  In connection with numerous X-ray and neutron investigations of some high
temperature superconductors (YBa$_2$Cu$_3$O$_{6+x}$ and related compounds) a
non-trivial part of the structure factor, coming from partly disordered
Cu-O-$\dots$-O-Cu chain fragments, situated within basal planes, CuO$_x$, can
be a subject of theoretical interest. Closely connected to such a diffusive
part of the structure factor are the correlation lengths, which are also
available in neutron and X-ray diffraction studies and depend on a degree of
oxygen disorder in a basal plane. The quantitative measure of such a disorder
can be associated with temperature of a sample anneal, $T_q$, at which oxygen
in a basal plane remains frozen-in high temperature equilibrium after a fast
quench of a sample to room or lower temperature. The structure factor evolution
with $x$ is vizualized in figures after the numerical calculations. The
theoretical approach employed in the paper has been developed for the
orthorhombic state of YBCO.
