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Description
(Text)
Based on surface acoustic waves (SAW), the reflective array compressor (RAC) is a high performance device widely used in modern radars. This thesis deals with another important application of RACs the Chirp Transform Spectrometer (CTS). The performances of the CTS, such as input bandwidth and frequency resolution, are mainly determined by the performance of the RAC filter used in it. Thus, the development of a large time bandwidth product RAC filter is essential for CTS technology in microwave atmospheric remote sensing for radio astronomy. This dissertation focuses on the design, modeling and development of the surface acoustic wave RAC type chirp filters using standard electron beam lithography (EBL) technology. Utilizing groove duty cycle weighting technique, two types of large time bandwidth product RAC filters with good performances were developed (one with BT = 4000 and the other with BT = 8000). Three other types of RAC filters with equal or higher time bandwidth products (BT = 7000, 12 000 and 16 000) were simulated. The maximum achievable time bandwidth product of RAC type filters was estimated to be about 16 000 for YZLiNbO3. Experiments were also performed for phase deviation compensations using 50 nm Al stripes between the two arrays of reflectors. The practical phase compensation coefficient was derived using the measurement results.
(Extract)
As shown above, the chirp transform consists of a multiplication of the signal to be analyzed with the down chirp signal, then a convolution with a matched up chirp signal and another multiplication with the down chirp signal as in the first one. Hence, this kind of system is called an M-C-M system. The purpose of the last multiplication is to correct the phase of the output of the chirp transformer. If we do not need the spectral phase of the signal, the last mixing can be omitted. Then only the power spectrum of the signal is calculated. The chirp transform with the last multiplication omitted is called an M-C system.
