  We present the results of a long-look monitoring of 3C273 with RXTE between
1996 and 2000. A total of 230 observations amounts to a net exposure of
845ksec, with this spectral and variability analysis of 3C273 covering the
longest observation period available at hard X-ray energies. Flux variations by
a factor of 4 have been detected over 4years, whereas less than 30% flux
variations have been observed for individual flares on time-scales of 3 days.
Two temporal methods, the power spectrum density (PSD) and the structure
function (SF), have been used to study the variability characteristics of
3C273. The hard X-ray photon spectra generally show a power-law shape with a
differential photon index of G = 1.6+-0.1. In 10 of 261 data segments,
exceptions to power-law behaviour have been found: (i) an additional soft
excess below 4keV, and (ii) a broad Fe fluorescent line feature with EW =
100-200 eV. Our new observations of these previously reported X-ray features
may imply that 3C273 is a unique object whose hard X-ray emission occasionally
contains a component which is not related to a beamed emission (Seyfert like),
but most hard X-rays are likely to originate in inverse Compton radiation from
the relativistic jet (blazar like). Multi-frequency spectra from radio to
gamma-ray are presented in addition to our RXTE results. The X-ray time
variability and spectral evolution are discussed in the framework of beamed,
synchrotron self-Compton picture. We consider the ``power balance'' (both
radiative and kinetic) between the accretion disk, sub-pc-scale jet, and the
10kpc-scale jet.
