1. ECE 1100: Introduction to Electrical and Computer Engineering Notes 11 Crystal Radio Principles Spring 2011 Wanda Wosik Associate Professor, ECE Dept. Slides developed by Dr. Jackson

2. Crystal Radio Schematic

3. Resonant Circuit Inductor L [Henrys] Capacitor C [Farads] Resonant frequency: Adjusting C allows you to “tune” the circuit to respond to a given frequency. This uses the principle of resonance.

4. Amplitude Modulation m(t) = audio (message) signal (voice) | m(t) | < 1 t Ex (t)Ex (t) High frequency signal (broadcast from antenna) t m(t)m(t) Audio message signal A0A0 Field transmitted from transmit antenna: Your ear cannot hear such a signal!

5. Detection t v (t)v (t) diode + - v(t)v(t) sinusoidal signal (from receive antenna) The diode only allows current to flow in one direction. S earphone

6. Low-Pass Filtering Peak of signal t Average value of signal: v A (t) (proportional to the amplitude) The total rectified signal (red part) consists of a sum of two parts: (1)The average v A ( t ) (2)A high-frequency part that has a zero average value. v (t)v (t)

7. Low-Pass Filtering (cont.) t t v A (t) t v H (t) Audio signal + DC offset High-frequency signal (zero average value) + v (t)v (t)

8. Your ear cannot respond to the high-frequency signal (it is beyond the frequency range of human hearing) or the DC offset. What your ear hears t m (t)m (t) Audio message signal Human hearing range: 20 [Hz] – 20 [kHz] Low-Pass Filtering (cont.)