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首页> 外文会议>International Kharkov Symposium Physics and Engineering of Millimeter and Sub-Millimeter Waves >HIGH-PRECISION MILLIMETER-WAVE SPECTROMETER: LAST IMPROVEMENTS
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HIGH-PRECISION MILLIMETER-WAVE SPECTROMETER: LAST IMPROVEMENTS

机译：高精度毫米波光谱仪：最后改进

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The automated millimeter-wave spectrometer at the Institute of Radio Astronomy of the NASU at Kharkov [1-3] was successfully applied for molecular spectra investigation during a number of years. The spectrometer is based on the millimeter-wave frequency synthesizer and covers frequency ranges of 49 - 149 GHz and 200 - 260 GHz with frequency doubler. The spectrometer is a usual absorption frequency-modulated spectrometer with lock-in detection. Current state of art of the spectrometer is presented in the figure 1. As a reference source of millimeter-wave frequency synthesizer we have applied the direct digital synthesizer (DDS) AD9851 from Analog Devices, Inc. Application of such kind of synthesizers gives a number of advantages in comparison with other methods of the frequency synthesis [4]. In order to minimize multiplication factor of DDS output we applied up-conversion, which allows us to convert the output signal of the AD9851 from a frequency range of 30 - 60 MHz (in our application) to 390 - 420 MHz, keeping its initial spectral purity. The narrow-band spectral purity of the DDS is very good. However, as it is known from the literature [4], the wideband output spectrum of a DDS contains a lot of rather intense spurious components that is why high factor multiplication (in our case it may be up to 600) of the DDS output signal usually produces too spurious spectrum. In order to overcome this obstacle we applied narrow-band adaptive filtering by means of narrow-band phase-lock loop of the klystron (its closed loop bandwidth is about 1 - 2 kHz). In order to provide wide-range continuous spectra record the klystron is equipped with special system which provides mechanical adjustment of its resonator (see fig.1). As an absorption cell we use quasioptic dielectric hollow waveguide. Presents of standing wave in an absorption cell leads to large variations of a baseline and gives a number of problems during spectra investigation, such as sensitivity and accuracy limitations. The most common method to suppress essentially variations of the baseline is application of the second harmonic lock-in detection [5]. In this case a spectrometer provides records in the form of the second derivative of true line-shape. Nevertheless in spite of rather low visible level of the standing wave its influence at the value of measured frequency stays quite essential.
机译：在Kharkov在Kharkov [1-3]的NASU射频天文学研究所的自动毫米波谱仪已成功应用于多年来的分子谱调查。光谱仪基于毫米波频率合成器，涵盖49 - 149 GHz和200 - 260 GHz的频率范围，频率倍增。光谱仪是一种常用的吸收频率调制光谱仪，具有锁定检测。光谱仪的当前现有技术在图1中示出。作为毫米波频率合成器的参考来源，我们已经应用了从模拟设备，Inc。的直接数字合成器（DDS）AD9851，Inc。这种合成器的应用给出了一个数字与频率合成的其他方法相比的优点[4]。为了最大限度地减少DDS输出的乘法系数，我们应用了上转换，这允许我们将AD9851的输出信号从30 - 60 MHz（在我们的应用程序中）到390-420MHz的频率范围转换为390-420 MHz，保持其初始光谱纯度。 DDS的窄带谱纯度非常好。然而，由于从文献[4]中已知，DDS的宽带输出频谱包含大量相当强烈的虚假组件，这是DDS输出信号的高因子乘法（在我们的情况下最多可达600个）的原因通常会产生太杂散的光谱。为了克服这种障碍，我们通过Klystron的窄带相锁环（其闭环带宽约为1 - 2kHz）施加窄带自适应滤波。为了提供广泛的连续光谱记录，Klystron配备了特殊的系统，该系统提供了其谐振器的机械调节（见图1）。作为一种吸收细胞，我们使用Quasioptic介电空心波导。在吸收细胞中的常设波存在导致基线的大变化，并在光谱调查期间在诸如灵敏度和精度限制的诸如敏感性和准确性限制的若干问题。抑制基线基本变化的最常见方法是应用第二次谐波锁定检测[5]。在这种情况下，光谱仪提供真正的线形状的第二导数的形式。然而，尽管存在相当低的常设波的可见水平，其对测量频率值的影响保持非常重要。

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