This is the diagram for the simplest receiver circuit, which is also commonly known as a 'crystal set' receiver. This circuit consists of an antenna, a tuned circuit, a diode or 'crystal' detector, and headphones (or earphones).

In this circuit, the antenna picks up the signal, causing a flow of current in the primary winding of T1, which is just a coupling transformer.  This primary current flow induces a voltage in T1's secondary winding, which develops a charge across capacitor C1.  Note that both the secondary winding of T1 and capacitor C1 form a series resonant circuit.

At the resonant frequency, the voltage developed across C1 is significantly higher than at other frequencies. This voltage is known as the resonant rise voltage or resonant step up voltage. Thus, when the frequency received is 'tuned' to the resonant circuit, the received signal is readily passed on to the rest of the circuit.

The diode or crystal rectifies the signal from the tuned circuit. Capacitor C2, on the other hand, 'filters out' the carrier signal, thereby allowing only the information signal to pass through to the attached listening device. What this means is that the simplest receiver must, at the minimum, be a demodulator.

See also:  Resonance; Simplest Transmitter

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