How Does a Superheterodyne Radio Work?Author: Warren Parks
A Historical Background and Theory of OperationThe superheterodyne receiver circuit made it's first appearance on the market in the mid 20's, and represented a great step forward in radio technology. This new circuit offered incredible sensitivity and selectivity, far superior to the regenerative and TRF designs. The better and cheaper superheterodyne circuit soon became the standard of radio receiver design, completely dominating the marketplace by the 30's. In fact, this revolutionary design is still manufactured by the millions today, in transistor form.
IntroductionThe purpose of this article is to aquaint the new radio hobbyist or collector with the basic ideas behind the superheterodyne receiver, without going into a lot of technical detail.
If you are interested in a more technical overview of the superheterodyne circuit and some troubleshooting tips, I strongly recommend that you read the excellent web article "Troubleshooting the Stages of a Superheterodyne Receiver" by Bill Harris. He provides a clear explanation of how each stage of the circuit works, complete with example schematics. Once you are familiar with the underlying principles presented in this article, and the technical details found there, you should be able to perform simple troubleshooting and repairs on a wide variety of superheterodyne sets.
Amplitude Modulation (AM)This is the method used to embed an audio sound wave into a high frequency radio "carrier wave". The carrier wave must be used, because it's high frequency gives it the ability to travel great distances when broadcasted from an antenna as a radio wave. The AM transmitter at the station generates this carrier wave at the broadcast frequency, encodes the audio program onto it, then amplifies it greatly for transmission.
The carrier wave (shown in blue) is between 550 Khz to 1.7 MHz for the regular AM broadcast band. The audio wave (shown superimposed in red for comparison) is a much lower frequency from about 40 Hz to 10 Khz. In the process of modulation, the amplitude (height) of the carrier wave is modified so that it's peaks follow the shape of the sound wave. The modulated shape of the carrier wave remains intact as it is transmitted and travels great distances. The radio receiver is designed to pick up this distant and weak signal, separate it from all of the other radio signals, amplify it greatly, and recover the sound wave back from the carrier wave. This audio is then amplified further, and reproduced by the speaker.
Before the Superheterodyne ...The greatest advantage of the superheterodyne radio circuit over the Tuned Radio Frequency (TRF) design was the increase in sensitivity. Listeners who used to struggle to hear distant stations with a even a long antenna could now hear them well with even a short antenna. More stations could be heard, and they sounded more clear because the off frequency noise was blocked better. This great performance was possible because of a fundamental improvement in the method used to amplify the received signal.
In a TRF system, the radio signal is picked up and amplified many times at its own broadcast frequency, which keep in mind is a pretty high one. The radio frequency that is tuned in is selected from the others by designing a series of amplifier stages (shown below) to prefer the selected frequency more than the rest. This is done by adding "resonant" (frequency selective) circuits to each amplifier. This made the total amplification much stronger for the one desired frequency, leaving the rest too weak to interfere.
Once the signal was strong enough, the audio wave was "detected" from the carrier, and sent to the audio amplifier and on to the speaker. The major problem with this design was that it was very difficult to design a radio frequency amplifier that was sensitive across the entire broadcast band. The simple three-electrode vacuum tubes that were available at the time had decent gain (amplification) at the lower frequencies, but had less gain at the higher radio frequencies. This was due to frequency dependent limitations internal to the tube that were difficult to overcome. If attempts were made to increase this gain, severe oscillation (squealing or motorboating) would result at the high end of the band. When the gain was reduced enough to prevent oscillation at the high frequencies, the sensitivity (overall amplification) was pretty low across the rest of the broadcast band. Thus many (3 or 4!) of these relatively low gain amplifier stages were required to step the signal up for acceptable sensitivity. TRF's are typically identified by the row of several identical amplifier tubes, beside a long multi-sectioned tuning capacitor that controlled which frequency is amplified.
Edwin H. Armstrong ...The superheterodyne circuit was actually invented in 1918, during the infancy of radio development. Edwin H. Armstrong, noted inventor of the regenerative radio circuit in 1912, was a Captain stationed with the American Expeditionary Forces in France during WWI. He was head of the Airplane Radio section of the Research and Inspection Division, Signal Corps, in Paris. It was their job to inspect British, French and German radio equipment, and make reports on what should be changed before the US copied the designs for our military use. (We were quite a bit behind in battlefield radio apparatus.)
