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Di vider
Thi s secti on i s on schemati c 15C .
I
t
p
rovi des α division whi ch, togeth er with the PLL Core
f
requ ency,
gives th e re quired out pu t fre quen cy. Α switch se lects the divi
ded
o utpu t, or t h e exter nal cloc k .
R
egister
U
311 is
l
oaded
f
rom the DBI wit h data t hat sets the
op erat ing sp eed of the d ivi ders. If
t
he sp eed is ab o v e 1ΟΜΗΖ , the
divi si on s are carried o ut at
E
C L le vels, and if bel o w 1OMHz, at
E
C L
an d TTL lev els . U301 and
U
303
p
erform the
n
ecessar y switc hin g.
Th e si g n al from t he PLL Core is first divi ded in U 302 and th en
converte d to C MOS by the Schmi tt
T
rigger,
U
305 D,
b
efore
b
eing
appli ed to t he prescaler
U
306 . Th e sig nal is then
d
ivi ded by t he
p
rogra mmable divi ders,
U
307 -
U
309 .
The output sig n al mu st be
E
C L compati bl e, regardl ess o f
d
ivi sion
rati o, so
i
t is
p
assed th rough U 311 to achie ve this .
Clock Inp U t
Thi s sectio n is on sc hematic 15D .
I
t condit i o n s ex ternally
a
pplied C loc k signals .
External Cloc k Signal
Th e extern al
C
lock sig nal enters t hrough J 401 . The inpu t impedance
is s wi tchabl e to either 50 Ω or α hi gh imp edance in th
e region
o
f
10 k Ω, us ing Q401, relay Κ 401 and assoc iated comp o ne nts .
Comp arato r
T
hreshold Ge n eration
1 . Α
d
i rect o u t put to the
P
eriod Board
HP 8110 Α Service Guide
U404 B i s α comp arator that shapes t he inco min g tri gger s ignal.
C R402 and C R 403
p
rovi de input over l oad
p
rotection for
U
404B .
The t hres h old at whi ch t he comp arator,
U
404 B
o
perates, is set by
th e linear 12- bit
D
- Α converter
U
402 . The voltage range of
t
he DAC
b
ein g condi ti oned by U 403 A to
m
ake it comp atible wit h U 404 B .
External Clock Distributio n. Α s pli tter,
U
405,
p
rovi des 3 out pu t
p
aths
for t he exter n al
c
l ock:
2 . An o ut put to t he PLL Core ci rcuitry to sub stitute the osc ill
ator
reference sign al
3 . An outpu t to α divi de- by-16 c ircuit to
m
eas u re the fre quen cy
o
f
the inpu t signal.
The d ivi de- by-16 circ ui t is
p
roduced by U 452 A
a
nd
U
452 B (dividin g
b y 4) an d the coun ter
U
453 . Thi s s ig n al
i
s availa ble
f
or
m
easur ing
exter nal
c
l ock frequ ency.
Functional Descr iptio n
7-
2 5
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