  We report results from a long-term monitoring data set obtained with the
Rossi X-ray Timing Explorer (RXTE) from the bright globular cluster binary
source 4U 1820-30. During this monitoring, the source intensity varied from a
flux of 2.9E-9 ergs/cm**2/s to 8.5E-9 ergs/cm**2/s in the 2-10 keV band, which
correspond to, respectively, a count rate of 1,600 cps and 4,500 cps in the
nominal 2-60 keV RXTE-PCA band. Its energy spectral state was observed to
encompass both the banana branches and the island state. Kilohertz QPOs were
observed to exist in the upper-banana and lower-banana branches, as well as in
the island state. In particular, we report that the kHz QPOs were present only
when the soft-color (defined as the flux ratio of the 3.9-5.0 keV band to the
1.7-3.9 keV band) was below 0.78 without regard to the overall count rate and
without regard to whether the source is in any one of the three branches or
states. Their centroid frequency was well correlated with the overall count
rate below 2,500 cps. When the count rate was above 2,500 cps, the QPO
frequency was consistent with a constant independent of the overall count rate.
We take this as further evidence that the inner edge of the accretion had
reached the marginally stable orbit, and therefore the constant 1060 +/- 20 Hz
is the marginally stable orbit frequency.
