Linearization of downconversion for IQ detection purposes.

The downconverter used in LLRF control system is a nonlinear device, thus not only the fundamental frequency but also its harmonics are present and sampled by ADC. The levels of higher order harmonics depend on the level of RF signal. For a downconverter used in FLASH LLRF system the harmonics are 30 dB below the fundamental frequency component (in the worst case - at maximal RF signal level). These harmonics produce errors in IQ detector of up to a few percents in amplitude and one degree in phase. This paper contains the analysis of the error and propose the calibration method allowing to obtain the correction curve for detected amplitude and phase.