Superheterodyne Receiver Block Diagram

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The superhet radio receiver is used in many forms of radio broadcast reception, two way radio communications and the like.

It is useful to have an understanding of the different signal blocks, their functions, and the overall signal flow, not only for the RF circuit design, but also from an operational viewpoint. It is possible to get the best performance by understanding its internal RF design and the functions of the various circuit blocks.

There are several different circuit blocks that make up the overall receiver, each one has its own function.

Whilst the superheterodyne receiver block diagram below is the most basic format, it serves to illustrate the operation and the challenges of the electronic circuit design. More complicated receivers with more complicated block diagrams are often seen as these radios are able to offer better performance and more facilities.

Whether they are used for broadcast use, monitoring or two way radio communications, the same principles are used, although for more demanding operation, the receivers tend to be more complicated.

Superheterodyne receiver circuit blocks

There are some key circuit blocks within the RF design of the basic superheterodyne receiver. Although more complicated receivers can be made, the basic RF circuit design is widely used – further blocks can add improved performance or additional functionality and their operation within the whole receiver is normally easy to determine once the basic block diagram is understood.

Often to provide improved performance additional frequency conversions are included to improve the separation between the wanted signal and the image response. Nevertheless the same basic principles are used and often the same types of circuit blocks are used, although the overall topology is different.

Normally low cost broadcast and other forms of radio receiver tend to have much simpler block diagrams and the higher performance radios used for professional radio communications and monitoring applications tend to have more complicated block diagrams.

The main types of circuit bock used in the RF circuit design for superhet receivers is given below.

• RF tuning & amplification:   This RF stage within the overall block diagram for the receiver provides initial tuning to remove the image signal. It also provides some amplification. There are many different approaches used within the RF circuit design for this block dependent its application.

  The electronic circuit design presents some challenges. Low cost broadcast radios may have an amplifying mixer circuit that gives some RF amplification. HF radios may not want too much RF gain because some of the very strong signals received could overload later stages. The RF design may incorporate some amplification as well as RF attenuation to overcome this issue. Radios for VHF and above will tend to use more gain to have a noise figure that is sufficiently low to receive the signal. Noise is a particular problem for VHF / UHF radio communications systems.

  If noise performance for the receiver is important, then this stage will be designed for optimum noise performance. This RF amplifier circuit block will also increase the signal level so that the noise introduced by later stages is at a lower level in comparison to the wanted signal.

  All radios will need a sufficiently high level of image rejection, and this is provided by the RF tuning. High IF frequencies enable the RF tuning to be more effective as the difference between the wanted signal and image is increased.

  Read more about . . . . RF amplifier & tuning.
  
  

• Local oscillator:   Like other areas of the RF circuit design, the local oscillator circuit block within the superhet radio can take a variety of forms.

  Early receivers used free running local oscillators. There was a considerable degree of RF circuit design expertise used with these oscillators in high performance superhet radios to ensure the lowest possible drift. High Q coils, low drift circuit configurations, heat management (because heat causes drift), etc . .

  Today most receivers use one or more of a variety of forms frequency synthesizer. The most common approach in the RF circuit design is to use a phase locked loop approach. Single and multi-loop synthesizers are used dependent upon the requirements, performance, cost and the like. Direct digital synthesizers are also being used increasingly.

  Whatever form of synthesizer is used in the RF design, they provide much greater levels of stability and enable frequencies to be programmed digitally in a variety of ways, normally using some form of microcontroller or microprocessor system. They are more complicated than the older variable frequency oscillators, requiring many more electronic components, but providing a very much higher level of performance.

Note on the Frequency Synthesizers:

RF frequency synthesizers enable stable signals to be produced and controlled by a programmable input. There are several different types of synthesizer: some based on phase locked loop techniques, and others use digital technology to create a waveform directly. Often complete synthesizers may incorporate one or more types of technology

Read more about Frequency Synthesizers.

• Mixer:   The mixer can be one of the key elements within the overall RF design of the receiver. Ensuring that the mixer performance matches that of the rest of the radio is particularly important.

  Both the local oscillator and incoming signal enter this block within the superheterodyne receiver. The wanted signal is converted to the intermediate frequency.

  The actual implementation requires that the minimum number of spurious signals are generated. In some very low cost broadcast receivers, self oscillating mixers that provide RF amplification from a single transistor and a few other electronic components may be used, these do not offer high performance. For a high performance radio used for two way radio communications an the like, much better performance is required. To achieve this mixer circuits such as balanced mixers, double balanced mixers, and the like may be seen within the oerall electronic circuit design.

