5.7. Pulse width modulation (PWM)
In PWM, the pulse width of the carrier varies with the instantaneous voltage of the modulating signal, and the
waveform shape of the carrier must be pulse. Before entering into PWM, users should firstly set the
frequency and amplitude of carrier wave under CHA continuous function.
Press
【
PW M
】
key, the keyboard indicator of “PWM” will be on and the CHA output phase width
modulation signal, the carrier wave is automatically set as pulse wave.
5.7.1. Pulse width deviation
Press
【
Menu
】
key and select “Devia” option, then set pulse width deviation value. It represents the
variation of carrier pulse width to the period of the pulse when the modulating waveform is with full amplitude
during PWM process, also the variation of the duty cycle. Name the maximum duty cycle of modulated
carrier as Dmax, and the minimum as Dmin, the pulse width deviation’s formula should be:
Pulse width deviation
＝
Dmax
－
Dmin
If Dmax=80%, Dmin=20%, the pulse width deviation is 60
％
. If Dmax=50%, Dmin=50%, the pulse
width deviation should be 0%. That is to say, when pulse width deviation is 0, the duty cycle of pulse wave is
50%.
5.7.2. Modulation frequency
Press
【
Menu
】
key, select “Mod_f” option and set modulation frequency value.
5.7.3. Modulation waveform
Press
【
Menu
】
key and select “Shape” option, set modulation waveform by setting waveform No.,
the modulation waveform may be any one of the 16 kinds of waveforms listed in the waveforms
table.
5.7.4. Modulation source
Press
【
Menu
】
key, select “Source” option and then set modulation source, there are only two
values to choose: 0 and 1. Set the value as 0 to choose internal modulation source, and set it as
1 to choose external modulation source. If external modulation source is chosen, connect a
modulation signal to the
《
Modulation In
》
port on the rear panel of the generator. W hen external
modulation signal reaching to full amplitude as ±5V, the display of pulse width deviation accords
with its real value. Or the display of pulse width deviation is incorrect.
5.7.5. Sync output
In PWM, the generator outputs a sync signal from the front panel “Sync” connector, which is a square wave
with TTL level and 50% duty cycle, its frequency is equal to modulating frequency and its phase is
referenced to the phase of the modulating signal.
In PW M, press
【
PW M
】
key, the “PWM” keyboard indicator will be off, and the generator exits
PWM function and returns back to continuous function.
5.8. Frequency shift keying (FSK)
In FSK, the frequency of the carrier shifts between “carrier frequency” and “hop frequency”
alternately, the rate at which the output shifts is determined by hop rate, all the 16 waveforms
listed in the waveforms table could be used as carrier wave. Before entering into FSK, users
should firstly set the waveform, frequency and amplitude value of the carrier wave under CHA
continuous function.
Press
【
FSK
】
key to light “FSK” keyboard indicator, and CHA will output FSK signal.
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