AM example: Amplitude Modulation - labAlive experiment

AM was the earliest modulation method used for transmitting audio in radio broadcasting. Amplitude modulation, Wikipedia.

To transmit an audio signal wireless it must be modulated by a high-frequency carrier so that antennas of practical size can be used.

AM modulator adds the carrier amplitude

\hat{s}

as DC offset to a low frequency message signal

m (t)

and multiplies this signal with a high-frequency carrier

\cos (ω t)

.

Original baseband signal.

AM modulated transmit signal. The modulating signal forms an envelope over the underlying high-frequency carrier wave.

The amplitude modulator generates a signal

$s (t) = [\hat{s} + m (t) \cdot k_{m}] \cdot \cos (ω t)$ .

Where

$\hat{s}$ carrier amplitude

$m (t)$ modulating signal

$k_{M}$ sensitivity of the AM modulator - a constant amplification factor

On the 24th of December 1906 Reginald Fessenden, a Canadian inventor, performed the worlds first amplitude modulated transmission. After a few years of research he sent a voice message, which was received by radio equipped ships over hundred miles away. Due to the very simple hardware amplitude modulation became very popular and was used for communication during the first world war. Though there are some disadvantages. AM is very prone to interferences and its need for bandwidth is relatively high.

Start

The amplitude modulated signal is shown in the oscilloscope and spectrum analyzer:

AM signal. The message signal's amplitude forms an envelope over the high-frequency carrier wave.

AM signal spectrum

Maximum and minimum amplitudes of the transmission signal's envelope determine the modulation depth:

m = \frac{s_{\max} - s_{\min}}{s_{\max} + s_{\min}} = \frac{1,5 - 0,5}{1,5 + 0,5} = 0,5

Experiment

The modulation index or modulation depth is defined as $m = \frac{m_{\max} \cdot k_{m}}{\hat{s}}$ .

Adjust the modulation index

m

.

Note that in this setup $m$ has the same value as $k_{m}$ as both the message signal and carrier amplitude are set to 1V.

Compare the AM modulated signal in the time and frequency domain for different modulation indexes.

	Oscilloscope	Spectrum
m=0,5
m=1
m>1

Next Steps

Switch off the carrier and create a double sideband modulated signal without carrier.

Switch off the DC offset and AM carrier.
Double Sideband Suppressed Carrier, time-domain	Double Sideband Suppressed Carrier spectrum

Change source and carrier signal parameters such as amplitude, frequency and waveform. Watch the effect on the modulated signal and spectrum.

Changing source signal parameters	Changing carrier parameters
Modulated signal with sawtooth as source signal	Spectrum of modulated signal with sawtooth as source signal

Modulation depth	AM transmission signal	Modulation trapeze
0.5
1.0
?