  We have developed a new phase referencing technique for high frequency VLBI
observations. In conventional phase referencing, one interleaves short scans on
a nearby phase calibrator between the target source observations. In fast
frequency switching described here, one observes the target source continuously
while switching rapidly between the target frequency and a lower reference
frequency. We demonstrate that the technique allows phase calibration almost
reaching the thermal noise limit and present the first detection of the AGN in
the FR I radio galaxy NGC 4261 at 86 GHz. Although point-like, this is the
weakest source ever detected with VLBI at this frequency.
