Home Audio and Video Basic concepts Circuits - analogue Electronic components Radio Ham radio Constructional techniques Testing and test equipment Links Bookshop

Related articles

Superheterodyne radio basics

Basics of RF mixing

Basic crystal set

Superhet or Superheterodyne Radio Receiver Basics

- an overview or introduction of the basics of the superhet or superheterodyne radio receiver.

One of the most common forms of radio receiver is the superhet or superheterodyne radio receiver. Virtually all broadcast radio receivers, as well as televisions, short wave receivers and commercial radios use the superheterodyne principle as the basis of their operation.

The superheterodyne radio technique is used in most radios found around the home. Virtually all transistor portable radios as well as television sets, hand portable radios and many others use the superheterodyne principle.

... the superheterodyne radio technique is used in most radios found around the home....

Despite the fact that the superheterodyne receiver is still in widespread use, the basic concept for it was developed back in 1918, and its invention is credited to a brilliant American engineer named Edwin Armstrong who constructed the first superhet radio.

The superheterodyne radio receiver, although more complicated than some other forms of radio set, offers many advantages in terms of performance, particularly its selectivity. In this way it is able to remove unwanted signals more effectively than other forms like the TRF (Tuned Radio Frequency) sets or even regenerative radios that were used particularly in the early days of radio.

Basic superheterodyne concepts

The superhet or superheterodyne radio operates by taking the signal on the incoming frequency, mixing it with a variable frequency locally generated signal to convert it down to a frequency where it can pass through a high performance fixed frequency filter before being demodulated to extract the required modulation or signal.

It is obviously necessary to look ay this in more detail to understand the concept behind what goes on, but the main process in the superheterodyne radio is that of mixing.

How the superheterodyne receiver works

In order to look at how a superhet or superheterodyne radio works, it is necessary to follow the signal through it. In this way the processes it undergoes can be viewed more closely.

The signal that is picked up by the antenna passes into the receiver and enters a mixer. Another locally generated signal, often called the local oscillator, is fed into the other port on the mixer and the two signals are mixed. As a result new signal are generated at the sum and difference frequencies.

The output from the mixer is passed into what is termed the intermediate frequency or IF stages where the signal is amplified and filtered. Any of the converted signals that fall within the passband of the IF filter will be able to pass through the filter and they will also be amplified by the amplifier stages. Any signals that fall outside the passband of the filter will be rejected.

Tuning the receiver is simply accomplished by changing the frequency of the local oscillator. This changes the incoming signal frequency for which signals are be converted down and able to pass through the filter.

It is often helpful to look at a real example to illustrate how the process works. To see how this operates in reality take the example of two signals, one at 1.0 MHz and another at 1.1 MHz. If the IF filter is centred at 0.25 MHz, and the local oscillator is set to 0.75 MHz, then the two signals generated by the mixer as a result of the 1.0 MHz signal fall at 0.25 MHz and 1.75 MHz. Naturally the 1.75 MHz signal is rejected, but the one at 0.25 MHz passes through the IF stages. The signal at 1.1 MHz produces a signal at 0.35 MHz and another at 1.85 MHz. Both of these fall outside bandwidth of the IF filter so the only signal to pass through the IF is that from the signal on 1.0 MHz.

The basic concept of the superheterodyne radio
using a mixer to convert the frequency of the incoming signal

If the local oscillator frequency is moved up by 0.1 MHz to 0.85 MHz then the signal at 1.1 MHz will give rise to a signal at 0.25 MHz and another at 1.95 MHz. As a result the signal at 1.1 MHz giving rise to the 0.25 MHz signal after mixing will pass through the filter. The signal at 1.0 MHz will give rise to a signal of 0.15 MHz at the IF and another at 1.85 MHz and both will be rejected. In this way the receiver acts as a variable frequency filter, and tuning is accomplished by varying the frequency of the local oscillator within the superhet or superheterodyne receiver.

The advantage of the superheterodyne radio 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.

Images in the superheterodyne radio

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 0.75 MHz and with an IF of 0.25 MHz, it has already been seen that a signal at 1.0 MHz mixes with the local oscillator to produce a signal at 0.25 MHz that will pass through the IF filter. However if a signal at 0.5 MHz enters the mixer it produces two mix products, namely one at the sum frequency which is 1.25 MHz, whilst the difference frequency appears at 0.25 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 acceptable value.

This RF tuning 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 0.25 MHz, the image will be 0.5 MHz away from the wanted frequency.

Using a tuned circuit to remove the image signal

Complete superheterodyne receiver

Having looked at the concepts behind the superheterodyne receiver it is helpful to look at a block diagram of a basic superhet. Signals enter the front end circuitry from the antenna. This contains the front end tuning for the superhet to remove the image signal and often includes an RF amplifier to amplify the signals before they enter the mixer. The level of this amplification is carefully calculated so that it does not overload the mixer when strong signals are present, but enables the signals to be amplified sufficiently to ensure a good signal to noise ratio is achieved.

The tuned and amplified signal then enters one port of the mixer. The local oscillator signal enters the other port. The local oscillator may consist of a variable frequency oscillator that can be tuned by altering the setting on a variable capacitor. Alternatively it may be a frequency synthesizer that will enable greater levels of stability and setting accuracy.

Once the signals leave the mixer they enter the IF stages. These stages contain most of the amplification in the receiver as well as the filtering that enables signals on one frequency to be separated from those on the next. Filters may consist simply of LC tuned transformers providing inter-stage coupling, or they may be much higher performance ceramic or even crystal filters, dependent upon what is required.

Once the signals have passed through the IF stages of the superheterodyne receiver, they need to be demodulated. Different demodulators are required for different types of transmission, and as a result some receivers may have a variety of demodulators that can be switched in to accommodate the different types of transmission that are to be encountered. The output from the demodulator is the recovered audio. This is passed into the audio stages where they are amplified and presented to the headphones or loudspeaker.

Block diagram of a basic superheterodyne receiver

The diagram above shows a very basic version of the superhet or superheterodyne receiver. Many sets these days are far more complicated. Some superhet radios have more than one frequency conversion, and other areas of additional circuitry to provide the required levels of performance. However the basic superheterodyne concept remains the same, using the idea of mixing the incoming signal with a locally generated oscillation to convert the signals to a new frequency.