Any discussion of the superheterodyne radio, a system still in use today, has to include mention of its inventor, Edwin Armstrong.  Armstrong was a brilliant electrical engineer who, early in the 20th Century, developed the circuits and receiving systems that enabled the Radio Age to grow from simple crystal sets and spark-gap transmitters into the massive communications networks we have today.  

     While he was still a student in college, Armstrong experimented with a primitive kind of electron tube called the Audion.  The Audion Tube had been developed by another famous engineer, Lee DeForest and it consisted of three elements, a hot wire cathode, a metal plate called the anode and in between the cathode and the plate was a grid work of wire called, of all things, a grid.    The Audion was very expensive, it would burn out quickly, it could not amplify sound and it had many other shortcomings too, but it could detect and amplify radio signals, which is something no other electronic device of the time could do.  The very first amplified radios used the Audion Tube, but they never became very popular because the Audion tube they used was so expensive and unreliable.  

      Armstrong discovered that he could make the Audion oscillate (produce a stable radio frequency) by feeding some of the tube's output power back into its input -- very much like how sound from a speaker gets into a microphone and causes a loud feedback.  He patented the design of this oscillator and today we know it as the Armstrong Oscillator.   Out of the Armstrong Oscillator, he developed his famous regenerative detector and patented it in 1912.  Soon after he graduated from college, America entered the Great War (World War One) and Armstrong joined the U.S. Army Signal Corps with the rank of Captain.  While serving in the Army, Armstrong offered the Government free use of his radio patents, including his regenerative detector.  Regenerative detector radios were far, far superior to the crystal radios then in use by the Army and Navy, but the military's crystal radios were already in service and they worked well enough for their purposes at the time, so the regenerative radio was not adopted to any great extent.  


     In 1917, while designing better radio direction finding equipment, Armstrong invented perhaps his most important contribution, the superheterodyne radio (patented in 1918).  Armstrong's new kind of radio was several years ahead of its time and I say that because the electron tubes (valves) available in 1917 just weren't good enough to make the superheterodyne practical.  The introduction of the superheterodyne would have to wait until after 1920 when Westinghouse began to manufacture a really good triode tube (the UV199).

     By the early to mid 1920s, powerful new transmitting tubes that could handle thousands of watts of power were being produced and AM stations were appearing everywhere.


     With all that wonderful music and entertainment to listen to, the radio craze was sweeping the industrialized world.  People wanted good performing, amplified radios to replace their crystal radios so that they could hear even far away stations and what was more, there were people who were willing to spend a lot of money to get one of these radios.  At this time, there were several amplified radio designs out there all based on high vacuum triode tubes and all of them were expensive.

     Armstrong's regenerative radio design was superior to all the others in terms cost and simplicity, but it too had its serious shortcomings.  Competing with and sometimes including elements of Armstrong's regenerative radio, were the Tuned Radio Frequency (TRF) radios.  Some of these TRF radios were quite powerful, very complex, extremely expensive and beautiful works of art.  The early Atwater Kent radios are a good example of what I'm talking about.  


     As successful as the early triode tubes were, everybody could see that there was a need for even better and cheaper tubes.  It was the shortcomings of those early tubes that spurred other electrical scientists and engineers to rapidly develop highly sophisticated new tubes.  It was those better tubes that finally made Armstrong's superheterodyne radio practical and, of course, those tubes also opened the door to further innovation in radio and television, making possible all the later developments that evolved into the modern radio communications infrastructure we have today.

     Finally, in 1924, the very first commercial superheterodyne radio was manufactured and sold by RCA (the Radiola model AR812), but it was not much of a success.  It was very complicated, it carried a big price tag and it was very expensive to operate (it took four batteries).  It really needed better tubes than the triode tubes that were available and so it might have been a mistake to manufacture and market it so early.  However, it did open the door, so to speak and not long after this, with the introduction of the first really good, affordable and sophisticated multi-element tubes, the superheterodyne radio design began to dominate all other radio designs, just as it does even to this day.  




