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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 frequency-shift keying (FSK), where the output frequency
“shifts” between two frequencies depending on the state of a digital
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 DDS process running in an internal digital signal
processor (DSP). 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) and is
then routed to the DSP. 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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