A gain element and a variable wavelength reflector form a resonator. Particular embodiments comprise: a buoyant main body having an axial channel with corresponding end surface openings; an retention member seat running around the main body surface; and at least one line retention member receivable and retainable by the seat, and operative with the seat to retain a line passing therebetween at two points. When the delay in the test signal matches the jitter in the locked frequency of the PLL, the respective delay of the test-signal is used to determine the effective locked frequency of the PLL. A second detachable portion at the end of the carton borders the first detachable portion and may also be detached from the carton to enlarge the opening in an alternative embodiment. Because the compensating means modifies the output of the temperature sensor in a closed loop using information derived from an echo signal, the frequency regulating means can regulate the frequency output by the frequency generator\u2014keeping it as close as possible to an ideal constant value\u2014despite changes in its temperature.