What’s the difference between an old TRF (Tuned Radio Frequency) radio from the 1920’s and early 1930’s and the “superhet” design which replaced it and is still with us today? And why were there two entirely different designs anyway?
We answered these questions in a Pacific Region workshop on
May 20th in
Early radios were very simple technically, mostly because their designs were limited by the technology of the day, the triode vacuum tube.
We demonstrated a 1925 battery set whose 3 tuned amplifier stages were all ganged together by belts so by adjusting one know we could tune all 3 radio frequency circuits at the same time – hence the term tuned radio frequency or TRF.
The early vacuum tubes were dependent solely on manipulation of filament current to control their amplifying capacities and tended to oscillate or whistle at the drop of a hat. This meant that the early operator had to fiddle with a whole collection of “modifier”, “selectivity”, “sensitivity” or “rheostat” knobs on the front panel, all to assist in tuning in a station while keeping the radio set from either distorting or howling. They worked but left a lot to be desired in reliability and ease of operation.
Enter the “superhet” design, short for superheterodyne, which means “mixing”. This mixing created a new frequency, the intermediate frequency or “I.F.” which improved the selectivity, sensitivity and reliability of radio because all signals coming into the radio were converted, by mixing an internally generated signal with the incoming to create only one signal to be amplified, the “I.F.”. Thus the entire design of the radio could be concentrated to maximize its performance by only having to amplify one frequency, not dozens.
The superhet design gave us radio at the turn of a key, so to speak – turn on the set, set the volume and dial in your favourite station. Rapid advancements in vacuum tube technology in the 1930’s made it all possible, we use the same “superhet” designed today, virtually unchanged from 75 years ago.
In our September 2007 workshop, we’ll take a detailed look at a typical radio receiver to probe the whys and wherefores of all those mysterious components.