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. 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. The "Wadley loop" was used in radio receivers from the 1950s to approximately 1980. The "Wadley loop" was mostly used in more expensive stationary radio receivers, but the "Wadley loop" was also used in a portable radio receiver (Barlow-Wadley XCR-30 Mark II). Overview In a traditional superheterodyne radio receiver, most oscillator drift and instability occur in the first frequency converter stage, because it is tunable and operating at a high frequency. Unlike other drift-reducing techniques (such as crystal control or frequency synthesis), the Wadley Loop does not attempt to stabilize the oscillator. Instead, it cancels the drift mathematically. Principles of operation The Wadley loop works by: combining the first oscillator with the received signal in a frequency mixer to translate it to an intermediate frequency that is above the receiver's tuning range, mixing the same oscillator with a comb of harmonics from a crystal oscillator, selecting one of the results of (2) with a band-pass filter, and mixing this with the IF signal from (1). Since the high-IF of part 1 drifts in the same direction and the same amount as the "synthetic oscillator" of part 3, when they are mixed in part 4, the drift terms cancel out and the result is a crystal-stable signal at a second intermediate frequency. However, the drift makes it impossible to use high-IF selectivity to reject undesired signals. Instead, the high IF is designed with a band-pass characteristic. Also, since the first oscillator is cancelled out, it cannot be used to tune a particular signal. Instead, it selects an entire band of signals - which one depends on which harmonic was chosen in part 3 above. The size of the band is equal to the spacing of the crystal harmonics. A conventionally tuned "back end" selects the desired signal from the band of signals presented at the second IF. Example Let