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3.6 Frequency modulation(FM)
Pressing the key of (AM/FM) and selecting "Freq Mod", the signal of frequency
modulation will be output from the end of "output A".
3.6.1 Setting of carrier frequency: Pressing the key of (Option 1) and selecting
"Carrie F", the carrier frequency will be changed to green. The setting of carrier
frequency can be carried by the number keys or adjusting of knob. For frequency
modulation, the signal of channel A is taken as carrier signal and the carrier
frequency is in fact the frequency of channel A. But for the clock signal of DDS
synthesizer is switched from fixed clock reference to controllable clock reference,
the frequency accuracy and stability of carrier wave will be reduced. The maximum
carrier frequency can reach to 10MHz only.
Setting of frequency modulation: Pressing the key of (Option 3)
and
3.6.2
selecting "FM Freq", the signal of frequency modulation will be changed to green.
The setting of carrier frequency can be carried by the number keys or adjusting of
knob. For frequency modulation, the signal of channel B is taken as modulation
signal and the modulation frequency is in fact the frequency of channel B. Generally
speaking, carrier frequency should be 10 times higher than modulation frequency.
3.6.3 Setting of offset of frequency modulation: Pressing the key of (selection 4)
and selecting "FM Devia", the offset of frequency modulation will be changed to
green. The setting of offset of frequency modulation can be carried by the number
keys or adjusting of knob. The offset of frequency modulation stands for the change
of carrier frequency in the frequency modulation process. In the waveform graph of
carrier wave on the top left of the screen, using the period change of carrier wave
will be more visual. PERD, in the graph, is the period value when the offset of
frequency modulation is 0, and, SHIFT is the maximum single peak change value in
frequency modulation. The offset of frequency modulation, DEVI, is expressed as
following:
DEVI% =100×SHIFT/PERD
In order to observe the frequency change clearly, the offset of frequency modulation
can be set much larger in the demonstration. But in practice, the offset value of
frequency modulation is under 5% in order to limit the bandwidth taken by the carrier
signal.
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