LeCroy LW110 User Manual page 64

Using The Instrument 3-34
2. Stop Sample Clock – This parameter specifies where the sweep
will stop. This parameter is available from within the Sweep
Modulation menu. The sweep stop programs the sample clock in
units of sample per second. Note that sweep stop does not
necessarily have to have lower value than the sweep stop
frequency. If you define sweep stop frequency higher than the
sweep start frequency, then the generator will sweep up. You can
also reverse the direction of the sweep by using the sweep direction
parameter.
In standard waveform mode, the Sclk parameter and the
number of waveform points are set automatically by the
instrument and can not be modified directly. During
sweep modulation, only the sample clock is changing
but the number of waveform points remain constant.
Therefore, computing the number of waveform points
used by the instrument is critical to the sweep operation.
For example, if you set your start frequency at 50 MHz,
the LW120 can use only two waveform points. While the
instrument can generate a fairly nice sine waveform at
50 MHz by using just two points (and special filters), the
same two points will definitely look like square
waveforms if the sample clock is reduced below 10
MS/s.
As a reverse example, if you sweep modulate the output
from a start frequency of 150 KHz, the generator
automatically sets the Sclk to 76.8 MS/s and the number
of waveform points to 512. If you try to sweep up from
this point, then your maximum frequency will be 195,312
KHz! (100MS/s / 512 Pts = 195,312 KHz). Therefore and
due to these limitations, it is recommended using the
sweep modulation
waveforms because you are in complete control of the
number of points and the sample clock frequency you
are using to generate your waveform.
It is also recommended to use the SINE OUT at the rear
panel because the start and stop frequency setting are
available at this output at nominal values, without
needing to compute values for different waveform types.
3. Sweep Type – Defines the steps of which the sample clock
increments, or decrements from start to stop frequencies. A choice
is provided between linear and logarithmic step. If you select linear
sweep the frequency range is divided to 1000 linear increments and
the sample clock hops through these frequency-increments within a
User Manual LW120
HINTS
in conjunction with arbitrary