  A physics package for a compact cold atomic clock is hereby presented. The
uniqueness of this package is its small dimensions that enable, for the first
time, implementation of a primary cold atomic clock in a standard package of 3U
height (=133mm). These dimensions are made possible by using an in-vacuum
Microwave (MW) Loop Gap Resonator (LGR) whose length and diameter can be
reduced from those of a typical resonator. Following our presentation of the
design, we analyze the homogeneity of the MW field amplitude and phase. We find
that the expected clock instability due to non-uniformity of the MW field is
$\sim1.5\cdot10^{-14}$/day. Adding other clock errors, we estimate the total
uncertainty of a cold atomic clock of this design will be around
$\sim2\cdot10^{-14}$/day which results in a time drift of a few
nanoseconds/day. Such a clock can serve as a primary-grade frequency reference,
and may replace the cesium beam atomic clock and the GPS-disciplined rubidium
atomic clocks.
