|Radio and Electronics (DED Philippinen, 66 p.)|
|6. RECEPTION OF RADIOSIGNALS (AM - TYPE)|
The reason for the effect stated at the end of the last chapter, can be explained very easily, if we have a closer look to the signal produced by the tuned circuit: This signal is actually a mixture of two signals - the carrierfrequency modulated by the audiofrequency.
- The earphone - when connected to the terminals of the tuned circuit - will be passed by a current with a frequency which is the carrierfrequency. If the diaphragm would be able to follow this carrierfrequency it would produce air pressure oscillations of a frequency far above the range of audiofrequencies, therefore we would not hear anything.
So the diaphragm cannot follow these high frequencies, it will be at rest. Therefore we cannot hear anything at all.
In order to be able to listen to anything, we have to remove the carrierfrequency from the modulated radiosignal (to discharge the audio signal).
The process of removing the carrierwave is called DEMODULATION and it is carried out by a circuit called DEMODULATOR or DETECTOR.
If we assemble the blocks which we came to know up to hear, we would be able to hear at least a strong radiostation which is near to our receiver.
Fig. 39 shows the system which can be achieved if we do so. By doing so we have got a very simple kind of radioreceiver called a CRYSTAL RADIO.
This was the type of radio which was used first during the first days of radio-technology.
But you can very easily imagine, why this kind of radio was not a satisfying one: The sound produced was very weak and only one person could hear something if he was lucky enough to receive a station which could deliver enough energy for his earphones.
How could this receiver be improved? Of course have you have heard something about the law of CONSERVATION OF ENERGY.
If you apply this law on our crystal receiver, you will find out very easily, that the transducer (the earphone) can only produce sound energy with a maximum which is limitted by the input energy of the aerial.
This means as well: If we want to increase the sound energy we have to add energy to the energy from the aerial. This energy must be supplied within our radio by a so called poer supply.