A system for the coherent conversion of the signals of the centimeter-band astronomical very long baseline radio interferometer of the Scientific-Research Institute of Radiophysics

Abstract

In the Scientific-Research Institute of Radiophysics we have developed specialized apparatus for the astronomical centimeter-band very long baseline radiointerferometer to solve a number of problems in astronomy, geodesy, and geodynamics [i]. The apparatus enables the main initial quantities (group delay and interference frequency) to be measured at the modern level of accuracy required. An important part of this apparatus is the system for the coherent conversion of the frequency of the received signals from the radio source. As is well known, this system can be constructed using an atc~nic frequency standard at each point of the very long baseline radio interferometer, the instability of which will determine the duration of the coherent storage of the interferometric signal. The problem reduces to transferring the highly stable properties of the signal of the frequency standard to the signals of the heterodynes, while ensuring their spectral purity. This is particularly important for an astronomical system for accurate and unambiguous measurements of interference frequency. In addition, the system for the coherent conversion of the signals of an astronomical very long baseline radio interferometer must carry out the conversion over a fairly broad frequency band, required to synthesize the corresponding broad effective frequency band, whose width to a large extent determines the final accuracy of the group-delav measurement [2]. i. Construction of the Centimeter-Band Coherent Signal Conversion System. The coherent signal conversion system we developed is a system with double freouency conversion with a receiving passband of 50 MHz, which converts the received centimeter-band signals into the operating band of the broadband frequency synthesis device (BFSD) of 30-76 MHz [3]. A block diagram of the system is shown in Fig. i. The signal of the first fixed-frequency heterodyne (FH) is generated directly from the 5-MHz signal from the Chi-69 rubidium frequency standard by multiplication, and the signal of the second heterodyne is obtained by multiplication of the output signal of a Ch6-58 frequency synthesizer, which is synchronized with the output signal of the same rubidium standard. The value of the second-heterodyne is set by the frequency synthesizer taking into account the previously calculated value of the differential Doppler frequency shift at the initial instant of each interferometer session of the observations of the space radio source. In existing very long base radio interferometer systems in the centimeter band, and some