The Wadley Drift Cancelling Loop

The Wadley Drift Cancelling Loop, or simply Wadley Loop, was invented by Dr. Trevor Wadley (1920-1981.) Wadley was a South African Engineer who had worked during WWII for the Telecommunications Research Establishment in England. Working later for RACAL Dr. Wadley designed around 1954 the RA17 Communications receiver which was the first to exploit his invention.

The Wadley Loop was subsequently incorporated in several solid-state receivers, starting from the famous Barlow-Wadley XCR-30. The XCR-30 was the first transistor portable to achieve a tuning accuracy of 5 kHZ over the 0.5-30 MHz spectrum. One knob was used to choose one of 30 1-MHz band in steps of 1 MHz. The second knob tuned inside the 1 MHz band. The XCR-30 was followed by the Yaesu FRG-7 and FRG-7000 and by the less famous DRAKE SSR-1, Realistic DX-300 and Standard C6500. With the exception of the XCR-30, all the others were tabletop units. The FRG-7 was probably the one having the best performances in terms of cross-modulation and overload.
The principle is shown in the figure. The values are those of the FRG-7, but the principle is the same for the other receivers (the IF values may change.)
The coarse tuning is done via a VFO covering the 30 MHz range from 55.5 to 84.5 MHz without bandswitching. The VFO output is sent to Mixer 1 of the receiver and to balanced Mixer 2. Mixer 2 mixes the VFO signal with the output of a 1 MHz harmonic generator (produced by a very stable crystal oscillator).
Of the multitude of harmonics only the one giving a mixing product close to 52.5 MHz is amplified. The amplified signal is then sent to to Mixer 3 where it is mixed with the output of the IF amplifier to produce a signal in the 2-3 MHz range. This signal is then tuned in a 2-3 MHz receiver where is converted to a standard 455 kHz. For this reasons all the Wadley Loop receivers are triple-conversion (signalwise.)
Why the name "drift cancelling loop" ? Suppose we want to tune a signal at 5050 kHZ. Then the MHz knob has to be set to 5 MHz. The VFO will produce a 60.5 MHz signal (55.5 + 5) from which, by subtraction, a 55.45 MHz signal is sent to the IF amplifier. Notice that the VFO frequency is added and the signal frequency is subtracted.
The balanced Mixer 2 mixes 60.5 MHz with the 8th harmonic (8 MHz) and produces a 52.5 MHz signal. Also in this case the VFO frequency is added. Finally the 52.5 MHz is subtracted from the 55.45 MHz signal from the IF amplifier producing a 2.95 MHz signal which is tuned in the last receiver.

So the 2.95 MHz signal is the result of an operation where the VFO frequency is first added and then subtracted from the input signal's frequency. This is the trick to achieve drift cancellation

Suppose now that the VFO is not exactly at 60.5 MHz or that it drifts 150 kHz upwards, and goes to 60.65 MHz. The IF signal is now at 55.60 MHz, still within the IF bandpass. The balanced mixer now produces a 52.65 MHz output, also within the amplifier bandpass (notice that if the drift is excessive the signal will fall outside the bandpass, causing the lock indicator to light up). So at the end the output signal is still 55.60-52.65=2.95 MHz. So despite the VFO drifted 150 kHz upwards the output did not move. The drift was cancelled.
Disadvantages The main disadvantage of the Wadley Loop is the triple-conversion which is implicit in its design. Multiple conversion receivers are prone to overload and cross-modulation effects in presence of strong signals. They violate the principle that all the selectivity must immediately follow the mixer. These effects can be reduced by careful design, for example using balanced mixers and special transistors or tubes, but never eliminated. A single-conversion receiver using a Phase Locked Synthesizer is in principle the best approach and this is the reason why the Wadley Loop eventually disappeared. On the other hand, with the Wadley Loop the noise problems that affect receivers with cheap synthesizers are not present, and this is a very good quality of the design.