For a reliable and robust operation of free-electron lasers with bunch-arrival time variations on the sub 10fs scale, the short-term and long-term stability of the cavity field is an important factor. For the European XFEL we used the established non-IQ field detection scheme in the MicroTCA.4 crate standard. We achieved for a single channel cavity field detection an accumulated short-term time jitter of <6.7fs (<3.4mdeg), respectively amplitude stability of <5.5E-5 within the noise bandwidth of 1MHz operating at 1.3GHz. To achieve this we present the detectors spurious free signal spectra, its channel scaling behavior, sub-components, signal integrity, EMC zone concepts and the packaging. To achieve the required long-term stability, we present a 2U 19" drift calibration module with a phase stability of <+-0.01deg_pp over several days by injecting the reference. Furthermore component variants and options to improve the stabilityare discussed
