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HP 811 OA Service Guide
P
owe
r F
ilte
r
s and Device
B
us Interface
7-24
F
unctio
n
al Desc
r
i
p
tion
T
his section is on schematic 15Α.
Α DBI (Device Bus Interface)
I
C is used to communicate with
t
he
Microprocessor . This
I
C is driven serially, buffered through U107.
An EEPROM stores specific information
a
bout the PLL Board,
such as its serial number and calibration data .
A
t switch-on the
Microprocessor compares its data with this EEPROM
a
nd if
differences show, uploads data
f
rom
t
he EEPROM . In this way the
HP 8110Α
b
ecomes aware of
c
hanges in the instrument (for example
α
P
C B oard change) .
Multiplexers
U
103 -
U
106 convert the
I
SIS CMOS output signals to
E
CL.
PLL
Co
r
e
This section is on schematic 15Β .
Υ 251 i s α
5
MHz oscillator .
I
t receives α stabilised
p
ower supply from
regulator
U
206 .
U
251 switches the output of the oscillator to drive
the
l
oop, or an external
c
lock supplied by the u ser .
The reference clock s ignal is applied to
U
203, the PLL IC . Inside this
I
C the clock signal is
p
assed t hrough α divider . Α second divider
p
rocesses the output
f
rom α V CO,
U
201, and the two divided signals
are phase-compared,
p
roducing an output voltage over an active
L ow-pass filter,
U
204B . The range of voltage output
f
rom this filter
i
s limited by U204A, to
m
ake it suitable
f
or controlling the
v
aractor
diodes CR201 - CR208 . These diodes vary in capacity to control
t
he
f
requency
o
f t he V CO, which is then fed
b
ack to the loop for
f
requency comparison. In this way the V CO is
l
ocked to α reference
signal,
p
roviding α conditioned output signal signal, regardless of
whether input is
f
rom the internal oscillator, or
f
rom an external
source .
U
202 splits the output of the VCO into two
p
aths, one
f
or
f
eedback
comparison, and one
f
or output to α
d
ivider chain (described in 15C) .
The
f
ollowing regulators
p
rovide onboard
p
ower supplies :
U 205
-5V for VCO
U
206
+5V for 5MHz oscillator
U
207
+5
V
, general
U208
+24
V
, general
U209
-5
V f
or
U
202 (splitter) and the corresponding output
pulldown
The VCO
p
ower supply (U205) can
b
e switched on by Q251, Q252,
and the 5MHz oscillator
p
ower supply can
b
e switched on by Q253
and Q254, under software control to
p
rovide
p
ower when the internal
oscillator is required .
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