Racal RA17 Technical & Circuit Description

The Racal RA17 used new technical advances including the Wadley Loop to eliminate drift and used a triple conversion superheterodyne format with 23 valves in the circuit.

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At the time of its launch in 1955, the Racal RA17 provided a quantum leap in technical performance over other receivers of the time.

Frequency stability was a major issue at the time - frequency synthesizers that are used widely today were not viable in the 1950s, and other schemes were cumbersome.

The Racal RA17 used a Wadley Loop circuit which was able to eliminate drift, and this gave it a significant technical advantage over other communications receivers of the time.

The RA17 circuit was nevertheless complicated and utilised a total of 23 valves / tubes, but performance in terms of spurious responses was still very good.

Technical solutions for the time

Some high performance radios used a crystal controlled first conversion with a free running variable frequency oscillator running at a lower frequency.

Typically this required a down-conversion to a wideband intermediate frequency, typically 1 Mc/s wide, and a VFO running at a frequency of possibly 5Mc/s or so, could then give the final conversion down to the final fixed frequency IF.

This approach meant that to tune between 1 Mc/s and 30 Mc/s, there would need to be 29 bands. This complicated the circuit and although it provided good performance, it still did not match the frequency stability performance of the Racal RA17.

RA17 signal path: technical description

The signal path can be seen along the top line in the circuit block diagram. The receiver adopted a conventional triple conversion approach, up-converting before finally down converting to the final IF where the majority of the selectivity was provided.

The up-conversion process and circuit meant that the RA17 was able to provide a very high degree of image response rejection, a distinct advantage as plenty of signals were present within the HF portion of the spectrum, and image signals could be a problem, especially for commercial and government use.

Looking at the block diagram in more detail, it can be seen that the signal enters on the left hand side and enters an RF amplifier with variable gain - this is provided manually by an RF gain control and using the AVC. A 30Mc/s low pass filter is also used to prevent signals above 30 Mc/s entering the receiver chain.

Racal RA17 radio receiver block diagram
Racal RA17 radio receiver block diagram

Having passed through these stages, the signal is mixed with the first local oscillator and up-converter to the first IF circuit which is between 39.35 and 40.65Mc/s. It is this high first IF that gives the RA17 its excellent image performance - the image falls at 2 x the intermediate frequency away from the wanted signal, i.e. 80 Mc/s away.

The signals then undergo two further stages of downconversion at M2 and M3, first to an IF of between 2 and 3 Mc/s and next to 100kc/s where the main adjacent channel filtering occurs. A buffered output of the 100kc/s IF is available for external use if required.

Finally the signal is demodulated and the resulting audio amplified. A 3Ω out is provided for a loudspeaker and headphones and a 600Ω output is also available.

RA17 Wadley Loop technical & circuit description

The Wadley Loop circuit forms the basis of the scheme used within the RA17 that provides the exceptional frequency stability.

The first local oscillator circuit tunes between 40.5 and 69.5 Mc/s to enable the up-conversion to take place. As the first conversion is an up-conversion, it means that the local oscillator can cover the whole coverage of the receiver without the need for any band switching.

Normally a free running oscillator running at these frequencies would not be able to provide the stability required, but the technical advantages of the Wadley Loop circuit enable the whole system to provide frequency drift cancelling, and thereby provide the frequency stability required.

The Wadley Loop circuit starts with a 1Mc/s crystal oscillator which feeds a comb harmonic generator. This produces signal harmnics up to 32 M/s and beyond.

The mixer M4 converts the signals upwards by an amount determined by the signal from the first VFO. Dependent upon the tuning of the VFO one of the comb frequencies will pass through the 37.5MHz filter and amplifier and be applied to mixer M2 to converts the desired signal from the 40MHz first IF down to the 2-3MHz second IF.

In this way if the VFO is used to convert both upwards and downwards and in this way the drift cancels out.

Please note: the terminology kc/s and Mc/s has been used rather than kHz and MHz to retain commonality with the diagrams and literature of the time.

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