I could write a book about Edwin Armstrong, let alone a few small articles. He probably contributed more to the technological development of radio as we know it today, than anyone else. Born on December 18, 1890, in New York City, he graduated with a degree in electrical engineering from Columbia University in 1913 and went on to serve in the US Army Signal Corps. In 1918, while serving in France, he developed the concept for the heterodyne receiver. The circuit, like much of the development of early radio, was the subject of a substantial amount of patent litigation. Mr. Armstrong was involved in a number of radio patent related law suits apart from the heterodyne, one of the most notable being his challenge of Lee DeForest over the invention of the regenerative receiver (see my December 2004 article) which cost both men a substantial portion of their fortunes. DeForest won that case.
I consider the vacuum tube to be the most substantial invention of the early electronic age. As far as radio was concerned, the heterodyne circuit was the most substantial thing that you could do with the vacuum tube. The principle behind a heterodyne circuit is relatively simple. It uses a vacuum tube to mix a received signal from an antenna with a slightly different frequency created by an oscillator circuit in the radio. Four distinct signals will appear at the mixer tube’s output, each of the input signals, plus the sum and difference of the signals. For example, let’s say that you want to listen to a station on 800 kHz. You point your dial to 800, but what you have actually done is to tune your radio’s oscillator to 1255 kHz. This signal is mixed with the 800 kHz signal from the station and the output produces four signals, 800, 1255, 455 (the difference) and 2055 kHz (the sum). Next these signals are fed into a filter that will only pass the 455 kHz signal. The 455 kHz signal, known as the intermediate frequency, or I.F. is then amplified and sent to the detector which extracts the audio content.
In reality, the signals from all receivable stations are presented to the mixer but only one frequency, (in this case 800 kHz) will produce the 455 kHz signal that the filter will pass. So what does this accomplish? It makes it very easy for the receiver to reject adjacent stations. The heterodyne offers vastly improved selectivity over the TRF or the Regenerative circuits. Once the unwanted signals are filtered out, the following amplifier stages can be operated with very high gain without fear of interference or instability.
Armstrong was not the first to conceive of the heterodyne principle. Others used it to mix local signals with the received signal to produce a frequency in the audio range. This was used to make Morse code reception more intelligible. Mr. Armstrong was the first to use it to produce a second radio frequency specifically for the purpose of being filtered and amplified.
Mr. Armstrong sold his patent to RCA, and RCA registered the trademark “Super Heterodyne” which caught on as the name for the topology, “superhet”, for short. Why RCA chose not to implement the circuit until the late 1920s remains an unanswered question. There were probably a number of reasons with the foremost being the technology of the day. Early triode tubes were not well suited to be used as mixers. To build a superhet with these tubes would not have been cost effective and therefore unable to compete with the regenerative and TRF receivers. The type 224 tetrode introduced in the late 1920s was just what the superhet circuit needed to reduce the parts count while out performing the older topologies.
RCA strove to be the leader in technology. As long as their TRF receivers out performed the competition, why introduce a new circuit? The superhet was not a secret, amateur operators and serious radio hobbyists (those who could afford the parts) had been building receivers with the circuit since the early 1920s. When RCA finally chose to introduce the circuit to the general consumer, they did something quite out of character for the corporate giant. They licensed the rights to other radio manufacturers. In the past, it had been RCA policy to be the “first and only” for at least a model year or two.
With its vast improvement over TRF sets, the superhet would rapidly become the circuit of choice as it remains today, in fact, despite using transistors and even integrated circuits, the design concept of the superhet has changed very little over the past seventy five years. Every modern AM, FM, and TV still uses superhet technology. It is also used by two way radios and cell phones.
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This month’s radio is a 1931 Philco Model 70, a seven tube superheterodyne AM receiver in a pine and oak cathederal style cabinet. The model 70 is stereotypical of what most people think of when they envision an antique radio. It, and its close cousin, the Model 90, have probably been used on more movie and TV sets than any other vintage set. It features the same familiar controls as a modern set: volume, tone , and tuning. It is intended for use with an outdoor long wire antenna as was popular with the TRF receivers that preceded it. The superhet design was sensitive enough that the long wire antenna would soon be replaced with a simple coil antenna internal to the radio itself. The Philco was equipped with a large speaker and a frequency calibrated dial as well. The chassis was all inclusive, and did not have a separate power supply.
