  The combination of electronic sources operating at high frequencies and
modern microwave instrumentation has enabled the recent development of
chirped-pulse spectrometers for the millimetre and THz bands. This type of
instrument can operate at high resolution which is particularly suited to gas
phase rotational spectroscopy. The construction of a chirped pulse spectrometer
operating at 200 GHz is described in detail while attention is paid to the
phase stability and the data accumulation over many cycles. Validation using
carbonyl sulphide has allowed the detection limit of the instrument to be
established as function of the accumulation. A large number of OCS transitions
were identified using a 10 GHz chirped pulse and include the 6 most abundant
isotopologues, the weakest line corresponding to the fundamental R(17)
transition of 16 O 13 C 33 S with a line strength of 4.3 x 10-26 cm-1
/(molec.cm-2). The linearity of the system response for different degrees of
data accumulation and transition line strength was confirmed over 4 orders of
magnitudes. A simple analysis of the time domain data was demonstrated to
provide the line broadening coefficient without the need for conversion by a
Fourier transform. Finally, the pulse duration is discussed and optimal values
are given for both Doppler limited and collisional regimes.
