There are many circuits that can be used to make a crystal radio set. All work in basically the same way but optimise different elements of the operation.
Looking at how a crystal radio works reveals some of the basic principles of any radio including tuning, selectivity and demodulation or detection.
Crystal radio development
The crystal radio set was one of the first forms of radio used. Its simplicity and effectiveness when compared to previous forms of radio made it very popular.
The crystal radio also became the radio of choice for many enthusiasts when radio broadcasts started.
Note on Crystal Radio Set History:
The crystal radio set was first used in the first decade of the 1900s. Various scientists had seen that forms of crystal would rectify or detect radio waves. As broadcasting became popular in the 1920s the crystal radio was used almost exclusively until vacuum tube or thermionic valve technology enabled amplifiers to be introduced.
Read more about crystal radio history.
Crystal radio circuits & functions
There are very many different circuits that can be used for a crystal radio. Although there are differences in these circuits, the operation is basically the same.
A typical crystal radio circuit is shown below and from this it can be seen how a crystal radio works.
There are four main areas to the overall radio operation:
- Antenna / earth: Although the antenna / earth system is not actually part of the crystal radio, it is an essential element in seeing how the crystal radio works. The antenna picks up the signal which is used as the input to the radio. The earth connection is normally an integral part of the antenna. Typically a long wire is used for this.
- Tuning: The tuning is a key element in looking at how the radio works. It provides the selectivity, i.e. the ability to allow stations on the required frequency and reject others that are on different frequencies.
- Signal detector: The signal detector is the part of the circuit that extracts the modulation from the radio frequency signal and provides an audio frequency signal.
- Electrical to sound wave conversion: This is a transducer that converts the electrical signals to sound waves so that they can be heard.
Crystal radio operation: tuning
The tuning is one of the key areas of any radio set. To see how it works it is necessary to look at the inductor and capacitor L1 and VC1 in the diagram. Together these form a resonant circuit. It provides the receiver with selectivity.
The tuned circuit provides a passband within which signals are accepted. Outside this passband signals are rejected.
To change the frequency that is being received it is possible to change the value of either the inductor or the capacitor. In practice it is possible to make a variable capacitor than a variable inductor and therefore this approach is normally adopted.
When the capacitance value is changed, so the centre of the passband moves and stations can be tuned in.
Crystal radio: how demodulation works
Another of the key areas of the crystal radio is the demodulation process. In gaining an understanding of how a crystal radio works it is good to be able to grasp how this works.
Crystal radios are able to receive amplitude modulated signals. For these signals the actual amplitude of the waveform varies in line with the audio being carried by the signal.
A diode is used to rectify the signal and this only allows current to flow in one direction and this means that half the waveform is allowed through.
This is then filtered to remove the high frequency element of the signal and this is presented to the audio transducer.
Often no filtering is include because the transducer will not be able to respond to the radio frequency signal and this has the effect of the filtering.
Crystal radio transducer
A transducer is the name given to the item that converts the electrical signals to audio sounds that can be heard. For a crystal radio headphones are suitable – the signal level is not sufficient to drive a loudspeaker because of there crystal radio does not have any amplification and even with a good antenna the signal levels are not sufficient to drive a loudspeaker.
Typically high impedance headphones or an earpiece are needed. The low impedance ones common today are not suitable.
More Essential Radio Topics:
Radio Signals Modulation types & techniques Amplitude modulation Frequency modulation OFDM RF mixing Phase locked loops Frequency synthesizers Passive intermodulation RF attenuators RF filters Radio receiver types Superhet radio Receiver selectivity Receiver sensitivity Receiver strong signal handling
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