Heterodyne Receiver

    The primary aim for adopting heterodyne technique for rotational spectroscopy is to achieve maximum resolution and sensitivity. In this technique, we will down convert the source frequency by mixing it with nearby local oscillator (Optically Pumped Molecular Laser) frequency, the difference frequency of these two signals will carry the complete spectral and phase information of the source frequency. In our setup, we are planning to excite the NO₂ molecules (bounded in absorption metallic cell) by the blackbody source. Output of the cell (absorption band of the desired line) and local oscillator signal (OPML output) will go to the Martin-Puplett diplexer for the phase matching. The output of diplexer, which carries both source and LO frequency will be focused on the Whisker antenna of nonlinear mixer (Schottky Barrier Diode based on Gallium Arsenide) with the help of a suitable FIR lens. This mixer will generate intermediate frequency (IF) in microwave range with maximum 1.0 GHz bandwidth. Output of the mixer will be passed through the impedance matched filter, which is positioned at the output port of mixer. To strengthen the IF signal, it will be passed through the appropriate Low Noise Amplifier (LNA). To avoid any damage in mixer because of reflected power from the LNA, we will use a high quality microwave isolator between LNA and output port of mixer. Amplified output from LNA will go to the Spectrum Analyzer for further analysis.

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