AM Transmission of iPod Audio
Objective: To build a transmission system in which an audio signal from
iPod (or equivalent audio device) is AM modulated, demodulated, and then
played through a speaker.
Alexander and Sadiku, pp. 657-659, 836-838.
ed., pp. 88-93.
Scherz, pp. 384, 429, 617-619, 624-625, 627.
Note: Students are expected to bring the following components to the lab
(1) 741 op-amp
(1) 1N4148 Si diode
(2) 1.0 kΩ resistor
(1) Variable resistor (e.g., pot)
(1) 0.1 μF capacitor
(1) 1 μF capacitor
(1) Portable audio device. Any portable audio device with a 3.5 mm mini
headphone jack, which includes most MP3 and CD players, handheld game
devices, and laptops, will do.
The following components will be provided:
(1) 3.5 mm mini to BNC female adapter
(1) 10 Ω resistor
(1) 80 kΩ resistor (68+12k
(1) 220 μF capacitor
(1) 2.2 μF capacitor
(1) 10 μF capacitor
(1) 1 nF capacitor
(1) 47 nF capacitor
(1) LM386N low voltage audio power amplifier
(1) 741 op-amp
(1) Miniature PM speaker
Amplitude modulation (AM) of a message signal was discussed in lab 5. In
this lab, we are going to AM modulate an audio signal, demodulate it, and
then listen to it.
An audio signal
contains frequency components within the range 20 Hz
to 20 kHz (humans can only hear sounds with frequencies within that
range). If we want to transmit
efficiently over air without any
modulation, we would need an incredibly long antenna.
We can use a practical antenna size if we modulate the signal. In
modulation, the frequencies of
are shifted to higher frequencies in the
MHz range. AM modulation is done by multiplying
with a carrier
is carrier amplitude
is the carrier frequency.
frequency becomes new the center frequency of
. When you tune the
frequency of an AM station on a radio, you’re selecting the carrier
frequency of the station to which you want to listen, such as 570 kHz, 980
kHz, and 1070 kHz.
The modulated signal,
is obtained by multiplying a weighted
with an offset by a sinusoidal carrier signal
, Multiplying the signals, we