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SECTION 3
Operation
The Model 3930A
generates
square
waves
with a fixed
50% duty cycle by processing sine waves with a zero-
crossing comparator that has hysteresis. The level of the
generated square wave, therefore, alternates high and
low at approximately
0", +180", and ti60"
(within about
i2.3") depending on the past value of the phase. Thus, to
set the square-wave start/stop
level high, set the phase to
90". Similarly,
the phase should be set to -90' to set the
square-wave
start/stop
level low.
The Model 3930A also generates the synchronous
output
for the sine, triangle,
and sawtooth
waves by passing
each waveform
through a zero-crossing comparator that
has hysteresis. As a result, the synchronous
output level
changes at the same points as the square wave when the
phase is approximately
0", +180", or ti60"for
the sine or
triangle waves, or about f180" with the sawtooth wave.
When the Model 3930A is connected to the optional
Model 3933A Phase Shifter to form a multiphase
oscilla-
tor, the Model 3930A start/stop
phase setting is also the
phase offset. Refer to the Model 3933A Instruction
Man-
ual for multiphase
oscillation
phase setting information.
3.5.11
Frequency
Sweep Operation
The Model 3930A can be set up to generate frequency
sweeps over a variety
of ranges. The following
para-
graphs discuss the various aspects of programming
fre-
quency sweeps. Figure 3-14 shows sweep waveforms.
Setting the Sweep Range
You can set the sweep range in one of two ways: (1) by
setting the start and stop frequencies, or (2) by setting the
center and span frequencies. The actual sweep is gener-
ated from the start frequency to the stop frequency, and
the sweep direction is determined
by the relative values
of the start and stop frequencies. If the start frequency is
lower than the stop frequency,
an ascending-frequency
sweep will be generated. If the start frequency is higher
than the stop frequency, a descending-frequency
sweep
will be generated.
If you specify the center and span frequencies to set the
sweep range, the Model 3930A will automatically
con-
vert them into start and stop frequencies. See the specifi-
cations in Appendix
B for more details on the sweep
range and sweep width.
The Model 3930A determines
the sweep range as de-
scribed below when you change a frequency parameter:
When the start frequency is changed:
The stop frequency remains unchanged.
center frequency=
(start frequency+stop
frequency)/2
N'ortion of value below O.lmHz is truncated.)
Span frequency = start frequency-stop
frequency
When the stop frequency is changed:
The start frequency remains unchanged.
center frequency =
(start frequency + stop frequency)/2
(Portion of value below O.lmHz is truncated.)
span frequency = start frequency-stop
frequency
When the center frequency is changed:
The span frequency remains unchanged.
The relationship
between the start and stop frequency
value remains the same as it was before the center fre-
quency was changed:
start frequency =
(center frequency 3 span frequency/Z)
U'ortion of value below O.lmHz is truncated.)
stop frequency =
(center frequency + span frequency/2)
(Portion of value below O.lmHz is truncated.)
When the span frequency is changed:
The center frequency remains unchanged.
The relationship
between the start and stop frequency
values remains the same as it was before the span fre-
quency was changed:
start frequency =
(center frequency 7 span frequency/2)
(Part smaller than O.lmHz is truncated.)
stop frequency =
(center frequency * span frequency/21
(Part smaller than 0.lmH.x is truncated.)
The start and stop frequencies always correspond to the
opposite ends of the actual sweep range. When the span
frequency
is an odd number of O.lmHz units, the dis-
played center frequency will be 0.05mHz higher than the
3-29
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