Modern research in atmospheric chemistry requires highly sensitive techniques for the measurement of concentrations of free radicals which determine the rate of photochemical destruction of most atmospheric pollutants. Tunable diode-laser absorption spectroscopy (TDLAS) has already been successfully used for measurements of very low concentrations of stable gases, but further improvement in its sensitivity by signal averaging has been limited by the stability of the instrument. In this paper the concept of the Allan variance is utilized to analyze the stability of an existing frequency-modulated (FM) TDLAS instrument leading to a detection limit for NO2 of 34 pptv at 6 Hz detection bandwidth. The stability of the instrument allows averaging over 60 s. Taking into account the measuring cycle consisting of the determination of the sample spectra and zero air spectra as well as gas exchange in the absorption cell, the detection limit achievable with this particular instrument was 10 pptv within 25 s under laboratory conditions. Possibilities of further improvement of the detection limit are discussed.

The limits of signal averaging in atmospheric trace-gas monitoring by tunable diode-laser absorption spectroscopy (TDLAS)

The state-of-the art of terahertz heterodyne receivers are reviewed. Emphasis is placed on front-end components such as mixers and local oscillators. The back-end technology is described to a lesser extent. Recent developments, which are expected to have a major impact in the future, are also discussed.

Terahertz Heterodyne Receivers

In this paper, we report about the development, fabrication, and characterization of a balanced waveguide hot electron bolometer (HEB) receiver for the Atacama Pathfinder EXperiment telescope covering the frequency band of 1.25-1.39 THz. The receiver uses a quadrature balanced scheme and two HEB mixers, fabricated from 4- to 5-nm-thick NbN film deposited on crystalline quartz substrate with an MgO buffer layer in between. We employed a novel micromachining method to produce all-metal waveguide parts at submicrometer accuracy (the main-mode waveguide dimensions are 90 times 180 mum ). We present details on the mixer design and measurement results, including receiver noise performance, stability and ldquofirst-lightrdquo at the telescope site. The receiver yields a double-sideband noise temperature averaged over the RF band below 1200 K, and outstanding stability with a spectroscopic Allan time more than 200 s.

A 1.3-THz Balanced Waveguide HEB Mixer for the APEX Telescope

This paper gives an overview of measurement techniques used in the THz region of the electromagnetic spectrum, from about 100 GHz to several THz Currently available components necessary for THz metrology, such as sources, detectors and passives, are briefly described A discussion of power measurements, vector network analysis and antenna measurements as well as the limitations of these measurements at THz frequencies is given The paper concludes with a summary of available components and instrumentation for THz metrology at the time of writing

THz Metrology and Instrumentation

A microwave heterodyne spectrometer consists of a radiometer front-end and a real-time spectrum analyser back-end. Common spectrometer types are filterbanks, autocorrelators and acousto-optic spectrometers (AOS). A high-resolution chirp transform spectrometer using reflective array compressor (RAC) filters with a time bandwidth product of 6400 is presented. The spectrometer has 1600 channels within a 40 MHz input bandwidth and a linear dynamic range of 63 dB for a CW input. It seems to be very promising for future space applications.

https://iopscience.iop.org/article/10.1088/0957-0233/1/7/008/pdf

A high-resolution chirp transform spectrometer for microwave measurements

For the characterization of radiometer noise and stability a new and very practical method is introduced. It consists of the measurement of the "Allan-variance" well known for the characterization of the stability of frequency standards. The plot of the Allan variance versus integration time allows one to determine the different types of fluctuations of the radio-meter output signal. In particular, the best range of integration time for optimum use of the system can accurately be evaluated. Theoretical considerations and experimental results are presented.

Characterization and Measurement of Radiometer Stability

For the characterization of noise and stability of any measuring instrument a new and very practical method is introduced. It follows the principles of the "Allan variance" well estabilished for the characterization of the stability of frequency standards. The plot of the Allen variance versus integration time enables one to determine the different types of noise power spectra from the output of any instrument. In particular, the best range of the integration time for optimum use of the system can accurately be evaluated. Theoretical considerations and experimental results with components of the 3-m radiotelescope of Univ. of Cologne are presented.

Characterization and Measurement of System Stability

For the characterization of noise and stability of any measuring instrument a new and very practical method is introduced. It follows the principles of the "Allan variance" well estabilished for the characterization of the stability of frequency standards. The plot of

B. Vowinkel

Papers

Characterization and Measurement of Radiometer Stability

Characterization and Measurement of System Stability

of System Stability