Nuts and Volts - March 2008

Although cell phones,

global positioning

system receivers, satellite television systems, and the AM/FM radio in your car perform completely different functions, the receivers used in these systems are all based on a concept first developed by the American electrical engineer Edwin Armstrong during the waning days of the first World War. Almost a century after its introduction — except for sophisticated approaches such as software radio that involve advanced digital signal processing techniques — Armstrong’s “superheterodyne” or “superhet” design reigns supreme in communications electronics.

Continue reading to learn how to build a simple superhet receiver that demonstrates Armstrong’s concepts.

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March 2008

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A SIMPLE SUPERHET

Receiver Design Evolution

To understand the superhet’s superiority over previous designs or architectures, it is helpful to review a few of the difficulties of one receiver design that it replaced — that of the “Tuned Radio Frequency” or TRF receiver, shown in Figure 1. The TRF receiver came about from the common sense observation that since radio signals coming from the antenna are extremely weak, a more sensitive receiver might be obtained by amplifying the radio frequency (RF) signals immediately following the antenna. And given the poor performance of early vacuum tube amplifiers, if one stage provided insufficient gain, then cascading more stages should lead to a receiver that became progressively more sensitive (able to receive even weaker signals) and selective (able to select the desired signal and reject the others).

Unfortunately, these ideas

■ FIGURE 1. Tuned Radio Frequency Receiver (TRF) block diagram showing intra-stage feedback (dashed line) and inter-stage feedback (dotted line).

contain a number of troubling flaws that ultimately proved fatal to widespread application of this approach.

As far as increasing sensitivity goes, a problem immediately occurs because the radio frequency amplifiers necessary for each stage of the TRF receiver are inherently unstable. This is because in all electronic amplifiers — be they of the vacuum tube or transistor type — a small amount of capacitive coupling exists between the input and output of the device itself.

This intra-stage feedback is shown as the dashed line between the input and output of each stage in the figure. Several techniques, such as neutralization, may be employed to extend the frequency range of an individual TRF stage, but the difficulties of avoiding oscillation in tuneable, high-gain RF stages mount directly as the operating frequency increases.

Additionally, inter-stage feedback occurs when the output of one amplifier stage appears at the input of a preceding amplifier stage. The dotted lines in the figure show the multiple feedback paths that may exist in a TRF receiver with only a few stages. Either increasing the