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Chapter 7 Tutorial
Modulation
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7
Modulation
Modulation is the process of modifying a high-frequency signal (called the
carrier signal) with low-frequency information (called the modulating
signal). The carrier and modulating signals can have any waveshape,
but the carrier is usually a sine waveform.
The two most common types of modulation are amplitude modulation
(AM) and frequency modulation (FM). These two forms of modulation
modify the carrier’s amplitude or frequency, respectively, according to
the instantaneous value of the modulating signal. A third type of
modulation is phase modulation (PM), which is similar to FM except that
the phase of the carrier waveform is varied, rather than its frequency.
Another type of modulation is frequency-shift keying (FSK), where the
output frequency “shifts” between two frequencies depending on the
state of a digital modulating signal. Finally, pulse width modulation
(PWM), is provided for pulse waveforms only. In PWM, the pulse width
or duty cycle of the pulse waveform is varied according to the modulating
signal.
The function generator will accept an internal or external modulation
source. If you select the internal source, the modulated waveform is
generated by a secondary DDS synthesizer. If you select the external
source, the modulated waveform is controlled by the signal level present
on the function generator’s rear-panel Modulation In connector.
The external signal is sampled and digitized by an analog-to-digital
converter (ADC). With either modulation source, the result is a stream of
digital samples representing the modulating waveform.
Note that for FSK, the output frequency is determined by the signal level
present on the rear-panel Trig In connector.
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