  Delay line oscillators based on photonic components, such as the
optoelectronic oscillator (OEO), offer the potential for realization of phase
noise levels orders of magnitude lower than achievable by conventional
microwave sources. Fibre optic-based delay lines can realize the large delay
required for low phase noise systems whilst simultaneously achieving insertion
loss levels that can be compensated by available microwave and photonic
amplification technologies. However, the long fibre is vulnerable to
environmental perturbations such as mechanical vibrations and variations in
ambient temperature, which result in short term fluctuations and thermally
induced drift of the oscillation frequency. The phase shifter used
conventionally to adjust the frequency of an OEO to enable phase lock has a
finite range that is insufficient to compensate the delay change resulting from
operational temperature ranges. A solution to continuous tuning without
mode-hopping and to compensation of thermally induced frequency drift without
loss of lock of a time delay oscillator is proposed. The basic concept is to
introduce a tuning mechanism that works with Cartesian co-ordinates on the
complex plane and to avoid explicit use of polar co-ordinates. Consequently,
the transmission of the tuning component may traverse the unit circle in either
direction multiple times without range limitation to the phase. Thereby tuning
by mode-hopping is avoided and expedients to stabilisation, such as the use of
tunable lasers within a control loop or precision temperature stabilisation
measures are not required. The concept is verified by Simulink simulations. The
method has been experimentally tested successfully using a prototype OEO phase
locked to a system reference. Solid lock was maintained even when the OEO was
placed in an oven and cycled over a temperature range from ambient to 80
{\deg}C.
