Model 3580A
GENERAL
OPERATING
INFORMATION
Section III
3-149. In many applications relatively wide portions
of
the
spectrum being swept do not contain useful information.
The plot shown in Figure 3-27, for example, has a number
of
narrow spectral components
but
more than
98%
of
the
display
is
nothing but noise floor. Using a conventional
sweep at the optimum sweep rate, it took more than
15
minutes
to
trace the plot shown in Figure 3-27. Using the
3580A Adaptive Sweep feature, however, the same plot
(minus the noise floor)
was
traced in about
1.5
minutes
(Figure 3-28).
Figure 3-27. Plot Using Conventional Sweep (15 minutes).
3-150.
To
use
the
Adaptive Sweep feature, the operator
sets a baseline threshold using the frorit panel ADAPTIVE
SWEEP
control. The baseline threshold can be adjusted
anywhere from the bottom
of
the screen
to
approximately
70%
of
full scale. For the plot shown in Figure 3-28, the
baseline threshold
was
set about 10
dB
above the noise
floor.
Figure 3-28. Plot Using Adaptive Sweep.
3-151. At the beginning
of
the Adaptive Sweep, the
instrument sweeps at the rate selected by the
SWEEP
TIME
setting. This ensures that the zero response or any other
signal on or near the start frequency will be properly
detected. After the sweep passes through any initial
responses, the sweep rate
is
automatically increased
to
20
or 25 times the selected rate.
When
the sweep reaches a
response that rises above the baseline threshold, it backs up
slightly, pauses to allow the IF Filter
to
settle and then
sweeps slowly over the response at the panel-selected rate.
When
the response has been completely traced, the sweep
is
again speeded up until another response
is
encountered.
As
a result, the portions
of
the spectrum below the threshold
level are not displayed, but the spectral responses above the
threshold level
are
displayed just
as
they
are
using a
conventional sweep.
By
sweeping rapidly through unused
portions
of
the spectrum, the Adaptive Sweep greatly
reduces the overall measurement time.
3-152. Setting the Baseline Threshold.
When
setting the
baseline threshold for the Adaptive Sweep, the following
guidelines must
be
observed:
a. In the Linear amplitude mode the threshold must be
at least
60%
below the peak
of
the smallest
signal
to be
displayed. For example, if the peak
of
the smallest signal to
be
displayed
is
4 vertical divisions, the threshold must be at
least 2.4 divisions (0.6 X 4) below it. Similarly,
if
the peak
of
the smallest
signal
to
be
displayed
is
1 vertical division,
the threshold must
be
at least 0.6
of
a division below it.
b. In the Log amplitude mode, the threshold must be at
least 8 dB below the peak
of
the smallest
signal
to be
displayed.
3-153. The reason for setting the baseline threshold below
the peak of the smallest signal to be displayed
is
that the
responses
are
detected when the instrument
is
sweeping 20
to
25
times faster than the panel-selected rate. During these
fast sweeps the IF Filter does not have time to fully
respond.
As
a result, the signals applied to the internal
threshold detector
are
about 6
dB
(50%) lower in amplitude
than they
are
when sweeping at the optimum rate.
If
the
threshold
is
not more than 6
dB
below the peak
of
a signal,
that signal
will
not
be
detected and consequently, will not
be displayed.
NOTE
Adaptive Sweep cannot be used on the
0.05 Sec., 0.02
Sec.
and 0.01
Sec.
SWEEP
TIME settings.
3-154.
With
the
SWEEP
TIME control set
to
one position
slower than the optimum rate, the signal compression
during fast sweeps
is
approximately 3
dB
or 30%. This
allows the baseline threshold
to
be set 4
dB
or 45% below
the peak
of
the smallest signal
to
be displayed. The trade
off here
is
that the measurement time
is
considerably longer
than it is when sweeping at the optimum rate.
3-155. Adaptive Sweep, Log 1 dB Mode. The Adaptive
Sweep
is
difficult
to
use in the Log 1
dB
amplitude mode.
This
is
because the display range
is
only 10
dB
and, when
sweeping at the optimum rate, the baseline threshold must
be at least 8
dB
below the peak
of
the smallest signal to be
displayed. With the baseline threshold
at
the bottom
of
the
screen, signals more than 2
dB
below full scale will not be
displayed.
If
the Adaptive Sweep
is
to
be used in the Log
1
dB
mode, set the
SWEEP
TIME control one or two
positions slower than the optimum sweep rate. This will
reduce the amplitude compression during fast sweeps and
3-21
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