say we want to pick up signals from 0 to 30 MHz. This is divided into 30 1 MHz bands, which are then translated to a band at 44-45 MHz. To convert 0-1 MHz, the first oscillator must be 45 MHz; to convert 1-2 MHz it must be 46 MHz; and so on. Meanwhile, the first oscillator is also mixed with harmonics from a 1 MHz crystal and the results pass through a 42 MHz filter. Only one harmonic gets through. When the first oscillator is 45 MHz, it is the third harmonic, because 45 - 3 = 42. At 46 MHz, it is the fourth harmonic, and so on. The oscillator does not have to be exactly 45, 46, and so on, only close enough to get through the 42 MHz band-pass filter. Let's say it is 45.1 . Then we get 42.1 from the filter, and 45.1 - 42.1 is still 3. When the high IF is mixed with the 42 MHz, the result is a band of signals from 3 MHz to 2 MHz, from which the desired signal is selected. This can potentially be accomplished with a conventional superheterodyne back-end converting 3-2 MHz to 455 kHz and finally demodulating the signal back to audio. The overall receiver drift consists of the crystal's drift plus the 3 MHz back-end. When we're listening to a 30 MHz signal, this receiver is about ten times as stable as one using a high-frequency tunable VFO. To a new user, the feel of the first oscillator tuning control is counterintuitive. Although the knob moves in a continuous, analog fashion, its affect on the receiver operation is discrete, that is, the tuning advances in 1 MHz jumps. An example is Yaesu's FRG-7 communications receiver, which uses the system to remove local oscillator drift. The Racal RA17 and Realistic DX-302 also used the Wadley Loop in their design. An optical implementation of a Wadley Loop has recently been proposed. This allows a compact relatively unstable laser to be used as a local oscillator. The system's stability being obtained from a master 'comb source' (usually a pulsed laser, such as a mode-locked laser), possibly common to many receivers within an exchange. Notes Radio electronics South African inventions