Analysis of VLBI interferometer characteristics using zero-baseline lab prototype and RASFX correlator
A zero-baseline radio interferometer model was created in IAA RAS at 2017 in order to analyze VLBI radio interferometer characterictics in laboratory conditions. The model consists the radio telescope RT-13 tri-band and ultra-wideband receivers (UWB) heterodyne type receivers, broadband data acquisition system (BRAS), and RASFX software correlator. The radio interferometric sessions results in S/X/Ka-bands with 512~MHz channel bandwidth and 2-bits sampling are presented in this paper. The session duration was varied from single 5..20 minutes "scan" to 1 hour consisted of 120 10-seconds "scans". To simulate cosmic radio source the noise generator signal was injected to receivers. Obtained with RASFX correlator fringe characteristics were analyzed: signal-to-noise ratio (SNR), group delay, group delay rate, fringe phase, and its standard deviations. Allan deviation was calculated to find the origin of delay variations. The sinusoidal ripple of fringe delay due to the frequency inaccuracy of a LO and overlapping spectra from Nyquist zones was revealed. It was shown that digital band filtration reduces the measured delay variation of zero-baseline radio interferometer.