  We have analyzed a large set of RXTE/PCA scanning and slewing observations
performed between April 1996 and March 1999. We obtained the 3-20 keV spectrum
of the cosmic X-ray background (CXB) by subtracting Earth-occulted observations
from observations of the X-ray sky at high galactic latitude and far away from
sources. The sky coverage is approximately ~22600 sq.deg. The PCA spectrum of
CXB in 3-20 keV energy band is adequately approximated by a single power law
with photon index Gamma~1.4 and normalization at 1 keV ~9.5 phot/s/cm2/keV/sr.
Instrumental background uncertainty precludes accurate RXTE/PCA measurements of
the spectrum of cosmic X-ray background at energies above 15 keV and therefore
we can not detect the high energy cutoff observed by HEAO-1 A2 experiment. Deep
observations of the 6 high latitude points used to model the PCA background
provide a coarse measure of the spatial variation of the CXB. The CXB
variations are consistent with a fixed spectral shape and variable
normalization characterized by a fractional rms amplitude of ~7% on angular
scales of ~1 square deg.
