  We discuss the interpolation of the electric and magnetic fields within a
charge-conserving Particle-In-Cell scheme. The choice of the interpolation
procedure for the fields acting on a particle can be constrained by analyzing
conservation of the energy and the particle generalized momentum. The better
conservative properties are achieved, if the alternating-order form-factor is
used for interpolation, which combines the lower-order and higher-order
interpolation from integer and semi-integer points of a staggered grid. This
approach allows us to significantly reduce noise in the charge conserving
scheme and improves both the results quality and the computational efficiency.
