Wadley Loop
Encyclopedia
The Wadley Loop circuit was designed by Dr. Trevor Wadley
in the 1940s and was first used for a stable Wavemeter.
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.
Since the high-IF of part 1 drifts in the same direction, and the same amount, as the "synthetic oscillator" of part 3, when we mix them in part 4 the drift terms cancel out and the result is a crystal-stable signal at a second intermediate frequency.
But the drift makes it impossible to use high-IF selectivity
to reject undesired signals. Instead, the high IF is designed with a bandpass 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.
the signal back to audio. The overall receiver drift consists of the crystal's drift plus the 3 MHz back-end, so 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 effect on receiver operation is discrete
, that is, the tuning advances in 1 MHz jumps.
Trevor Wadley
Trevor Lloyd Wadley, was a South African electrical engineer, best known for his development of the Wadley Loop circuit for greater stability in communications receivers....
in the 1940s and was first used for a stable Wavemeter.
Overview
In a traditional superheterodyne radio receiver, most oscillator drift and instability occurs in the first frequency converter stage, because it is tunable and operating at a high frequency. In theory, if one can eliminate this drift, the receiver will be stable.Unlike other drift-reducing techniques (such as crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
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 mixerFrequency mixerIn electronics a mixer or frequency mixer is a nonlinear electrical circuit that creates new frequencies from two signals applied to it. In its most common application, two signals at frequencies f1 and f2 are applied to a mixer, and it produces new signals at the sum f1 + f2 and difference f1 -...
to translate it to an intermediate frequencyIntermediate frequencyIn communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...
that is above the receiver's tuning range, - mixing the same oscillator with a comb of harmonics from a crystal oscillatorCrystal oscillatorA crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency...
, - selecting one of the results of (2) with a band-pass filterBand-pass filterA band-pass filter is a device that passes frequencies within a certain range and rejects frequencies outside that range.Optical band-pass filters are of common usage....
, 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 we mix them in part 4 the drift terms cancel out and the result is a crystal-stable signal at a second intermediate frequency.
But the drift makes it impossible to use high-IF selectivity
Electronic selectivity
Selectivity is a measure of the performance of a radio receiver to respond only to the radio signal it is tuned to and reject other signals nearby in frequency, such as another broadcast on an adjacent channel....
to reject undesired signals. Instead, the high IF is designed with a bandpass 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's go through an example. Let's say we want to pick up signals from 0 to 30 MHz. We'll divide this into 30 1 MHz bands, and translate them 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, we also mix the first oscillator with harmonics from a 1 MHz crystal and put the result 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's 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 bandpass 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 we mix the high IF with the 42 MHz, we get a band of signals from 3 MHz to 2 MHz, from which we select the desired signal, perhaps with a conventional superheterodyne back-end converting 3-2 MHz to 455 kHz and finally demodulatingDemodulation
Demodulation is the act of extracting the original information-bearing signal from a modulated carrier wave.A demodulator is an electronic circuit that is used to recover the information content from the modulated carrier wave.These terms are traditionally used in connection with radio receivers,...
the signal back to audio. The overall receiver drift consists of the crystal's drift plus the 3 MHz back-end, so 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 effect on receiver operation is discrete
Discrete frequency
A discrete frequency domain is a frequency domain that is discrete rather than continuous.For example, the discrete Fourier transform maps a function having a discrete time domain into one having a discrete frequency domain...
, that is, the tuning advances in 1 MHz jumps.