•Waveguide phase reference can be compensated directly in quadrature raw data.•Vessel features can be observed along plasma reflection in reflectometry data.•X-mode first fringe estimation algorithms should use amplitude and spectral details.•X-mode lower cut off reflection influences first fringe estimation algorithms. The new multichannel X-mode reflectometer installed on ASDEX Upgrade measures the plasma density profile evolution at different positions in front of the ICRF antenna. The reflectometer operates in the extended U-band (40–68GHz) microwave region, measuring density profiles up to 2×1019m−3 with magnetic fields between 1.5T and 2.7T. In this heterodyne reflectometer architecture, the signal reflected by the plasma is down-shifted and quadrature detectors demodulate its in-phase and quadrature (IQ) components. In this work we analysed the acquired IQ signals from the different reflectometer antennas, and describe the waveguide dispersion calibration and filtering of the raw signal. The effect of spurious reflections, such as the multiple reflections from the ICRF antenna metal straps, are analysed and taken into account on the data processing software. In high plasma density and high magnetic field scenarios, both the lower and upper X-mode cut off frequencies are detected in the probing range. The first fringe (FF) reflection of the upper cut off indicates the start of the plasma signal and must be determined robustly to reduce density profile errors. Existing FF estimation algorithms based on signal amplitude and spectral information may still fail in the presence of both upper and lower cut off reflections.