HP 3580A Operating And Service Manual page 36

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
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