  Active galactic nuclei (AGN) are known for irregular variability on all time
scales, down to intra-day variability with relative variations of a few percent
within minutes to hours. On such short timescales, unexplored territory, such
as the possible existence of a shortest characteristic time scale of activity
and the shape of the high frequency end of AGN power spectra, still exists. We
present the results of AGN single-dish fast photometry performed with the
Korean VLBI Network (KVN). Observations were done in a "anti-correlated" mode
using two antennas, with always at least one antenna pointing at the target.
This results in an effective time resolution of less than three minutes. We
used all four KVN frequencies, 22, 43, 86, and 129 GHz, in order to trace
spectral variability, if any. We were able to derive high-quality light curves
for 3C 111, 3C 454.3, and BL Lacertae at 22 and 43 GHz, and for 3C 279 at 86
GHz, between May 2012 and April 2013. We performed a detailed statistical
analysis in order to assess the levels of variability and the corresponding
upper limits. We found upper limits on flux variability ranging from $\sim$1.6%
to $\sim$7.6%. The upper limits on the derived brightness temperatures exceed
the inverse Compton limit by three to six orders of magnitude. From our
results, plus comparison with data obtained by the University of Michigan Radio
Astronomy Observatory, we conclude that we have not detected source-intrinsic
variability which would have to occur at sub-per cent levels.
