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Transcript
4.10 Analog Modulation
Analog modulation is a signal changing in level and frequency on a continual
basis. This occurs when a microphone or telephone is used with sound waves
being converted into electrical signals.
Figure 4.36 Analog modulation
The analog transmitter is a LED that emits an unchanging light intensity. A
LED normally emits an unchanging light level or a steady signal. The signal
has to be modulated by using the audio output from a radio to change the light
levels slightly.
Figure 4.37 Audio signal added to the DC voltage driving the LED. The result
is an analog signal with a DC offset.
The detector is a phototransistor that converts the light into electrical signals
that can be seen on the oscilloscope and heard through the amplifier.
Acton Instruments - ANU
Page 1
5/12/2017
Activity
Construct a fibre audio link based on analog modulation by reference to figure
4.38.
An oscilloscope is useful to visualise the waveforms.
Fibre
Analog in
Analog out
Photodiode 1
Photodiode 2
BNC & LED
BNC & LED
trig
volt/div
volt/div
ch1
ch2
secs/div
Speaker
Radio
Figure 4.38 Analog modulation
Tune in the radio station and display the signal on the oscilloscope. Try
VOLTS/DIV on 0.5volts/div and SECS/DIV on 5msecs/div.
To display the Phototransistor signal move the DC-GND-AC slider to AC.
This blocks the DC voltage and allows the VOLTS/DIV setting to be increased
to display the signal.
Acton Instruments - ANU
Page 2
5/12/2017