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W
h en t he Μ 153 switch is set to "Ιπ t", the TCXO is
energized. The TCXO outpu t is input to α
p
hase detector,
t
he othe r inpu t of w h ic h is derived fr om
t
h e Μ30-1/ Α
Crystal Reference, which serves as
t
h e VCO in
t
he
p
hase-
locked
loop .
Any
d
ifference in freq uen cy or ph ase
produces an erro r voltage w h ich co rrects the Μ30-1/4
Crystal
R
eference frequ en cy to
k
eep it ph ase locked to the
TCXO .
Wh en
t
he M1 53 switc h is set to " E xt", the t une d am plifier
is energize d and
t
he TCXO is turned off.
T
he tun ed
a mp lifier acce p ts the exte rn al
r
efe r e n ce signal, a mp lifies it
(If the exte rn al refe rence is 5 MHz,
t
he t uned
a
mp lifier
converts it to 10 MHz), and inpu ts it to
t
h e ph ase
d
etecto r.
The phase-locked loop
t
hen operates in t h e same ma nne r as
it did fo r
t
he TCXO, and
t
he Μ30-1/4
C
rystal Reference is
ph ase locked to
t
h e exte rnal
r
efere nce sou r ce.
Ι(
t
he
extern al
reference
is
not
with in
accuracy
specifications, the ph ase-locked loop will not
b
e able to
lock onto
t
he extern al signal . When this occurs, the
comφra τor senses
t
he excessive error
v
oltage from
t
h e
ph ase detector and activates
t
he module and instrument
unlock indicators.
If the M1 53 switch is set to "Εχτ"
b
ut no (or insufficie n t)
exte rnal signal is available, the
r
eference monitor senses the
condition and disa b les t he phase-locked loop .
CA L I BR ATIO N
Tank circuit
L
4/C12
Is
br
oa
d
-t
un
e
d
α[
t
h
e facto
r
y,
and
s
hou l d r equ ir e no adjustment.
An adjustme n t t ri mmer is
p
rovi ded on
t
h e TCXO to adjust
the 10 MHz reference frequ ency . T h is adjustme n t s h ou l d
be perform ed at 3 to 6
m
onth Intervals to compensate fo r
aging of the TCXO .
To adjust the TCXO frequency, first ensure the instrument
h
as
b
een
t
urne d off for at least 3
h
ours and
h
as stabilized
in α - 25' C ambient tempe rat ure. Remove
t
he instru-
ment's to p cove r and
t
he Ml 53 .
m
odule cover. Set t he
ins/ Εχί switch to "I ns" and co nnect α frequ ency co un ter
to t h e rear-panel REF IN/OUT co nn ecto r. (Ensure that
the co unter is properly warmed up and h as at least 0.1
p
pm
αεευιυεγ with 1 Hz
r
esolution at 10 ΜΗτ. " Turn the
in strument on and
a
djust t h e TCXO Fr equency Ad ju st ment
to 10 .000000 MHz ± t h e offset indicated on the calibration
tag a dj acent to t h e Fr equency Adjust ment.
( Exa mple-
If t h e tag
r
eads "Fred Set at 25° -3.0 Hz",
a
djust the
f req u ency to 9.999997 MHz) . T h is adj ustment should
be
p
erformed wit h in 5
m
i nu tes after turn-on. Replace t h e
m
odule and i nstrum ent covers .
TROUBLE SHOOTI NG
If the
M153 unlock indicator is lit, t h e fa u lt
m
ay
b
e in
t
he
Μ153, bu t may also
b
e in
t
he Μ30-1/4 Crystal
R
eference
m
odule or in
t
he external source . If either the M30-1/4 or
t
he exte rnal so urce is not within its
p
rescri b ed
s
pecification
limits, the MI 53 may not be able to accomp lish p hase lock,
and
t
he unlock in dicator will ligh t. If t h e un lock occurs
only when t he Μ 153 switc h is set to "Ext", suspect the
external so urce or
t
he t un ed
a
mp l ifier in
t
h e Μ 153. If t h e
unlock occurs only when t h e Μ 153 switch is set to "Ιητ",
recali brate t h e TCXO . If the unlock occurs in both switch
positions (wit h external source present), t he
p
ro blem is
p
ro b ab ly α
d
efective M1 53 or an out-of-specification
M30-1/4 .
Cali brate the Μ30-1/4 as follows: C h ec k the DC voltage
pr ese nt on
p
in 11 of
m
odu le Μ 153. This ist h e PLL co rr ec-
tion voltage. Th e unlock i ndicato r will light w h e n
t
h is
voltage exceeds - t4 V, If the inst rumen t is un locke d
after α 5 mi n ute warm-up
p
erio d, i mm ediately set the PLL
correctio n voltage to .3 .5 V using
t
he Μ301/4 Frequ e n cy
Adjust. If the In st ru ment beco mes
u
nlocked after 15
m
inu tes, allow
t
he instrument 2ο warm up until it
h
as
b
ee n
on for at least 2
h
ours and set t he PILL co rrection voltage
to +3 .5 V .
ΝΟΤΕ
It may be necessary to
r
epeat t h e above pr ocedure several
times i n or der to ph ase lock
t
he Μ30-1/4 at all te mpera.
tures. Do not adjust the Μ30-1/4 F requency Adjust for
0 V, si nce this co u ld produce an un locke d con dition at
extreme tem p eratures .
8/5 Α- 3
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