Speaker
Description
The transition of VLBI systems towards ultra-wideband receivers represents a decisive step for both radio astronomy and space geodesy, where achieving millimeter-level accuracy critically depends on highly reliable signal conversion and processing chains. Within this context, we present the design, development, and validation of a 4–8 GHz downconverter conceived for cryogenic receivers compatible with VGOS and BRAND. Its purpose is to translate broadband RF signals into a DC–4 GHz intermediate frequency, ensuring full compatibility with the DBBC3 backend and enabling next-generation observations within the RAEGE network.
The proposed design integrates commercially available surface mount devices (SMD) with custom components specifically developed at Yebes Observatory. All elements are mounted in a precision-machined aluminum housing that guarantees mechanical stability, optimal impedance matching, and high electromagnetic isolation between polarizations. The signal chain incorporates isolators, mixers, low-pass filters, amplifiers, and programmable digital attenuators, carefully configured to achieve stable conversion gain, controllable output levels, and minimal additional noise.
Extensive measurements demonstrate that the prototype satisfies the requirements of VGOS and BRAND. Experimental results confirm stable and adjustable gain in both polarizations, adequate linearity with input compression points (Pi1dB) up to +13.4 dBm, noise figure consistently below 25 dB across the band, and excellent cross-polarization rejection exceeding 44 dB. In addition, the phase stability was characterized through dedicated measurements at several frequency points across the 4–8 GHz RF band, with integration times of 5, 10, and 30 minutes. The results show an average peak-to-peak phase variation of approximately 4.3°, 6.5°, and 6.9°, respectively, consistently observed at all tested frequencies. Since VGOS scans last 30 s, stability was verified over extended integration times to ensure phase coherence well beyond the operational requirements. These results validate the downconverter as a reliable and efficient building block for ultra-wideband VLBI receivers.
Beyond technical validation, this development represents a significant milestone towards the implementation of a new generation of downconverters for ultra-wideband receivers in the RAEGE network.
