Appendix
640
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
“Direct Digital Synthesis”).
Since the rear-panel Modulation In connector is decoupled, you can use
the 81150A / 81160A 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.
BW 2 x (Modulating Signal Bandwidth) For narrowband FM
BW 2 x (Deviation + Modulating Signal Bandwidth) For wideband FM
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 x (75 kHz + 15 kHz) = 180 kHz.
Channel spacing is 200 kHz.
Frequency Modulation
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