  Microcalorimeters are used by National Metrology Institutes (NMI) for the
realization of the Radiofrequency and Microwaves (RF&MW) primary power
standard. Since they are not available on the market, NMIs have to design their
own systems. It involves the design of complex mechanical structures and of low
noise data acquisition systems. Resulting setups allow precision measurements
of RF&MW power by applying difficult and time consuming measurement techniques.
Weaknesses to be addressed in order to improve the performance, whether in the
mechanical structure or in the data acquisition electronics, are not easy to
identify. Anyway, they can be found by analyzing the microcalorimeter
measurement uncertainty. In recent years, several improvements have been made
on the INRIM (Istituto Nazionale di Ricerca Metrologica, Italy) coaxial
microcalorimeter with the aim of making its operation more stable and to reduce
the measurement uncertainty. Its weak points are here analyzed by means of a
comprehensive uncertainty budget. Details are also given on how uncertainty
contributions have been determined. The method here applied is general and can
be conveniently used in many other fields or applications provided that the
analytical model is known. It is useful while designing an experiment and/or a
posteriori, to identify the parts of the apparatus and of the measurement
set-up that need to be optimized.
