crystal radio receiver basics | crystal radio circuit
This application note covers crystal radio receiver basics.This receiver circuit is used in crystal radio design.
Figure below mentions circuit of crystal radio receiver consisting of antenna, transformer (T1), tuned capacitor (C1), fixed capacitor (C2), diode and headphone.
Fig-1: Crystal Radio Circuit
As shown antenna picks the signal and will cause flow of current in the primary coil of transformer. This will generate voltage in the secondary coil of transformer. This is due to generation of magnetic field in the secondary due to flow of current in the primary windings.
Each turn of secondary coil induces voltage and hence they act as tiny voltage generators. The effect is depicted at the right side in the figure. It can be represented as signal generator in series with secondry coil.
As shown, secondary coil along with capacitor C1 will form series resonance circuit. When this circuit is tuned to resonance, high voltage will get produced across capacitor. This is referred as resonant step up voltage. This tuned circuit with higher Q can select desired radio frequency.
In this crystal radio receiver circuit, diode or crystal does rectification of input signal. The capacitor C2 filters out the RF carrier from received modulated signal. This process will leave original baseband information after removal of carrier. This signal can easily be listened using earphones.
This crystal radio receiver does not provide selectivity as desired in the modern wireless receiver circuits and hence TRF receiver circuits are designed and used.
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