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Another reason to be familiar with the history of radio receivers, is that there are holes in technology along the way down the historical path — often more than you might think; there are areas which have been under-developed due to reasons such as impracticability at the time of the design. The size of components, for instance, might once (in the days of early valve radio receivers) have been too large or too expensive for a certain design, but might now be much smaller and cheaper. Development in the components and other design elements opens the door for these holes in technology to be filled, and elements from old designs to find a place in modern and future radio receiver designs. Other early designs did not “take off” due to the lack of technical knowhow, and lack of components (such as sharp crystal filters) which exist today. One good example of this is what was known as the “Superinfragenerator” concept. 1 2 Though this concept offers a simple and effective mode for digital communication, it was never taken up again after its development. Now that we’ve discussed why it might be important to look at the history of radio ideas, let’s examine some early developments in radio design.
Early Radio Development Selectivity Selectivity is the ability of a radio receiver to sort out one frequency from another, and for the transmitter to keep the signal within the confines of what is needed for the receiver to recover the original signal. Figure 1 is a graphi-
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The description of a modern receiver is quite different from that of a receiver of thirty years ago. The philosophy of what is important in a radio receiver is constantly changing. This is due to changes in technology which enable ever better performance. It seems to me to be a good idea to see why that is so, and a good way to do that is to delve into the history of radio receivers. Knowing the history of radio receivers can make allows you to see how and why things have changed with time, and allows a designer to better spot a good design. It may also be useful to use parts of older designs to complement a new design. For instance, oscillators, which are used in most radio receivers have changed over time to the point where noise generated within is likely to shift the design philosophy for a new generation of radio receivers. This will be covered in a future article.
cal representation of the selectivity of cascaded tuned circuits. Selectivity was an early issue to be conquered by the budding wireless industry. This was because all stations within listening range at that time would interfere with each other because they all broadcast on a very wide frequency band using spark transmitters. See the black trace of Figure 2. This severely limited the usefulness of the new wireless technology.
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Figure 1: The cumulative effect on selectivity of one, two, and three tuned circuits. The Y axis is the amount of attenuation on a signal which passes through a tuned circuit. Notice that on the resonant frequency, very little attenuation occurs. However outside of the resonant frequency, a signal is attenuated more when more tuned circuits are cascaded. Each may be separated by an amplifier.
Average Output Power 10dB/Division
Why look at history?
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Frequency (MHz) Figure 2: Spectrum of a 4.5 MHz Spark Transmitter. In black and of a clean signal from a valve transmitter in green. Output power is shown on a log scale, and is relative.
One of the causes of the loss of so many lives aboard the Titanic was the lack of selectivity of its radio receivers and in fact to the broad frequency spectrum of the transmitted signals of that time as well. Titanic’s radio operator was trying to receive telegraphic signals from New York,
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