The operation of the first PLL stabilization system of millimeter and submillimeter-wave "Istok" made BWOs in the range up to 350 GHz was demonstrated by our group in 1970. Then the operation range of the system was extended step by step up to 500 GHz in 1976, up to 600 GHz in 1979, up to 820 GHz in 1984. The next considerable step towards a high frequency range we made after joint development with the Institute of Electronic Measurements "Kvarz" (Nizhny Novgorod) of a series of commercial millimeter-wave frequency synthesizers covering all together the 35-178 GHz [Ref.12 (1994)] (Device 005). The synthesizers are based on packetized BWOs from "Istok" stabilized by a PLL system against a stable radio signal. The synthesizers were used as a source of reference radiation for stabilization of submillimeter-wave tubes as well as primary radiation sources. In 1994 working in collaboration with Cologne's University (Germany) we introduced the BWO PLL system operating up to 970 GHz [Ref.1 (1994)]. Later the range of the setup was extended up to the range of highest frequency tube - 1267 GHz [Ref. 13, 18 (1995), Ref. 15 (1996), Ref. 1 (1997)]. In 1999 [Ref. 10 (1999)] we developed the first PLL system for MM-SubMM radiation sources allowing fast phase continuous digital frequency sweeping. Later [Ref. 2 (2006)] we proposed the approach, which extends the range of the fast continuous phase frequency sweeping up to the full operating range of BWO. In 2009 we demonstrated for the first time the possibility of phase stabilization of primary subTHz radiation source against components of terahertz frequency comb produced by down converting optical comb generated by a femtosecond laser [Ref. 3 (2010)].

For more details see the papers [Ref. 7 (1996), Ref. 5 (1998), Ref. 6 (2012)].