Radio Physics and Radio Astronomy (Sep 2020)
THE RT-32 RADIO TELESCOPE CONSTRUCTION BASED ON THE MARK-4B ANTENNA SYSTEM. 3. LOCAL OSCILLATORS AND SELF-NOISE OF THE RECEIVING SYSTEM
Abstract
Purpose: High resolution investigation of spectral lines of space sources requires low intrinsic noise of the radio telescope receiving system. It is provided with both input cryogenic amplifiers and low phase noise of local oscillators. To make spectral studies, it is neces ary to be able to tune the frequencies of local oscillators with a small frequency step. The paper presents the results of developing the frequency synthesizers, which simultaneously provide both a very high frequency resolution and low level of phase noise. The results of measurements of natural noise of the RT-32 radio telescope radio receiving systems are given also. Design/methodology/approach: The RT-32 receiving systems are constructed as heterodyne receivers with two stages of frequency conversion. Tuning of receiving systems with a frequency step of 10 or 20 MHz is provided by local oscillators of the first frequency conversion stage, and precise tuning is provided due to the ultra-high resolution 0.0001 MHz) of DDS-based (direct digital synthesizer) local oscillators of the second frequency conversion stage. Findings: It is shown that the application of direct digital synthesizers is possible only with the low values of frequency multiplication factors, as well as under the conditions of careful filtering of all reference signals. The parameters of the local oscillators were measured with the N9951A spectrum analyzer (Keysight Technologies) with the high resolution and wide dynamic range. To measure the radio telescope receiving system noise characteristics, a special matched loads with the possibility of cooling down to the liquid nitrogen temperature were made. The noise temperature measurements were made in different cross sections of the RT-32 receiving system. Comparison of such measurements in different configurations makes it possible to provide a preliminary estimation of the RT-32 self noise in the C- and K-bands. Conclusions: The results of measurements of self noise of radio receiving systems and phase noise of local oscillators of the RT-32 radio telescope show that within the C-band the radio telescope is capable to perform high-sensitive studies in both a wide frequency band and a narrow frequency band with the high spectral resolution. Within the K-band, the natural noise is comparable (≈60÷80 K) with the external noise that also allows studying the radiation of maser radio sources.
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