superheterodyne

The superheterodyne is a circuit arrangement that is the basis of almost all modern radio and television receivers. Through the years, there have been various improvements and versions, but the fundamental circuit is unchanged from when it first appeared.

HISTORY

Major Edwin Howard Armstrong, inventor also of the regenerative receiver and of FM, is credited as the inventor of the superheterodyne radio receiver circuit. During World War I, the receivers then in use by the Allied forces were unreliable and unable to intercept German radio signals on a consistent basis. These receivers were the Tuned Radio-Frequency (TRF) type – the signal as received went straight through the receiver, was demodulated, and sent to the audio amplifier without very much more processing. This circuit, though somewhat unstable, was fine for casual listening, but not for demanding conditions. Also, the Germans were using what were at the time “high frequencies” for communications, in the 500 to 3500 kilohertz (kHz) range. Ordinary radio receivers, even those modified for military service, were ill-equipped to handle such frequencies.

Armstrong, at the time (1918) a member of the U. S. Army Signal Corps, was commissioned to solve the problem. He knew from prior experiments that a radio frequency amplifier was more efficient when it only had to handle one frequency, rather than a range of frequencies (as in the TRF receiver). If a way could be found, the Major reasoned, to convert all incoming stations to a single frequency, then the problem would resolve down to one of amplification.

Further experimentation revealed that a vacuum tube configured as a signal detector could also act as a mixer in the presence of two radio frequencies. Armstrong now had his means of getting the single frequency, and it only remained to send that signal through a carefully tuned amplifier, thus creating the superheterodyne circuit. After some successful demonstrations, the Signal Corps wasted no time and certified the superheterodyne circuit for communications use. Armstrong released the circuit for general use after war’s end, although radio manufacturers were slow to employ it in commercial receivers. It wasn’t until the early 1930s that most receivers offered for commercial sale were superheterodynes.

BUILDING BLOCKS OF THE SUPERHETERODYNE

There are many variations of the superheterodyne circuit, but most have in common these stages:

• The Converter (called a mixer if there is a separate local oscillator)
• The Intermediate Frequency Amplifier, fixed-tuned to a single frequency
• The Demodulator
• The First Audio Amplifier
• The Output Audio Amplifier

The Converter stage has two functions: to generate a local oscillator signal, and to mix (or heterodyne) the signal coming from the antenna with that local oscillator signal. In most superheterodyne receivers, the local oscillator signal’s frequency will be kept constantly above the incoming signal by a value equal to the incoming signal plus the fixed frequency to which the next stage, the Intermediate Frequency amplifier (IF amplifier), is tuned. For every frequency, or channel, the receiver is capable of receiving, the receiver maintains this relationship between the incoming signal and the oscillator.

At the output of the converter, there will then appear four frequencies: the original signal, the local oscillator signal, and two new frequencies created when the mixer multiplies or heterodynes the original signal with the oscillator signal. These new frequencies are equal to (original signal + oscillator signal) and (original signal - oscillator signal)

In the United States, the intermediate frequency of a superheterodyne receiver has been standardized at a value of 455 kHz. Receiver manufacturers have used other intermediate frequencies (most notably 262 kHz for older automobile receivers), but for the last few decades 455 kHz has been in general use.

As an example, if we wish to receive a station whose broadcast frequency is 1500 kHz, the converter will mix that signal with a locally-generated oscillator signal of 1955 kHz. As noted, at the output of the converter there will appear those frequencies and the two new frequencies, 3455 kHz,and 455 kHz. It is the 455 kHz frequency we are interested in, since the following stage (the IF amplifier) is fixed-tuned to that value. At this point, the original signal has been converted to a frequency of 455 kHz and will be passed to the IF amplifier.

Since it is easier to design and tune an amplifier to amplify only one frequency, especially lower frequencies, the IF amplifier can handle the 455 kHz signal with ease and maximum efficiency. Most superheterodynes have at least one IF amplifier; more elaborate receivers may have two or more stages. Also, the IF amplifier provides most of the selectivity of the radio (selectivity refers to a receiver’s ability to pick out and receive one selected signal, to the exclusion of all others).

The Demodulator, or Detector stage also has two functions. Its primary function is to convert the IF signal to audio that can be further amplified and presented as loudspeaker output. The demodulator is usually a simple diode detector that rectifies the signal by stripping off the carrier, leaving just the audio portion of the signal.

A portion of this recovered audio will be filtered by a time-constant network and fed back to the converter and IF stages for automatic volume control (AVC) purposes. This keeps the audio output level of the receiver relatively constant, and prevents it from booming as the receiver is tuned. AVC saves the listener from constantly having to adjust the volume control.

Finally, the audio signal is passed to the First Audio Frequency Amplifier. This amplifier brings the signal up to a level sufficient to properly drive the Output Audio Amplifier, which in turn will produce sound at the loudspeaker.

There have been, since its invention, improvements to the generic superheterodyne circuit. A common one is to include one or more Radio Frequency Amplifiers (RF amplifier) ahead of the converter. In this position, an RF amplifier will improve the sensitivity of the receiver, but there can be a penalty in the form of increased reception of atmospheric noise. A well-designed RF amplifier can also provide a small improvement to the receiver’s selectivity.

Some professional superheterodyne receivers include a Beat-Frequency Oscillator (BFO), a separate oscillator operating at the same value as the IF frequency. This component makes possible reception of morse code and single-sideband (SSB) signals.

For nearly a hundred years, the superheterodyne circuit has proven its worth. It has been refined and carried into the digital era, but no other receiving circuit arrangement has thus far supplanted it.

SOURCES

Thanks to whereismymind for some important technical revisions regarding the action of the mixer!