UNLOCK, unlocked = refers to the condition in a PLL (phased locked loop)
where one or more elements of the loop become defective or incorrect,
preventing the phase detector from supplying the correct control voltage to
the oscillator for the intended frequency of operation.
VARACTOR = a diode whose capacitance can be varied in step by the
amount of DC voltage applied to it. The higher the voltage, the lower the
capacitance. See also VCO.
VCO = "Voltage Controlled Oscillator," an oscillator whose frequency is
varied by DC voltage applied to varactor diodes, which change capacitance
in step with the voltage level.
ZENER DIODE = a diode designed to have a very specific reverse
breakdown voltage. This property makes the Zener diode ideal for simple
voltage regulators or precise voltage drop elements.