     The following then is a simplified essay on how a typical tube type superheterodyne radio works.  The basic principles discussed are also true for later radios using transistors and integrated circuits.

The Antenna Circuit

     As the diagram shows, the superheterodyne consists of just a few basic sections with each section made up of its own electronic circuit.  The first section to be discussed is the antenna circuit.  This circuit's job is to select a relatively narrow band of signals from among the wide band of signals that are captured by the antenna.  It is just one signal within this band that is the radio station we want to listen to, but the radio must first "filter out" or "tune out" the other signals on the antenna so that only this narrow band (containing our station) gets through to the other parts of the radio.

     By the way, you may already know that each broadcast band AM station transmits its signal on just one assigned frequency, but the broadcast band itself covers a wide spectrum containing dozens and dozens of stations.  The spectrum begins at 550,000 cycles per second (or 55 kilo Hertz or 55 KHz) and extends all the way to 1,600,000 cycles per second (1.6 mega Hertz or 1.6 MHz).  It is just a narrow "slice" of this band of signals that we want our radio tuned to at any one time, so it is the job of the antenna circuit to "select" this "slice" and reject the rest of the band.  For example, if our station is transmitting at 1,400,000 cycles per second (1.4 MHz), then we only want a "slice" of the band that contains this frequency and we want that "slice" to be as narrow as possible so that it doesn't include too many other unwanted stations.  In practical application, the "slice" is still pretty wide, but narrow enough so that our radio's later circuits can pick our station out of the "slice."

     The antenna circuit consists of an antenna coupling transformer and a resonating tuned circuit that actually selects the narrow band of signals that contains our station.  We "tune" the antenna circuit when we turn the tuning dial and change the value of the tuning condenser. By changing the "resonate frequency" of the antenna tuned circuit,  only that band of signals containing the signal we want to hear gets through the antenna stage and all the other signals that come in on our antenna wire are rejected (or "tuned out").  It is important that only a single, relatively narrow band of signals gets through the antenna tuning stage or else a lot of interference by unwanted signals (called "images") occurs and makes our radio useless.


     I'd like to draw your attention to the fact that the tuning mechanism actually has two variable condensers (capacitors) on the same shaft and when you change the value of one condenser, you change the value of the other one too.  The second tuning condenser controls the frequency of the Local Oscillator.

The Local Oscillator

     The word 'heterodyne' in the word superheterodyne means to 'mix' or 'blend' with another signal to produce a third signal that other parts of the radio can use.  This other signal that is mixed or heterodyned with the antenna signal to produce the third signal is produced by electronic circuits called the Local Oscillator (or LO).  This is a 'tunable' oscillator meaning that the dual capacitor, I mentioned above, changes value when you turn the dial to cause the Local Oscillator to change frequency.  For example, the LO of a typical AM radio can change its output frequency from 1,005,000 cycles per second (1.005 MHz) to 2,055,000 cycles per second (2.055 MHz) over its tuning range.  The LO frequency is created by oscillator coils and capacitors and is powered by the lower half of the first tube in the radio called the "first detector/oscillator tube."  The first detector/oscillator (also known as the mixer/oscillator) is usually a specially built tube called a "pentagrid converter" tube.

The Mixer -- also called The First Detector

     The actual mixing (or heterodyning) takes place in the top half of the first detector/oscillator tube.  This part of the tube is more commonly called the "mixer stage" which is a very descriptive phrase, because that is exactly what it does, it mixes. The mixer stage takes the very weak antenna signals that are in that band of frequencies I mentioned earlier and mixes them with the LO signal from the bottom part of the tube.  Armstrong discovered that when you mix (or heterodyne) signals together in a mixer tube, you get all kinds of things out of it, and one of the things you get is something very useful.  When you mix two signals, you get the original LO signal -- which you no longer need, you get the original antenna signal -- which you no longer need, you get the LO signal plus the antenna signal -- which you can't use because it is too high in frequency, but finally and most importantly, you get the LO signal MINUS the antenna signal.  This minus signal is very important because it is the signal the next stage, the Intermediate Frequency Amplifier will use.

