The Wadley Loop HF Receivers

By Chris Arthur VK3CAE

 

See more on RA-117's

A problem that was faced by the designers and constructors of early communications receivers was that of maintaining frequency stability. Prior to WW2 achieving this aim required precision tuned circuits contained in large metal enclosures, this helped in minimising drift effects but it was difficult to maintain over several octaves. The advent of crystal controlled oscillators provided constructors with a far better frequency stable source and when used with a heterodyne oscillator could maintain high stability over a small tuning range of not more than 500 to 1000 KHz. Obviously one would need 27 crystals to cover the entire HF band (3 - 30 MHz), so we see how the need for frequency synthesis using only one or two crystals arose.

The Phase Locked loop is by no means a recent addition to Radio Receivers and Transceivers. Such a system has been in use for over 30 years now but during the 60's and early 70's the cost of implementing such complexity into a receiver or transceiver would only see specialist use such as military or high end commercial applications. Since the advent of Large Scale Integration (LSI) Integrated Circuits in the later part of the 1970's, both cost and complexity has reduced to the point were PLL controlled receivers are now common place. A system that put high performance frequency control into the reach of the Amateur and SWL'er before LSI was the Wadley Loop.

During WW2, while working on a wavemeter project a "Dr Trevor Wadley" developed a unique circuit for canceling frequency drift, a circuit later to be known as the Wadley Loop. The first well known implementation of Wadley's Loop was during the 1950's in the development of the Racal RA-17, as shown above. The RA-17 and later RA-117 where quite unique receivers, gaining extensive use by the British military from the late 1950's and through the 60's, in fact the 117 made it into the early 70's...

How does this system work?
There were several different frequency schemes used by manufacturers who employed the Wadley Loop but the basic principle of operation is applicable to all. A further comment worth making is that the Wadley loop should not be confused with a phase locked loop, both systems are quite different in operation. The following operational description is for the Barlow Wadley XCR-30.

The Loop system uses a single 1 MHz crystal to generate harmonics at 1 Meg intervals, a double mixing process with a tunable 45.5 to 75.5 MHz oscillator provides 30 tunable ranges from 500 KHz to 30 MHz. The outputs of the first mixer (42.5 MHz) and second  mixer (45 MHz) are amplified and then fed to a third mixer. With the 45 MHz Amp having a flat band-pass range of 1 MHz the output of our 3rd mixer is 2.5 MHz +- 500 kHz. This 2 to 3 MHz signal is then amplified and tuned with a 2.455 to 3.455 MHz mixer/oscillator, passing through a 455 kHz IF to the AM and SSB detectors.

The Barlow Wadley XCR-30

Who used the Wadley Loop?
During the mid 1970's, Wadley's loop system was employed by several manufacturers, such as Yaesu (FRG-7), Drake (SSR-1) and Realistic (DX-300) but little is known of the actual Barlow Wadley Receivers and for that matter the Barlows Television Company.

 To find out more about Barlows and in particular the XCR-30 HF receiver a visit to "The Barlow Wadley XCR-30 Home Page" is a must...

One of the lesser known implementations of the Wadley Loop is in the Drake SSR-1 Communications Receiver, Drake did not openly acknowledge the Wadley Loop System within the SSR-1. This was however a very nice radio and I have recently added one to my collection of 70's receivers.
 


VK3CAE (Ex VK3JEG)