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. Alternatively, the samples that are generated on
the other channel can be selected as the modulating signal (2 channel
version only). This allows even more detailed control over the modulation
signal as provided by the 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.
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