  We demonstrate the capabilities of a novel frequency-domain terahertz
spectrometer based on a comb-locked frequency synthesizer, which provides
absolute frequency calibration. The inherent stability and repeatability of the
scans allow for the combination of fast data acquisition with an average
time-limited signal-to-noise ratio. We demonstrate kilohertz level frequency
resolution in terahertz precision spectroscopy of ultrahigh quality
whispering-gallery-mode resonators. Spectra covering multiple free spectral
ranges (>36GHz) with sub-20kHz resolution are acquired in 5s. We analyse the
coupling behaviour and temperature tuning of single resonances and, for the
first time, observe minute red and blue shifts of different mode families. The
experimental results are supported with finite element simulations.
