts behind noise characterization are deceptively
simple, and there are many places where one can make mistakes, either in
measurement or interpretation and analysis. In this article we cover the basics
of noise performance characterization, and the special problems it presents in
parametric amplifiers with limited power handling capability. We illustrate the
issues with three specific examples: a high-electron mobility transistor
amplifier, a Josephson traveling-wave parametric amplifier, and a Josephson
parametric amplifier. We emphasize the use of a 50-$\Omega$ shot noise tunnel
junction (SNTJ) as a broadband noise source, demonstrating its utility for
cryogenic amplifier amplifications. These practical examples highlight the role
of loss as well as the additional parametric amplifier `idler' input mode.
