Tektronix FG 504

T h
eo
r y
of O p eratio
n - F
G 504 (S NΒ 040000 & UP)
T h
e trigger i n put signal isfed i n to i n verter U 1225 B . Th is
integrated circuit is
an
E
C
L line
receiver . ltisco nn ectedto
operate as α Sc h mitt trigger for trigger sh api n g p ur p oses .
The i n verte d output is fed to t he in p ut of one
of
two
D type
flip flops ( U 1295 B ) . T he C i n put of t h e ot h er D fli p flo p
( U 1295A) is con n ecte d to t he sync signal from t he
oscillator . B ot h D in p uts are always h ig h . T h ey are
connecte d to +5 V t h roug h
C
R 1312 .
On
t h
e p ositive-
going transitio n of either, t he sync sig n al for U 1295A
or
t he triggering signal for U 1295 B , t h e h ig hs οη t he D inputs
are transferred
to t
h
e Q out p uts . The Q out pu ts co n nect to
t he i np ut of U1225A, w h ic h is also an E C L li ne receiver .
T he lows on t h e Q termi n als create α h ig h on p i n 3 of
U 1225A, w h ic h is co n nected to t he reset i np ut of b ot h fli p
flo ps . T h is actio n resets both fli p flops . T hey are now
ready to re p eat t h
e previously described cycle .
T h
e Q out
p
uts are conn ecte d t hroug h α low pass filter
com p ose d of R 1324, R 1320, an d C1322
to
t he in p ut of
U 1340 . If t he fre q uency of t he sync h ro n izing sig n al is
h ig h er t han t he triggeri ng sig n al, t he resulti n g curren t
differe nce at t he i np ut of U 1340 causes t he output of
U 1340 to go negative . T h is actio n is reverse d (+ i nput
terminal p ositive) if t h e fre q
uency of t
h
e triggeri ng signal
is lower t han t he sync signal . T he ph ase co n trol sets t he
voltage at t h e i n put of U 1340 so t h at wide differences i n
fre q uency are wit h in t he capture range of t h e circuitry .
T h e voltage at t h e outp
ut of U 1340 ad j usts t
h
e fre q ue
n
cy
of t he voltage controlle d oscillator to t he triggeri n g
fre quency . T h is voltage swings between ±12 V .
Swee p Circuitry (Diagram 6)
T h e swee p trigge r is ap plie d to pin 13 of U 1225C . T h e
TR
IG LEVEL co n trol a d j usts t he do level at p in 13 of
U 1225C, an E C L li n e receiver . P rotective diodes C R 1370
a n d C R 1372 are also connected to p in 13 . Wh en t he
voltage at p i n 13 reac h es about +3 .5 V, p in 15 goes from
about +3 .5 V to about +4 .0 V . T h is positive-going step is
d iffere n tiate d t h roug h C1392 to t he base of Q1400 . (T he
MAN
TR
IG button also
creates
α p ositive ste p at t
h
e base
of Q1400 .) Transistors Q1400 and Q1410 com p ose α
Sc h mitt trigger . T he base of Q1400 goes positive, cau sing
t h
e base of
Q1410
to go negative .
Wh
e
n
op erati n g i n t he
TR IG S WEEP mode, t h e collector of Q1410 rises, tur n i ng
Q1420 off . T h is actio n tur ns fet Q1430 off, allowi n g t he
i n tegrator formed by U 1445, Q1450, and t h eir associated
compone n ts to ram p up . T h e slope of t he positive-going
ram
p
is determin ed by t he setti ng of t he V A R co n trol, t he
resista nce i n series wit h its wi p er, and t h e value of t he
capacitor connected from pin 2 to t h e emitter of Q1450 .
