HP 8555A Operating And Service Manual page 25

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