There was a strong need to improve the efficiency of radio frequency amplifiers at the very high frequencies (of that time anyway, in the range of where the AM broadcast band is today) in order to keep up with the German technology. To this end, Mr. Armstrong chose a rather novel approach. Rather than attack the problem by improving tube performance, he sought a method of reducing the carrier frequency of the incoming radio signal before amplifying it. This way, the amplifier could have a much higher gain using fewer tubes, and without oscillation. It was also desirable that this reduced carrier frequency somehow be kept constant for all received radio stations. The amplifier could then be designed to have the highest gain and work best for a narrow band around that one frequency. This would also allow the tonal quality of the radio to be improved, and remain more consistant in quality across the broadcast band. All of these desirable qualities are accomplished by the superheterodyne design.
Armstrong was an incredible inventor, and probably the most important individual contributor to vacuum tube, AM and FM radio technologies. If you have the time, it is well worth reading about his life and accomplishments. Mike Katzdorn has assembled an excellent collection of historical documents and good biography, which is a great place to start.
Edwin H. Armstrong (right) with assistant and friend Harry W. Houck (left) with three superhet designs -
The Signal Corps. set built in France (left), the pre-production 2nd harmonic superhet (center) and the
production 2nd harmonic superhet, the 1924 Radiola AR-812 (right).
The Superheterodyne Conversion ..."Superheterodyne" is a fancy term that means to mix two frequencies together to produce a different frequency output. Thanks to the wonderous laws of physics, if two waveforms of different frequency are combined in the proper way, as in a superhet "mixer" circuit, an output will be produced that contains four waveforms at different frequencies. Two of the waveforms will be at the original two frequencies. A third waveform will have a frequency equal to the sum of the two original frequencies, and a fourth that is equal to the difference of the two. You can easily observe this effect yourself, by simultaneously striking two piano keys that are close together. You will hear a "wah-wah-wah" as the notes interfere. The frequency of this "beat" signal is equal to the difference between the two notes- lower if the notes are close, and higher if they are farther apart.
This is the magic behind the superheterodyne circuit. The received radio frequency carrier wave is mixed with a local signal of different frequency to generate a useful beat frequency. The local signal is generated inside the radio, in an adjustable frequency circuit called a local oscillator. The frequency of this oscillator signal is designed to change as you tune the radio, in such a way that it's frequency always stays a small but fixed difference higher than the radio carrier frequency that you are tuning in. For example:
This is exactly what was needed! Because the difference between the carrier and oscillator frequencies remains constant, so does the frequency of the difference signal, no matter what station the radio is tuned to. The audio signal that was modulated onto the original broadcast carrier is also present in this new carrier frequency. Since it is a constant lower frequency, it is much easier to amplify. This very special difference frequency is called the "Intermediate Frequency" or IF for short. The other resultant frequencies produced by the conversion are discarded.
The Superheterodyne Circuit
The basic superheterodyne receiver has the following structure:
The antenna picks up the radio signal and connects it to the preselector stage, which is either a simple tuned radio frequency amplifier or a filter circuit. This circuit will amplify the desired radio signal a little, and reduce some of the off frequency noise caused by the other radio stations. The local oscillator circuit generates the slightly higher frequency local signal that will be combined with the incoming radio signal in the mixer stage. The frequency of this local oscillator is able to maintain the fixed distance above the radio frequency because of the way the tuning capacitor is built. It has two capacitor sections that are mechanically connected to the same shaft and move together. One half controls the preselector, the other half the oscillator frequency. This "single knob tuning" design was quite an improvement over very early superhets, which had a separate knob for each.
The radio and local frequencies are combined in the mixer or "1st detector", which also serves to amplify the output signals. The useful intermediate frequency output is isolated and amplified by the IF amplifier stage, which also rejects the other undesired result frequencies. The amplifier does this by using "IF transformers" in the input and output that are resonant at and tuned to allow only this one frequency to pass. This further reduces the off-frequency noise while greatly amplifying the IF signal. Usually the IF frequency is either 455 Khz or 465 Khz in most sets, but in very early sets it may be as low as 130 Khz.
Once amplified, the IF frequency, which still contains the original audio program, is sent to the 2nd detector stage for demodulation. This is the process of separating the audio frequency (AF) from the IF carrier and sending it on to be amplified by the audio amplifier. The 2nd detector also generates the Automatic Volume Control (AVC) voltage which varies with the strength of the signal. This system is designed to adjust the sensitivity of the receiver stages to keep all stations at roughly the same volume. Without this feature, close stations would boom in really loud, while weak stations would be really quiet.
In the diagram below, you can see graphically how the audio is demodulated from the amplified IF carrier. The 2nd detector rectifies the waveform, removing the bottom or negative half of the wave.
The resulting waveform is sent through a low pass filter, which is designed to block the high IF carrier frequency, but pass the much lower audio frequency band. This separates off the audio wave, shown in red at the right. This audio signal is then amplified by the audio amplifier stage, which provides enough power to drive the speaker and reproduce the original audio of the radio broadcast.
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