Superheterodyne Radio Receiver Block Diagram

Here is a block diagram of a typical superheterodyne (superhet) radio receiver, together with theory and notes explaining each block. I have kept the theory very simple and at introductory level for beginners, however at some point there will be another article taking it further. If you like playing with radios then there is a great section on this site.

RF Amplifier

The radio frequency (RF) amplifier block amplifies the signal received from the tuned circuit. At this stage, the frequency of the signal is extremely higher than audio frequencies and a special transistor is used to increase the amplitude of the RF signal. Amplification at this stage improves the sensitivity of the radio allowing the reception of weaker signals from distant stations.

Although a radio has many amplifier blocks, each amplifies signals at a different frequency, and has special transistors that operate over that frequency.

Mixer Block

To understand the mixer and its reason for existing, we have to consider components such as the silicon transistor and its limitations. The transistor works well at amplifying audio frequencies; however, it is not very good when it comes to radio frequencies, which are extremely high in comparison. Therefore, what is required is a way to lower the radio frequency to one that is manageable by a silicon transistor, and they managed a way to do this using a process known as mixing. The physics of this process is that when you mix to waves together, you create a diffraction pattern where in some places the two waves add, and in others, they subtract. Mathematically, it is the product of two frequencies, resulting in the creation of harmonics of lower frequencies and higher frequencies. Therefore you obtain a spectrum of frequencies from the mixer block. It turns out that the lower frequency harmonic wave is still like the original radio wave with the information we need but of a much lower frequency, and therefore we use this one in the following stages of our radio.

Over the years, someone decided to give this part of the mixing process a name calling it heterodyning. The name is not that important but the physics behind it, which "experts" seldom understand is.

Knowing this, we know, without a doubt, that the output from the mixer will consist of two main products.

• Sum of the radio signal frequency and local oscillator frequency
• Difference of the radio signal frequency and local oscillator frequency

If the local oscillator frequency were to be 1455 kHz, and incoming radio signal frequency was 1000 kHz, then the mixer block will produce two IF frequencies, which will be 2455 kHz and 455 KHz. In the interests of keeping things simple, we choose the lowest of the two frequencies to pass to the next stage in the IF amplifier block.

Since the chosen IF is 455 kHz, the other frequency 2455 kHz is the image frequency, which we reject using filters.

The image, or second channel response, is simply one set of resultants due to the product sum of the oscillator frequency and radio signal frequency. In reality, if we were to look at the signal in the mixer block using a spectrum analyser, we would see a spectrum of resultants together with their mirror images.

Local Oscillator

The local oscillator block is responsible for generating high frequency sine waves. It is local to the radio receiver to mean part of the receiver circuitry. This signal feeds into the mixer block for mixing with the incoming radio frequency signal. The local oscillator frequency is usually slightly above the incoming radio frequency. To achieve this, the oscillator circuit links mechanically to the tuning capacitor so that as you move the tuning capacitor, the oscillator frequency follows the carrier frequency of the station. This is why tuning capacitors have two stages, where one stage connects to the ferrite coil for tuning the stations, whilst the other connects to the oscillator circuitry.

Since the oscillator frequency follows the carrier frequency of the incoming radio signal, it just so happens that the difference of the two frequencies is always the same irrespective of which station is tuned. Therefore the IF output signal from the mixer is always on the same frequency, and this is a big advantage because we can then design the IF amplifier block so that it amplifies a signal of one particular frequency and reject anything else such as noise. This is why the IF amplifier block usually consists of band pass filters and high gain amplifiers.

IF Amplifier

The intermediate frequency (IF) amplifier, amplifies the radio signal coming from the mixer. It rejects all harmonics produced by the mixer stage and allows only the lower one. The lower harmonic still consists of the carrier and the information we want. It is neither at radio frequency level, nor audio frequency level but in between or intermediate. The IF block, is the central stage, consisting of high-gain amplifier stages that amplify within a narrow frequency range. It ensures only the signal is amplified and not any other RF noise. Consequently, this type of block consists of band pass filters. Modern transistor radios usually have an IF band pass of 455 kHz for AM reception, and 10.7 MHz for FM reception.

Whilst an AM pocket radio might have one IF stage, larger radios have many IF stages referred to as an IF strip. For example, the Zenith Transoceanic 3000-1 has three IF stages, each stage consisting of a transistor and tuned coils. An advantage of having multiple IF stages, is that it improves selectivity.

Detector

If you have been playing with crystal radios, then you might already have come across this term where radio engineers refer to a germanium diode as a detector. A germanium diode in a crystal radio set does the same thing because of its non-linear response.

The detector block is where the demodulation process takes place. Demodulation is the process that retrieves the original modulating wave. This is where the carrier wave is separated from the information it is carrying, which is usually the original audio signal.

The function of the detector is to strip the carrier from the original audio signal, and it does this by utilising a germanium diode to rectify the AM carrier. We use this type of diode because it has a very low forward voltage of approximately 0.2 V, and the signal amplitude at this stage has to be above this otherwise it cannot be detected.

The rectification process (half-wave rectification) eliminates the negative half of the carrier leaving only the positive half. The half-rectified signal then finally passes through a RC filter to strip the carrier away from the audio signal.

AGC

In this block diagram, there is an automatic gain control (AGC) at the radio frequency (RF) stage, known as RF AGC. When radio signals are from nearby sources, the signal is very strong, and when they are from distant locations the signal is very weak in amplitude. The IF stage consists of extremely high-gain amplifiers, however they can become overloaded if the input signal was too strong. Therefore, the purpose of the automatic gain control block is to adjust the gain of the IF amplifier block so that it remains within a specific operating range.

AF Amplifier

The audio frequency (AF) block is responsible for amplifying the weak AF signal by increasing its amplitude. When the amplitude is sufficiently high enough, it can feed a power amplifier to drive a loudspeaker.

Loudspeaker

The loudspeaker is an electro-acoustic transducer, which radiates acoustic power into the air with the same waveform as the electrical input signal. It is a device, which converts electrical energy into mechanical energy in the form of vibrations. It usually consists of a paper cone diaphragm, which sets the air in motion to create sound waves.