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Positive sweeping（0）：The frequency of output starts from the start
frequency adding by frequency step, restarts to sweep from the start frequency after
it gets the end frequency.
negative sweeping（1）：The frequency of output starts from the end frequency
subtracting by frequency step, restarts to sweep from the end frequency after it gets
the start frequency.
To-and-fro sweeping（2）：The frequency of output starts from the start
frequency adding by frequency step, restarts to sweep from the end frequency
subtracting by frequency step after it gets the end frequency.
Press the key of 【Menu】and select “Sweeping mode”，the sequence number
and the name of the current sweeping mode will be displayed. The setting of sweeping
mode can be carried by the numeric keys or adjusting of knob.
3.4.4 Setting of interval time： As the start frequency, end frequency and frequency
step are settled, the interval time for each frequency step can be decided by the
requirement of sweeping velocity. The shorter the interval is, the faster the sweeping
velocity is. The longer the interval is, the slower the sweeping velocity is. But the
real interval is the settled interval time plus the runtime of the control software.
When the interval time is short, the runtime of software cannot be neglected because
the difference is large between the real interval time and settled interval time.
Pressing the key of 【Menu】 and selecting “Interval time”， the present interval
time value will be displayed. The setting of interval time can be carried by the
numeric keys or adjusting of knob.
3.5 Frequency modulation(FM)
Press the key of 【FM】 to light the indicator light of “FM”. The instrument will
be in the function of “ Frequency modulation” and the signal of frequency
modulation will be output from the end of “Output A”.
3.5.1 Setting of carrier frequency： Press the key of 【Menu】 and select “Carrier
frequency”，the carrier frequency will be displayed. The setting of carrier frequency
can be carried by the numeric keys or adjusting of knob. In 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
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