Superheterodyne Radio Receiver Tutorial

An introduction or tutorial about the basics of how the superhet or superheterodyne radio receiver works and how it can be used in radio communications systems.

The superhet radio or to give it its full name the superheterodyne receiver is one of the most popular forms of receiver in use today in a variety of applications from broadcast receivers to two way radio communications links as well as many mobile radio communications systems.

Although other forms of radio receiver are used, the superheterodyne receiver is one of the most widely used forms. Although initially developed in the early days of radio, or wireless technology, the superhet or superheterodyne receiver offers significant advantages in many applications. Naturally the basic concept has been developed since its early days, and more complicated and sophisticated versions are used, but the basic concept still remains the same.

Superheterodyne receiver history
This form of receiver is based around the idea of mixing signals in a non-linear fashion. This idea was first noticed when beats were detected between two signals. R A Fessenden was the first person to notice this and he patented the idea in 1901.

However the idea lay dormant for some years as most receivers consisted of detectors and tuned circuits. The diode thermionic valve or vacuum tube was invented by Ambrose Fleming in 1904, and then a third grid was added by Lee de Forest. Although early valves or tubes were in use, they were very unstable and it was difficult to gain much useful performance from them.

A young engineer named Edwin Armstrong started to utilise the power of the vacuum tube or thermionic valve, inventing the regenerative receiver around 1910. This provided a considerable increase in useful gain over what was previously attained.

It was the onset of the Great War in 1914 that gave fresh impetus to radio receiver design. There was a requirement for sensitive radio receivers for a variety of tasks. The first major step was taken by a Frenchman named Lucien Levy. At the time the performance of valves was very poor at frequencies above 100 kHz or so, and he devised a system for reducing the frequency of the incoming signal using the system of beats - the signal could then be tuned and amplified more effectively at a lower frequency.

Edwin Armstrong came to the fore again, by developing the superhet or superheterodyne receiver as we know it today with a fixed frequency intermediate frequency filter and a variable local oscillator. His idea was developed in 1918, right at the end of the war, and as a result it was not widely used.

After the war it was discovered that similar receivers were postulated by the Germans, but none were actually made. As a result, Edwin Armstrong was credited with the invention.

The superheterodyne receiver was not used initially, as it was felt that many valves in the set did not contribute to providing signal gain, and valves were expensive. However as the number of broadcast stations increased and selectivity became an issue, along with the falling cost of thermionic valves, use of the superhet receiver started to grow in the late 1920s and early 1930s. Since then it has been in widespread use.

Mixing and the superhet receiver

The idea of the superheterodyne receiver revolves around the process of mixing. Here RF mixers are used to multiply two signals together. (This is not the same as mixers used in audio desks where the signals are added together). When two signals are multiplied together the output is the product of the instantaneous level of the signal at one input and the instantaneous level of the signal at the other input. It is found that the output contains signals at frequencies other than the two input frequencies. New signals are seen at frequencies that are the sum and difference of the two input signals, i.e. if the two input frequencies are f1 and f2, then new signals are seen at frequencies of (f1+f2) and (f1-f2). To take an example, if two signals, one at a frequency of 5 MHz and another at a frequency of 6 MHz are mixed together then new signals at frequencies of 11 MHz and 1 MHz are generated.

Concept of the superheterodyne receiver

In the superhet radio, the received signal enters one inputs of the mixer. A locally generated signal (local oscillator signal) is fed into the other. The result is that new signals are generated. These are applied to a fixed frequency intermediate frequency (IF) amplifier and filter. Any signals that are converted down and then fall within the pass-band of the IF amplifier will be amplified and passed on to the next stages. Those that fall outside the pass-band of the IF are rejected. Tuning is accomplished very simply by varying the frequency of the local oscillator. The advantage of this process is that very selective fixed frequency filters can be used and these far out perform any variable frequency ones. They are also normally at a lower frequency than the incoming signal and again this enables their performance to be better and less costly.

To see how this operates in reality take the example of two signals, one at 6 MHz and another at 6.1 MHz. Also take the example of an IF situated at 1 MHz. If the local oscillator is set to 5 MHz, then the two signals generated by the mixer as a result of the 6 MHz signal fall at 1 MHz and 11 MHz. Naturally the 11 MHz signal is rejected, but the one at 1 MHz passes through the IF stages. The signal at 6.1 MHz produces a signal at 1.1 MHz (and 11.1 MHz) and this falls outside bandwidth of the IF so the only signal to pass through the IF is that from the signal on 6 MHz.

If the local oscillator frequency is moved up by 0.1 MHz to 5.1 MHz then the signal at 6.1 MHz will give rise to a signal at 1 MHz and this will pass through the IF. The signal at 6 MHz will give rise to a signal of 0.9 MHz at the IF and will be rejected. In this way the receiver acts as a variable frequency filter, and tuning is accomplished.

Image responses
The basic concept of the superheterodyne receiver appears to be fine, but there is a problem. There are two signals that can enter the IF. With the local oscillator set to 5 MHz and with an IF it has already been seen that a signal at 6 MHz mixes with the local oscillator to produce a signal at 1 MHz that will pass through the IF filter. However if a signal at 4 MHz enters the mixer it produces two mix products, namely one at the sum frequency which is 10 MHz, whilst the difference frequency appears at 1 MHz. This would prove to be a problem because it is perfectly possible for two signals on completely different frequencies to enter the IF. The unwanted frequency is known as the image. Fortunately it is possible to place a tuned circuit before the mixer to prevent the signal entering the mixer, or more correctly reduce its level to an acceptable value.

Fortunately this tuned circuit does not need to be very sharp. It does not need to reject signals on adjacent channels, but instead it needs to reject signals on the image frequency. These will be separated from the wanted channel by a frequency equal to twice the IF. In other words with an IF at 1 MHz, the image will be 2 MHz away from the wanted frequency.

While radio communications technology has advanced enormously since the first introduction of the superheterodyne radio receiver, it is still very widely used for many radio communications applications.

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