The "Wadley-drift-canceling-loop", also known as a "Wadley loop", is a system of two oscillators, a frequency synthesizer, and two frequency mixers in the radio-frequency signal path. The system was designed by Dr. Trevor Wadley in the 1940s in South Africa and the circuit was first used for a stable wavemeter. (A wavemeter is used for measuring the wavelength and therefore also the frequency of a signal) There is no regulation loop in a "Wadley-loop", which is why the term is in quotation marks, but the circuit topology is not known under more-correct names.