The High Resolution Chirp Transform Spectrometer for the Sofia-Great Instrument

Abstract

In this paper, we present the design of a high resolution Chirp Transform Spectrometer (CTS) which is part of the GREAT (German REceiver for Astronomy at Terahertz frequencies) instrument onboard SOFIA, the Stratospheric Observatory For Infrared Astronomy. The new spectrometer will provide unique spectral resolving power and linearity response, since the analog Fourier transform performed by the CTS spectrometer was improved through a new design, that we call "Adaptive Digital Chirp Processor (ADCP)". The principle behind the ADCP consists on digitally generating the dispersive signal which adapts to the compressor dispersive properties, achieving maximum spectral resolution and higher dynamic range. Excellent test results have been obtained such as a white noise dynamic range of 30 dB, and a spectral resolution (FWHM) of 41.68 kHz which would mean if analyzing signals with the high frequency band receiver on the GREAT instrument (4.7 THz) a spectral resolving power (λ/ λ) higher than 10 8 .

GREAT: the SOFIA high-frequency heterodyne instrument

“…The IF-processor is capable of stacking four AOS-bands, thus forming two 4 GHz wide bands. The two CTS, however, only have 220 MHz of bandwidth but a spectral resolution of 56 kHz (Villanueva & Hartogh 2004). The fast advancements in digital signal processing during recent years made it possible to equip GREAT with FFT spectrometers (Klein et al 2006(Klein et al , 2012: each detector channel is serviced by an AFFT spectrometer with 1.5 GHz instantaneous bandwidth and resolution of 212 kHz (ENBW).…”

Atmospheric ozone above Troll station, Antarctica observed by a ground based microwave radiometer

“…In the front-end heterodyne receiver, the incoming radiation is combined with an independent local oscillator signal at 255.6 GHz by a Superconductor-Insulator-Superconductor mixer cooled to 4 K. The intermediate frequency (IF) signal in the 4 to 6 GHz range is then amplified. For O 3 measurements the signal is further down-converted to 2.1 GHz and analyzed using a Chirp Transform Spectrometer (CTS) (Hartogh and Hartmann, 1990;Villanueva and Hartogh, 2004;Villanueva et al, 2006) with a 28 kHz resolution and 220 MHz bandwidth.…”

Application of Superconducting Hot-Electron Bolometer Mixers for Terahertz-Band Astronomy

“…GREAT uses waveguide HEB mixer detectors cooled at 4 K with cryogenic amplifiers in a liquid-heliumcooled cryostat, giving a hold time of >15 h [45]. The LO source system consists of solid-state cascading multiplier chains from Virginia Diodes, Inc. Utilized as backends are an acousto-optical spectrometer covering two 4-GHz-wide bands, a chirp transform spectrometer with 220-MHz bandwidth and 56-kHz resolution [46], and a XFFTS.…”