Wadley Loop
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The Wadley Loop circuit was designed by Dr. Trevor Wadley in the 1940's and was first used for a stable Wavemeter.
[edit] 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 control or frequency synthesis), the Wadley Loop does not attempt to stabilize the oscillator. Instead, it cancels the drift mathematically. It does this by
(1) 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,
(2) mixing the same oscillator with a sheaf of harmonics from a crystal oscillator,
(3) selecting one of the results of (2) with a band-pass filter, and
(4) 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 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.
Let's go through an example[1]. Let's say we want to pick up signals from 0 to 30MHz. We'll divide this into 30 1MHz bands, and translate them to a band at 44-45MHz. To convert 0-1MHz, the first oscillator must be 45MHz, to convert 1-2Mhz it must be 46Mhz, and so on. Meanwhile, we also mix the first oscillator with harmonics from a 1MHz crystal and put the result through a 42MHz filter. Only one harmonic gets through. When the first oscillator is 45MHz, it is the third harmonic, because 45 - 3 = 42. At 46Mhz, 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 42MHz 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 42MHz, we get a band of signals from 3MHz to 2MHz, from which we select the desired signal, perhaps with a conventional superheterodyne back-end converting 3-2MHz to 455kHz and finally demodulating the signal back to audio. The overall receiver drift consists of the crystal's drift plus the 3MHz back-end, so when we're listening to a 30MHz 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 1MHz jumps.