As t he ram p rises, t h e voltage at pin 3 of U 1470 also
rises . Wh en t h is voltage reac h es t he voltage at p i n 2
of
U 1470, t h e output immediately switc hes p ositive . T he
p ositive ste p at t he outp ut of U 1470 tur ns fet Q1430 on,
disc h argi n g t h e timi n g capacito r san d returning t he sweep
to
t he starti n g level . Wh en pi n 3
of U
1470
goes lower
t han
3-4
pi n 2, du r i n g t h e swee p fall, C R 1470 turns οη . T h is closes
t h e p ositive feedbac k loop for U 1470 and t h e output
switc hes imme d iately to ab o u t -13 V . If t h e sweep is
ru nn i ng i n t he triggere d mo d e, t h is n egative transition is
differentiated by C1394,
w h
ic
h
resets
t h e
Sc
h mitt trigger
circuit composed of Q1400 an d Q1410 . T he cycle t hen
re p eats at t he next trigger pulse . 1 η t hefree ru nn i ng mo d e,
w hen pin 6 of U 1470 goes n egative, Q1430 is turned off
an d t he free ru
nn
i n g cycle re p eats .
L
ogarit
h mic Conve r ter
In t
h
e bi
p
olar
tra n sistor, t he relatio n s h i p b etween t he
collector
curren t a nd t he emitter-to-base voltage is
p recisely logarit h mic over α wide curre n t ra n ge .
T h e li
n
ear
ram p
is fed to pi
n
2 of U 1480 t h roug h
tem
p
erature
compe n sati n g resistor R T1456 . Operational
am p lifier U 1480 is used as an atte n uator (gai n is co n -
siderably less t h an one) a nd as an inve r ter . R esistor
R T1456
decreases
i n value wit h i n
creasi
n
gtemperature to
compe n sate
for
t he decreasi n g gai n wit h temperature of
U 1495A . T r ansistors U 1495A an d Β are on α commo n
subst r ate so t h at t h eir base-to-emitter voltages remain
co
n
stant
wit h tem p erature, assumi ng eq ual collector
curren ts . T he o p eratio n of t h is circuit can be easily
un derstoo d if it is assu me d p i n3 of U 1490 is grou nd rat he r
t han returned to
t h
e out
pu t
of U
1480 . P i n 3
of
U1490 is
actually co nnected to t he output of U 1480 to improve t he
tem p erature ch aracteristics of t h e circuit wit h out in-
tro d uci ng sig n ificant error i n t h e linear to logarit h mic
co n version .
As t he ram p voltage at t he base of U 1495A chan ges
linearly, t he emitter voltage of U 1495A cha n ges t h roug h
t h e action of U 1490to mai n tai n α consta n t currentt h roug h
R 1496 . T h e li near emitter voltage chan ge of U 1495
Α
drives
t
h
e
emitter
of U
1495 B , causi ng t hecurren t drive for U1500
to
become logarit h mic . T h is logarit h mic current is co n -
verted to α voltage c han ge by U 1500 and fe d t h roug h t he
L OG L I N S WP switc h to t he fre q uency co n trol circuitry .
T
h e out
p
ut ram
p
c
h
anges
from 0 V
to
10 V . Wh e n
op erati n g i n t he STO P FRE Q mo d e, t he sweep outp u t line
co nn ects to 10 V p rovi d edby U
1495D
a
n
d C o p
erating
as a
sim p le voltage divider .
T h
is p rovides α
fixe d voltage
to
t he
sto
p
fre quency p oten tiometer and only t he sto p frequency
is out p utte d .
P owerSu pp lies
T here are five regulate d power su pp lies : +5 V, +15 V,
-15 V, +25 V,
a
nd
-25 V .
T h
e -15 V, +25 V, an d -25 V
sup p lies refere nce to t he +15 V sup p ly . T h e +5 V su pp ly
obtai ns its unregulate d voltage from
the +11 .5 V ac supply
i n t h e p ower mo d ule . T he +5 V supply is series regulate d
by U 1690 w h ic h co n tai n s its own inter n al refere nce and
provides sh ort circuit curren t limiting at approximately
750 m A .