The different noise contributions in a coherent tunable far infrared spectrometer were measured. On the basis of this quantitative analysis, several experimental schemes to enhance the sensitivity for spectroscopy in this spectral region are proposed. In particular, a full theoretical analysis for differential detection is developed. A double-detector scheme is theoretically analyzed and it is shown that a noise reduction of up to 17 dB is achievable with this configuration. The present work should make possible quantitative forecasting of sensitivity enhancement for new, different experimental configurations. ©1998 American Institute of Physics.