Superhet Radio Tutorial Includes:
Superhet radio Superhet theory Image response Block diagram / overall receiver Design evolution Double & multi-conversion superhet Specifications
See also: Radio types
Having looked at the concepts it is helpful to look at a superheterodyne receiver block diagram of a basic superhet. In this way it is possible to see the overall operation of the receiver.
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. More complicated receivers with more complicated block diagrams are often seen as these radios are able to offer better performance and more facilities.
Superheterodyne receiver circuit blocks
There are some key circuit blocks that form the basic superheterodyne receiver. Although more complicated receivers can be made, the basic circuit 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.
- 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. 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.
Read more about . . . . RF amplifier & tuning.
- Local oscillator: The local oscillator circuit block can take a variety of forms. Early receivers used free running local oscillators. Today most receivers use frequency synthesizers, normally based around phase locked loops. These provide much greater levels of stability and enable frequencies to be programmed in a variety of ways.
- Mixer: Both the local oscillator and incoming signal enter this block within the superheterodyne receiver. The wanted signal is converted to the intermediate frequency.
Read more about . . . . RF mixer.
- IF amplifier & filter: This superheterodyne receiver block provides the majority of gain and selectivity. High performance filters like crystal filters may be used, although LC or ceramic filters may be used within domestic radios.
Read more about . . . . IF amplifier & filter.
- Demodulator: The superheterodyne receiver block diagram only shows one demodulator, but in reality radios may have one or more demodulators dependent upon the type of signals being receiver.
- Automatic Gain Control, AGC: An automatic gain control is incorporated into most superhet radios. Its fubnction 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.
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.
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.
More Essential Radio Topics:
Radio Signals Modulation types & techniques Amplitude modulation Frequency modulation RF mixing Phase locked loops Frequency synthesizers Passive intermodulation RF attenuators RF filters Radio receiver types Superhet radio Radio receiver selectivity Radio receiver sensitivity Receiver strong signal handling
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