Model8555A
such
that
the
calibration
of
the
instrument
is
impaired. On some
control
settings
it
is acceptable
for
the
DISPLAY UNCAL light
to
be
"on"
if
the
light subsequently goes
"off"
when
either
the
SCAN TIME
PER
DIVISION
or
SCAN WIDTH
PER
DIVISION
control
is
switched
one
position
counterclockwise.
The
indicator lamp
bulb
is
replaceable from
the
front
panel. HP
Part
Number
2140-0259,
incandescent lamp,
12
volt, .06
ampere,
type
Tl
bulb.
Tum
plastic lens cover
counterclockwise
to
remove cover.
3-23. BAND Switch Lever. Selects frequency scale
from
a set
of
14
frequency scales.
The
band
switch
lever also controls a shaft
encoder
on
the
fre-
quency scale drum
that
performs several functions:
a. Controls
attenuation
of
the
1st
LO (YIG-
tuned
osc.) tuning ramp to maintain scan
width
calibration when using harmonic mixing.
b.
Optimizes
the
bias for
the
input
mixer
to
match
the
harmonic
number
(n)
of
the
YIG-tuned
oscillator. (n
= harmonic number,
shown
on left of
each frequency scale.)
c. Controls
the
overall gain
of
the
RF
Sec-
tion
to
maintain absolute calibration when using
internal mixing.
Operation
d. Controls switching
of
the
IF
signal
path.
Bypassing
and
disabling
the
second converter
on
the
1
+and
1-
(550
MHz
IF
bands). Bypassing
the
first
converter
when using external mixing on
the
n=6, 10-
and
10+
frequency bands.
e. Provides n
±
information
to
signal identi-
fication circuit.
f.
Provides frequency information
to
acces-
sory
equipment.
3-24.
TUNING
STABILIZER
Switch.
ON/OFF
control for
tuning
stabilization circuit. The
tuning
stabilizer locks
the
first
LO
(YIG)
to
a 1 MHz
voltage-controlled crystal oscillator
(VCXO)
to
reduce residual
FM
of
the
first LO. The circuitry
is
enabled when
the
switch
is
set
to
ON and
the
SCAl'J WIDTH switch is in
the
ZERO or
blue
color-coded
PER
DIVISION positions.
3-25.
SIGNAL
IDENTIFIER
Switch.
ON/OFF
control
for
signal-identification circuit. The signal
identifier
circuit
provides a
method
of determining
which
harmonic
of
the
1st
LO
is
mixing
with
the
input
signal
to
give
the
display on
the
CRT. The
circuitry is enabled when
the
switch
is
ON
and
SCAN WIDTH
PER
DIVISION control
is
set
to
1 MHz
or
below. (See Signal Identification Tech-
nique, Figure 3-5).
3-3
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