  We describe a testbed to characterize the optical response of compact
superconducting on-chip spectrometers in development for the Experiment for
Cryogenic Large-Aperture Intensity Mapping (EXCLAIM) mission. EXCLAIM is a
balloonborne far-infrared experiment to probe the CO and CII emission lines in
galaxies from redshift 3.5 to the present. The spectrometer, called u-Spec,
comprises a diffraction grating on a silicon chip coupled to kinetic inductance
detectors (KIDs) read out via a single microwave feedline. We use a prototype
spectrometer for EXCLAIM to demonstrate our ability to characterize the
spectrometers spectral response using a photomixer source. We utilize an
on-chip reference detector to normalize relative to spectral structure from the
off-chip optics and a silicon etalon to calibrate the absolute frequency.
