HP 435B Service Manual

SERV IC ES HEE T 5
PR I N CI PLE S O F O PER ATIO N
General
Th e Α 3asse mbly provid es α 50 ±5 MHz o u t pu t at 1 m W ±0 . 7% . Th e
osc ill ato r ou t pu t i s h e ld co n sta n t byan AL C loop m ad e up o f α
peak d etecto r C R 2 a nd co mp arato r U 2 . Th eco mp a r atorrefe r e n ce
inpu t i s fr o m α v e ry sta bl e +5 V p o w er s upply co mp osed o f U 1,
VR 1, VR 2, Q2, a n d t he ir assoc i ate d co mp o n e n ts . Th e LEVEL
ADJ co n t r o l R 4 sets t h e co mp a r ato r r e fer e n ce whi c h co n tr o l s t h e
osc ill ato r f ee dbac k l eve l a nd t h e r e by co n t r o l s t h e Α 3 asse mbly
P O W ER REFEREN CE O UTPUT l e v e l .
50 MH z Oscillator
Th e
osc
ill
ato
r
cir c ui t i s m ad e up o f co mm o n e mi tte r a mplif ie r Q1
a nd i ts assoc i ated
co
mp
o n e n ts . R es i sto r s R 10, R 11, R 12 a nd R 13
bi as Q1 f or
an
e mi tte r
c
urr
e n t o f a ppr o xim ate ly 5 mA . Th e
n- n et w o rk t un e d
c
ir
c
ui t
(C9, L 2, C10 a nd C11) d ete rmin es t h e
op e r at in g fr e qu e n c y . Th
e a
mplifi
e r ac gain is set by t h e ope r at in g
c ir c ui t imp e d an ceacross t h
e t un edc ir c ui t and t h e e mi tte r r es i sto r
R 15 (w hi c h i s ac co
upl e
d
to g
r
o
und
by C 12) . Th epos it iv e f ee db ac k
r e quir e d to s u sta i
n
osc ill
at
i o n i
s satisfied in t hi s c ir c ui t . Ph ase
s hif t o f 180° i s αc ha r
acte
ri
st
i
co
f
b ot h co mm o n -e mi tte r t r a n s i sto r
amplifiers and 7r-network t un e d c ir c ui ts . Thi s f ee db ac k i s coupl e d
t hro ugh C9 a nd C10, b ac k to t h
e b ase o
f
Q1 .
A L C L
oo
p
A t t h e pos i t iv e p ea k o f eac h c y c l e, c u rr e n t m o m e n
ta
rily
flows
fr o m t he f ee db ack loop t hr ou gh p ea k d etecto r di o d e
C
R 2 to
C7 .
Th e r es ul ta n t sto r e d c h a r ge i s coupl e d, as α d o inpu t vo l tage,
to
pin 3 o f U 2 . Th e d etecto r ou t pu t i s co mp a r e d to α v e ry stab l e
r efe r e n ce inpu t by co mp a r ato r U 2 . Any diff e r e n ce b etwee n t h e
co mp a r ato r 's inpu t v o l tages pr o du ces an e rr o r vo l tage at t h e d o
o u t pu t . Th e co mp arator o u t pu t i s coupl e d to α r eacta n ce vo l tage
divid e r, ca p ac i to r C9 a nd v aracto r C R 3 . As t h e er r o r o u t pu t v o l t-
age goes m o r e p os i t iv e t h e ca p ac i t iv e r eacta n ce o f C R 3 d ec r eases,
whi c h r e du ces t h e osc ill ato r fee db ack . Conv e rse ly, α m o r e n ega-
t iv e o u t pu t v o l tage w ill in c r ease t h e f ee db ack . For exa mpl e, if t h e
osc ill ato r ou t pu t we r e to s udd e nly in c r ease, t h e d etecto r o u t pu t
wo uld b
eco m e m o r e pos i t iv e . Th e co mp arator ou tp u t w ould
b eco m e m
o
r
e p os
i
t iv e, α
l
owe r C R 3 r eacta n ce wo uld d ec r ease t h e
f ee db ac k
to Q1
whi
c
h f
o r ces t h e osc ill ato r o u t pu t l e v e l b ac k to i ts
o ri g in a l l eve l . If t h
e R 4 LEVEL ADJ co n t r o l we r e ad j u ste d f o r α
m o r e pos i t iv e r e f
e
r e n
ce vo
l
tage, t h e co mp a r ato r o u t pu t w ould go
m o r e n egat iv
e, t
h
e
f
ee
db
ac
k
wo uld in c r ease, a ll o win g t h e osc ill a-
to r ou t pu t
to
in
c
r
ease . Th e r e
f
o r e, t h e p ea kd etecto r o u t pu t w ould
in c r ease un
t
il i
t e qu a
l
s t h e co mp a r ato r r e f e r e n ce l e v e l inpu t, t hu s
esta bli s hin g
α
hi g
h e r l e v e l e d -o u t pu t s i g n a l fro m t h e osc ill ato r .
P ower Supply
Ρ /
Ο
Α 4
S
ERV
IC
E
S HE E
T
14

HP 435B Specifications

General

Brand	HP
Model	435B
Category	Measuring Instruments
Language	English