User Manual LW120
time interval which is set by the sweep time parameter. Likewise,
using the logarithmic sweep type, the frequency span between the
start and stop frequencies is divided by 1000 logarithmic steps.
4. Sweep Direction
– Defines the direction of the sweep. UP sets
sweep direction from start frequency to stop frequency; DOWN
reverses the sweep direction so the output sweeps from stop
frequency to start frequency.
5. Sweep Time
– Defines the time interval that it takes for the
LW120 output to execute one complete sweep. The sweep time is
generated with an asynchronous free-running time interval
generator that has programmable intervals from 1 ms to 1000
seconds.
Refer to Figures 3-24 and Figure 3-25 throughout the following
description to learn how to program the LW120 to generate sweep
modulation.
Example 1, Generating
Sweep Using Standard
Waveforms
This example will show how to generate sweep modulation using
the standard waveforms. The start frequency will be set to 100 Hz
and the stop frequency to 25 KHz. We’ll be using linear sweep in
the down direction in 10 ms. We’ll monitor the sweep modulation
from the front-panel outputs, then compare the results to the rear-
panel sine output connector.
1. Press the FREQUENCY button. Modify Freq to display
100.0000Hz. Observe and note that the Sclk parameter is
showing 102.4000KS/s. In standard waveform mode, the Sclk
parameter is set automatically by the instrument and can not be
modified directly. The information that we need from this display
is the Sclk value and the number of points used for generating
the current waveform. In this case, the number of points is
computed from the relationship
N = Sclk/Freq = 102.4KS/s / 100Hz = 1024 points.
2. Now it is time to check if the number of points will not conflict at
the stop frequency. We can check this by dividing the maximum
possible sample frequency by the number of points from the
above calculations.
Fstop_max = 100MS/s / 1024Pts = 97.65625KHz
In this case we should be safe because we plan to sweep to
25KHz only.
3. Now we have to compute the sample clock frequency at the
stop point. Knowing already the number of points we have in
this waveform, the sample frequency at the stop point is
computed using the following equation:
Fstop = 1024Pts x 25KHz = 25.6MS/s.
4. Press the UTILITY button
5. Select and press the Modulations option
Using The Instrument 3-35