     Let me give an example of how this mixing works.  Say that there is a radio station that is broadcasting on 1400 on the AM dial.  That means that the frequency of its signal is oscillating at 1.4 million cycles per second (known as 1.4 Mega Hertz or 1.4 MHz).  If our 1.4 MHz signal is "mixed" ("heterodyned") with a LO signal of 1.855 MHz, the subtraction result would be 455 thousand cycles per second (known as 455 kilo Hertz or 455 KHz).
 In math terms: 1,855,000 (the LO frequency) - 1,400,000 (the station frequency) = 455,000 (the Intermediate Frequency).  

     All the other stations in the band of frequencies from the antenna tuning circuit are either above or below this 455 KHz signal.  For example, if there is another station at 1440 on the dial, the result of mixing would be 415 KHz (1.855 MHz - 1.440 MHz = 0.415 MHz or 415 KHz), but we don't want to hear that station, we only want to hear the one that has been transformed to that 455 KHz frequency and somehow "tune out" or "reject" all other frequencies.  

     Armstrong called those frequencies down like this "supersonic" frequencies (a term that has a different meaning today) because they were far above what can possibly be heard.  They are lower in frequency than the original signal, but still way too high in frequency for a person to hear -- the "sound" waves coming out of the mixer was thus  "super -- sonic."  Today we don't consider this frequency a "sonic" wave anymore, but consider it a low frequency radio wave.

     The next stage to processes this supersonic signal (radio wave) is called the Intermediate Frequency Amplifier (also known as the IF amplifier or simply as the IF).

The Intermediate Frequency Amplifier or IF

     The neat thing about the new supersonic frequency (radio wave) that has been created through heterodyning is that this signal is low enough in frequency that a really narrow "slice" can now be taken and all the close by stations are "sliced out" or eliminated.  The Intermediate Frequency amplifier (in an ordinary radio) has two or more very sharply tuned transformers that are almost always tuned to 455,000 cycles per second (455 KHz).  This 455 KHz frequency was chosen many years ago as an ideal IF frequency because it is easy to get really sharp, narrow tuning down there and there are other technical reasons for that frequency too, but we don't need to get into that.  Just remember that 455 KHz (or something very close) is the IF frequency your radio will most likely be tuned to.  

     The input to this stage is called "the first IF transformer and it consists of tuned circuits that select only signals at or very near 455 KHz.  Take a close look at the diagram of the 1st IF transformer (XFMR) and you will see two "tuned circuits" consisting of coils and their associated variable condensers.  The two tuned circuits are "lightly coupled" to each other through an air gap.  This arrangement makes for very sharp tuning.  The tuning is so sharp that most of the unwanted, but nearby signals are blocked while letting our station to get through, but this comes at a price because some of the signal we want to hear is weakened by this filtering.  Since we have to make up for the losses in the first IF transformer (and in the second IF transformer too), a tube with a lot of amplification is used.  This tube is called the IF amplifier tube. The IF amplifier tube is a special kind of tube that not only boosts our signal way up, its amplification can also be controlled so that if the signal is too loud, it can be cut back automatically.  Technically, the special kinds of tubes that can be controlled this way are called "variable transconductance" or "variable mu" tubes (some authors call them "remote cut-off tubes too), but let's not worry about that now.

     After the signal gets boosted by the IF amplifier tube, it goes through another transformer called, of all things, the second IF transformer, where the signal we want to listen to is made even purer with even fewer unwanted signals to cause interference.  

