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Pulse and Pulse-trains
Pulse Set-up10
The means by which pulse period is set-up in the hardware requires an understanding
because it affects the setting resolution of both pulse width and delay. Pulse is actually a
particular form of arbitrary waveform made up of between 4 and 50,000 points; each
point has a minimum time of 25.00 ns corresponding to the fastest clock frequency of 40
MHz.
At short pulse periods, i.e. for waveforms with a small number of points, the setting
resolution is, however, much better than 25.00 ns because the time-per-point is adjusted
as well as the number of points; since the pulse width and delay are also defined in terms
of the same point time, varying the time-per-point affects their resolution.
For example, if the period is set to 500 ns, the minimum pulse width, when set to 25.00
ns, will in fact be 25.00 ns; 20 points at 25.00 ns each exactly defines the 500 ns period.
However, if the period is set to 499·0 ns, 20 points at the minimum point time of 25.00 ns
will be too long so 19 points are used and the point time is adjusted to 26.26 ns
(499·0/19); 26.26 ns is now the increment size used when changing the pulse width and
delay.
For periods above 1·25 ms the maximum number of points in the waveform (50,000)
becomes the factor determining pulse width and delay resolution. For example, with the
period set to 100 ms, the smallest pulse width and delay increment is 2 µs (100
ms/50,000). This may appear to cause significant errors at extreme settings (for example,
setting 100 ns in the above case will still give an actual width of 2 µs) but in practical
terms a 1 in 50,000 resolution (0·002%) is normally acceptable.
The pulse period can be adjusted irrespective of the pulse width and delay setting (for
example it can be set smaller than the programmed pulse width) because, unlike a
conventional pulse generator, pulse width and delay are adjusted proportionally as the
period is changed. For example, if, from the default pulse settings of 100 µs period and
50 µs width, the period is changed to 60 µs the pulse width actual changes to 30 µs
even though the program width is still 50µs. To get a 50µs width with the period at 60
µs the width must be re-entered as 50 µs after the period has been changed.
Period can also be changed from the PULSE PERIOD screen, called by pressing the
FREQ key with pulse mode selected:
PULSE PERIOD
1
00·0 us
freq period
The new setting can be entered either as a period in the way already described or as a
frequency by first pressing the freq soft-key. However, changing the period or
frequency from this screen is slightly different from changing period on the pulse
setup screen. When changing from this screen the number of points in the waveform is
never changed (just as with a true arb) which means that the shortest period that can be
set is the number of waveform points times 25.00 ns. To achieve faster frequencies (up to
the specification limit) the period must be changed from the pulse set-up screen;
changing the frequency from this screen causes the number of points to be reduced as the
period is reduced (for periods less than 1·25 ms).
10-3
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