Wadley Loop - Principles of Operation

Principles of Operation

The Wadley Loop works 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 comb 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.

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