  Techniques to improve the data quality of interferometric radio observations
are considered. Fundaments of fringe frequencies in the uv-plane are discussed
and filters are used to attenuate radio-frequency interference (RFI) and
off-axis sources. Several new applications of filters are introduced and
tested. A low-pass filter in time and frequency direction on single baseline
data is successfully used to lower the noise in the area of interest and to
remove sidelobes coming from unmodelled off-axis sources and RFI. Related side
effects of data integration, averaging and gridding are analysed, and shown to
be able to cause ghosts and an increase in noise, especially when using long
baselines or interferometric elements that have a large field of view. A novel
projected fringe low-pass filter is shown to be potentially useful for first
order source separation. Initial tests show that the filters can be several
factors faster compared to common source separation techniques such as peeling
and a variant of peeling that is currently being tested on LOFAR observations
called "demixed peeling". Further testing is required to support the
performance of the filters.