  Read more about . . . . RF mixer.
  
  

• IF amplifier & filter:   This superheterodyne receiver block provides the majority of gain and selectivity. Often comparatively little gain will be provided in the previous blocks of the RF circuit design of the radio. The IF stages are where the main gain is provided. Being fixed in frequency, it is much easier to achieve high levels of gain and overall performance.

  Originally the IF stage might have included a number of different transistors, FETs or thermionic valves / vacuum tubes and other electronic components, but nowadays it is possible to obtain integrated circuits that contain a complete IF strip.

  This circuit block of the radio also provides the adjacent channel selectivity. High performance filters like crystal filters may be used, although LC or ceramic filters may be used within domestic radios. The type of filter will depend upon the radio RF design and its application.

  Also within a multi-conversion superhet, the IF may be on a number of different frequencies, typically the earlier stages are at higher frequencies to provide higher levels of image rejection, and later ones at lower frequencies to provide gain and adjacent channel selectivity.

  Read more about . . . . IF amplifier & filter.
  
  

• Demodulator:   The superheterodyne receiver block diagram only shows one demodulator, but in reality many radio RF designs may have one or more demodulators dependent upon the type of signals being receiver. Those radios used for professional radio communications applications and monitoring may need to be able to demodulate a variety of modulation schemes and waveforms and this may require a number of different demodulators that can be switched in as appropriate.

  Even many broadcast radios will have AM and FM, but professional radios used for monitoring and two way radio communications may require a larger variety in some instances. Having a variety of demodulators will enable many different signal modes to be received and increase the capability of the radio.

• Automatic Gain Control, AGC:   An automatic gain control is incorporated into most superhet radio block diagrams. The function of this circuit block is to reduce the gain for strong signals so that the audio level is maintained for amplitude sensitive forms of modulation, and also to prevent overloading.

  Although the basic concept is the same through all radio RF circuit designs, there are some variations in the implementation and the electronic circuit design required. Some of the key variations are the time constant of the AGC system. For AM and the like a relatively slow time constant is acceptable. For SSB, a shorter time constant is needed so that the envelope of the SSB signal is followed.

  There are also variations in the way the AGC voltage is derived, and where it is applied. Often it is applied to the IF circuit blocks first and then to the RF circuit block. In this way the best signal to noise ratio is preserved. Generally the AGC is relatively easy to implement, having relatively few electronic components.

  Read more about . . . . Automatic Gain Control, AGC.
  
  

• Audio amplifier:   Once demodulated, the recovered audio is applied to an audio amplifier block to be amplified to the required level for loudspeakers or headphones. Alternatively the recovered modulation may be used for other applications whereupon it is processed in the required way by a specific circuit block.

  In many ways, this circuit block within the superheterodyne radio is the most straightforward. For many applications, the audio amplifier will involved some straightforward electronic circuit design, especially if the audio is applied to simple headphones or a loudspeaker. For two way radio communication applications, the audio bandwidth may need to be limited to the "telecommunications" bandwidth of about 300 Hz to 3.3 kHz. Audio filters could be employed as well.

  For applications requiring a higher quality output, more thought may need to be applied during the electronic circuit design to achieving high fidelity performance.

  Whatever the radio, there can be different requirements for this circuit block.

Superheterodyne receiver block diagram explanation

Signals enter the receiver from the antenna and are applied to the RF amplifier where they are tuned to remove the image signal and also reduce the general level of unwanted signals on other frequencies that are not required.

The signals are then applied to the mixer along with the local oscillator where the wanted signal is converted down to the intermediate frequency. Here significant levels of amplification are applied and the signals are filtered. This filtering selects signals on one channel against those on the next. It is much larger than that employed in the front end.The advantage of the IF filter as opposed to RF filtering is that the filter can be designed for a fixed frequency. This allows for much better tuning. Variable filters are never able to provide the same level of selectivity that can be provided by fixed frequency ones.

Once filtered the next block in the superheterodyne receiver is the demodulator. This could be for amplitude modulation, single sideband, frequency modulation, or indeed any form of modulation. It is also possible to switch different demodulators in according to the mode being received.

The final element in the superheterodyne receiver block diagram is shown as an audio amplifier, although this could be any form of circuit block that is used to process or amplified the demodulated signal.

Block diagram summary

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.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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