ECE 1100: Introduction to Electrical and Computer Engineering

1
ECE 1100 Introduction toElectrical and Computer
Engineering
Spring 2008
David R. Jackson Professor, ECE Dept.
Notes 14
Crystal Radio Principles
2
Crystal Radio Schematic
3
Resonant Circuit
Resonant frequency
Inductor L Henrys
Capacitor C Farads
Adjusting C allows you to tune the circuit to
respond to a given frequency.
This uses the principle of resonance.
4
Amplitude Modulation
Field transmitted from transmit antenna
m(t) lt 1
m(t) audio (message) signal (voice)
Ex (t)
A0
High frequency signal (broadcast from antenna)
Your ear cannot hear such a signal!
t
5
Detection
The diode only allows current to flow in one
direction.
sinusoidal signal (from receive antenna)
diode
S

v (t)
v(t)
-
t
earphone
6
Low-Pass Filtering
Peak of signal
v (t)
Average value of signal vA (t)
(proportional to the amplitude)
t

• The total rectified signal (red part) consists of
  a sum of two parts
• The average vA (t)
• A high-frequency part that has a zero average
  value.

7
Low-Pass Filtering (cont.)
v (t)
t

Audio signal DC offset
High-frequency signal (zero average value)
8
Low-Pass Filtering (cont.)
Your ear cannot respond to the high-frequency
signal (it is beyond the frequency range of human
hearing) or the DC offset.
Human hearing range 20 Hz 20 kHz