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The amount of amplitude modulation is called the modulation depth which refers to
the portion of the amplitude range that will be used by the modulation.
For example, a depth setting of 80% causes the amplitude to vary from 10% to
90% of the amplitude setting (90% – 10% = 80%) with either an internal or a
full-scale (±5V) external modulating signal.
Frequency Modulation (FM)
For FM, the function generator uses modulation samples to modify the output
frequency of the instrument by changing the content of the PIR (see
Digital Synthesis"
connector is dc-coupled, you can use the 33210A to emulate a voltage-controlled
oscillator (VCO).
The variation in frequency of the modulated waveform from the carrier frequency
is called the frequency deviation. Waveforms with frequency deviations less than
1% of the modulating signal's bandwidth are referred to as narrowband FM.
Waveforms with larger deviations are referred to as wideband FM. The bandwidth
of the modulated signal can be approximated by the following equations.
In the United States, commercial FM stations usually have a modulation
bandwidth of 15 kHz and deviation of 75 kHz, making them "wideband".
Therefore, the modulated bandwidth is: 2 × (75 kHz + 15 kHz) = 180 kHz. Channel
spacing is 200 kHz.
Frequency Modulation
Keysight 33210A User's Guide
on page 341). Note that since the rear-panel Modulation In
≅
BW
2 × (Modulating Signal Bandwidth) For narrowband FM
≅
BW
2 × (Deviation + Modulating Signal Bandwidth) For wideband FM
Tutorial
"Direct
Modulating Signal
Modulated Carrier
7
359
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