  We present a new broadband radio frequency catalogue across 0.12 GHz $\le \nu
\le$ 20 GHz created by combining data from the Murchison Widefield Array
Commissioning Survey, the Australia Telescope 20 Gigahertz survey, and the
literature. Our catalogue consists of 1285 sources limited by $S_{20\,\rm{GHz}}
>$ 40 mJy at $5\sigma$, and contains flux density measurements (or estimates)
and uncertainties at 0.074, 0.080, 0.119, 0.150, 0.180, 0.408, 0.843, 1.4, 4.8,
8.6, and 20 GHz. We fit a second-order polynomial in log-log space to the
spectral energy distributions of all these sources in order to characterise
their broadband emission. For the 994 sources which are well-described by a
linear or quadratic model we present a new diagnostic plot arranging sources by
the linear and curvature terms. We demonstrate the advantages of such a plot
over the traditional radio colour-colour diagram. We also present astrophysical
descriptions of the sources found in each segment of this new parameter space
and discuss the utility of these plots in the upcoming era of large area, deep,
broadband radio surveys.
