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SWEEPING THE DISPLAY
Horizontal sweep for the display is either internal or
from an external sweep source. Sweep rate and source are
selected by the TIME/ DIV switch . When the TIME/ DIV
switch is in the AUTO position , the sweep rate is
controlled by an internal microcomputer.
When the TIME/DIV is in the EXT position , a signal
source of
to
+10 volts ,
applied to the EXT IN
HORIZ/TRIG connector , will sweep the crt beam the full
10 division graticule span . The input is dc coupled ,
sensitivity is 1 V/div . External input impedance is about
10 ΚΩ .
The beam can be positioned by the MANUAL SCAN
control when the TIME/DIV is in the MNL position (see
Manual Scan of the Spectrum that follows).
MANUAL SCAN OF THE SPECTRUM
Manual scan is used to examine a particular point or
portion of a display such as one of the null points of a
frequency modulation spectrum or where a slow sweep of
the full span would take unnecessarily long . When the
3-6
TIME/DIV control is set in the MNL position , the display
may be swept with the MANUAL SCAN control . The sweep
scan is usually first calibrated in one of the timed sweep
positions. Note that with a wide span/div and/or a narrow
resolution bandwidth setting , it is possible to scan too
rapidly to achieve an accurate display . Also, digital
storage can give unpredictable results when used with the
MNL SCAN mode . Digital storage is updated only when
scanning toward the right.
REFERENCE LEVEL , RF ATTENUATION ,
AND VERTICAL DISPLAY
A change in the REFERENCE LEVEL control requests
the microcomputer to change the display reference
level
the absolute amplitude represented by the top of
the crt graticule . The microcomputer selects the gain
distribution (IF gain and input RF attenuation ) for the new
reference level according to the setting of the FINE,
VERTICAL DISPLAY mode , MIN RF ATTEN dB , and MIN
NOISE/MIN DISTORTION selectors.
The amount of attenuation between the RF INPUT and
the first mixer, set by the microcomputer, is based on the
reference level requested and the mode of the MIN RF
492 Operator's Handbook
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