The Second Detector

     At the output of the second IF transformer, our signal is ready to listen to, but we can't possibly hear it because at 455 KHz it is far too high in frequency for the human ear to hear (it is "supersonic," remember?).  Our signal must then be "detected" by an AM detector tube.  This tube is called the "second detector" because it follows the first detector tube that originally mixed the antenna signal with the LO signal and it is the one that "detects" the sound that is contained in the 455 KHz IF signal we have just been discussing.  The second detector is really an old fashioned "Fleming Valve" detector that was invented by a scientist named Ambrose Fleming back in 1906.  The detector has the ability to turn radio frequency (455 KHz) signals into undulating direct current waves that we can actually hear as sound.

Automatic Volume Control

     Now, what happens if we are tuning across the dial and we suddenly tune in a very powerful signal or we are listening to a distant station who's signal gets stronger and then weaker then stronger again?  Well to keep the volume nearly constant, the second detector tube is arranged to put out a direct current voltage that is proportional to how strong the station's signal  is.  A very strong signal will put out a large negative voltage, but a weak station will put out a small negative voltage.  The voltage that is produce by the second detector is called the Automatic Volume Control voltage (also called the AVC or the AGC) and that voltage is fed back to the IF amplifier tube.  As I mentioned, the IF amplifier tube is a "variable transconductance" tube which means that if the negative voltage is high, the amplification will be very low, but if the negative voltage is low, the amplification will be very high.  This means that on strong signals that produce a lot of negative voltage, the amplification is low resulting in the sound being cut back automatically, but if the station is weak, the amplification of the tube will be high which will boost up the sound so that it is about the same as the sound level of the stronger station.  

     At the bottom of the diagram of the second detector shown above, note that the average voltage is negative.  The stronger the station, the more negative that voltage is and by feeding back that negative voltage, the amplification (or "gain") of the IF amplifier is controlled.  Strong signals are cut down and weak signals get maximum amplification so that the sound level doesn't vary all that much.

Audio Amplification Stages

     The problem with the electrical sound waves produced by the second detector is that they are too weak to drive a speaker or to even be heard.  The other half of the second detector tube is actually a very sensitive audio amplifier which is wired to a volume control.  From the volume control, the audio wave is amplified way up by the audio power amplifier tube which is wired through a transformer to the speaker and it is the vibrations of the speaker that we hear as our station's sound.

     Electron tubes need high current at low voltage to cause their cathodes to glow red hot.  These high temperatures cause electrons to "boil off" of hot cathodes in a process technically known as "thermionic emission."  This is the very same process that was noticed by scientists working for Edison in the 1880s and is also called "the Edison Effect."  It is this "thermionic emission" of electrons that enables an electron tube to do what it does.  For most radios, it doesn't matter if this current is AC or DC, all it has to do is cause the cathode to get red hot.  

     To make these electrons flow in the other parts of the tubes, a radio needs a supply of high voltage and this high voltage must be DC because AC won't work.  The voltage needs to be positive (+) to attract the electrons (-) that are boiled off the hot cathode.  These positive voltages (called B+ voltage) can be as high as 350 volts in some radios, but is usually between 250 and 110 volts in most tube radios.  In many older radios, this high voltage comes from a transformer, but the so-called All American Five radios have no transformer and the high voltage comes directly from the 120 volt AC wall socket.  Because there is no transformer to "isolate" the radio from the AC of the wall socket, touching any part of the inside of the radio will result in a terrible (and sometimes fatal) shock.  Many people have been electrocuted by AA5 type radios over the years, so if you have one, I urge you to read and follow my AA 5 safety suggestions found elsewhere on my website.  

     The high voltage AC that comes from the wall socket or from a transformer has to be turned into a positive (+) high voltage DC for the tubes to operate, as I explained before.  Turning AC into DC is called "rectification" and it is done by the "rectifier tube."  The rectifier tube is another example of a Fleming Valve, only it is designed to rectify high voltages and currents.  In later tube radios, the rectifier tube is replaced by a selenium rectifier so that they only have four tubes instead of the usual five tubes.

The End