7.6.1 Setting of carrier frequency
:
Pressing the key of “AM/FM” 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 amplitude
modulation, the signal of channel A is taken as carrier signal and the carrier frequency is in fact the
frequency of channel A.
7.6.2 Setting of frequency modulation
:
Pressing the key of “AM/FM” and selecting “FM Freq”, the signal of frequency modulation will be changed to
green. The setting of frequency modulation can be carried by the number keys or adjusting of knob. For
amplitude 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.
7.6.3 Setting of depth of amplitude modulation
:
Pressing the key of “AM/FM” and selecting “AM Depth”, the depth of amplitude modulation will be changed
to green. The depth of amplitude modulation can be settled by the number keys or adjusting of knob. During
the process of amplitude modulation, the amplitude of carrier wave varies periodically with the modulation
signal. The depth of amplitude modulation stands for the change of amplitude of carrier wave. For example,
100% means that the maximum amplitude of the carrier is 100% of the settled value and the minimum is 0%
, i.e., 100%-0%=100%. Depth of 0% means that both the maximum and the minimum amplitudes of the
carrier are 50% of the settled value, i.e., 50%-50%=0%. Similarly, depth of 120% means that 110% -(-
10%)=120%. There is another expression for the depth of amplitude modulation. In the waveform graph on
the top left of the screen, if the maximum Vpp of the modulation carrier is A and the minimum Vpp of the
modulation carrier is B, the depth of amplitude modulation, DEPTH, is expressed as follows:
DEPTH% =100×(A-B)/(A+B)
The two expressions are coincident virtually but the second is more clear and easy to be understood.
In order to observe the amplitude clearly, the depth of amplitude modulation can be set much larger in the
demonstration, even larger than 100%. But in practice, the depth value of amplitude modulation is under
50% in order that the modulation signal will not be distortion. This kind of modulation carrier is called bi-side
carrier wave, which is a modulation way used in most radio stations.
7.6.4 Setting of modulation waveform:
Since the signal of channel B is taken as the modulation signal, the modulation waveform is, in fact, the
waveform of channel B. Pressing the key of “AM/FM” and selecting “Mod Shape”, the serial number of
channel B will be changed to green. The setting of modulation waveform can be carried by entering the
serial number by the number keys or adjusting of knob.
7.6.5 Exterior modulation:
Both frequency modulation and amplitude use exterior modulation signals. There is a port of “AM/FM Input”
on the rear panel, which can import exterior modulation signals. Pressing the key of “AM/FM” and selecting
“EXT Mod”, the offset of frequency modulation or depth of amplitude modulation will disappear and the
interior modulation signal will be off automatically. The frequency of exterior modulation signal should fit that
of carrier signal. The amplitude of exterior signal should be adjusted according to the requirements of offset
of frequency modulation or depth of amplitude modulation. The larger the amplitude of the exterior
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