  X-ray calorimeters routinely achieve very high spectral resolution, typically
a few eV full width at half maximum (FWHM). Measurements of calorimeter line
shapes are usually dominated by the natural linewidth of most laboratory
calibration sources. This compounds the data acquisition time necessary to
statistically sample the instrumental line broadening, and can add systematic
uncertainty if the intrinsic line shape of the source is not well known. To
address these issues, we have built a simple, compact monochromatic x-ray
source using channel cut crystals. A commercial x-ray tube illuminates a pair
of channel cut crystals which are aligned in a dispersive configuration to
select the \kaone line of the x-ray tube anode material. The entire device,
including x-ray tube, can be easily hand carried by one person and may be
positioned manually or using a mechanical translation stage. The output
monochromatic beam provides a collimated image of the anode spot with
magnification of unity in the dispersion direction (typically 100-200 $\mu$m
for the x-ray tubes used here), and is unfocused in the cross-dispersion
direction, so that the source image in the detector plane appears as a line. We
measured output count rates as high as 10 count/s/pixel for the Hitomi Soft
X-ray Spectrometer, which had 819 $\mu$m square pixels. We implemented
different monochromator designs for energies of 5.4 keV (one design) and 8.0
keV (two designs) which have effective theoretical FWHM energy resolution of
0.125, 0.197, and 0.086 eV, respectively; these are well-suited for optimal
calibration measurements of state-of-the art x-ray calorimeters. We measured an
upper limit for the energy resolution of our \crkaone monochromator of 0.7 eV
FWHM at 5.4 keV, consistent with the theoretical prediction of 0.125 eV.
