The CP-FTS (Chirped Pulse Fourier Transform Spectrometer)

The CAS laboratories operate a broadband Chirped-Pulse Fourier Transform Spectrometer (CP-FTS) that covers the frequency regions 6–40 GHz, 75–110 GHz and 150–220 GHz. In this type of spectrometer, a molecular sample is polarised by a chirped pulse (~1 µs) after which the subsequent free induction decay (FID) is recorded with an instantaneous spectral bandwidth of several GHz. The spectrum is obtained by Fourier Transformation of the FID. The capability to record broad scans with high frequency resolution and high dynamic range on microsecond timescales makes this instrument very suitable for studying the spectra of astrophysically relevant molecules, in particular if they obey dense spectra (e.g. complex organic molecules, COMs) or if frequency predictions are very uncertain. The electronics are housed in a portable rack to facilitate coupling with a variety of chambers/cells depending on the sample to be studied. In addtition to a 1.5 m long waveguide sample cell and the absorption cells of the CASAC spectrometer, the spectrometer can be coupled with the supersonic jet apparatus to study unstable and short-lived molecules.

Setup

The chirped pulse is generated by a 5 GHz arbitrary waveform generator (Keysight, M8190A). The signal is upconverted by an IQ modulator, subsequently amplified by a solid state amplifier (6-18 GHz with 20 W, 18-26 GHz with 4W and 26-40GHz with 1 W) and fed into the sample chamber (e.g. waveguide), where it interacts with the molecular sample. On the detector side, the molecular signal (FID) is first amplified (low noise amplifier) and downconverted in frequency (IQ mixer), before beeing recorded using either a 25 GHz digital scope or 2.5 GHz digitizer card (Acqiris U5310A). A signal generator (Keysight E8257D) acts as local oscillator. Its signal is split by a power divider and fed into the IQ modulator and the IQ mixer.

Frequency extensions to 75-110 GHz and 150-220 GHz are available. The chirped pulse, generated at frequencies between 12-18 GHz is transformed into the 3mm wavelenght range using an active sextupler, which is followed by a medium power amplifier (250 mW) and eventually a doubler. The detected signal is first amplified using low noise amplifiers and then downconverted by either a harmonic (75-110 GHz) or an sub-harmonic mixer (150-220 GHz). A second active sextupler upconverts the signal of the signal generator to provide the LO signal. 

Spectra