Tektronix FG 504

R 790 . T h e differential o u tput from t h e m u ltiplier is a pp lied
to t h e i np ut transistors Q825 and 0805 of t heoutput buffe r
amplifier . T h is outp ut amplifier p rovi des gain as
well as
differential to si n gle-e
n
ded out
p
ut co
n version . T h is is an
ope
r
atio
n al am p lifier . T he collector of Q825 directly drives
t he b ase of outp ut emitter follower Q830 . T he collector of
Q805, wit h level sh ifti n g, d rives t h e base of Q840 .
T h
is
transistor provi des ph ase
inversion . Its collector drivest he
base
of
emitter follower ou t p ut tra n sistor Q835 . Feed back
ta kes place t h ro ugh R 822 . T he single-e n ded output
goes
to t h e fu
n
ction
switc h .
Amplitude
M
o
d ulating I np ut Circuitry
T he am p litu d e mo d ulating sig
n al is applied toth
einput
of operational am
p lifier U 710 . T he out p ut at p in 6 d rives
t he tra n sco nd ucta
n
ce
multiplier and t he base of Q720 .
T h is t r ansistor d rives t h e AM in d icator lam p . W it h no
modulating sig n al, t he voltage at t h e output of U
710 is
about +15 V . T h e i nd icator
lam p lig h ts w h en t h e voltage is
about
+10 V an d 100% modulation occurs w hen t he
voltage
r
eac
hes about +5 V .
Out p ut Amplifier (Diagram 4)
T h is circuitry is an inverti ng o p erational amplifier . The
i np u t stage, d rive n by emitter followers Q906 and
Q914,
co n sists of p airs of differe
n tial am p lifiers Q930 a nd Q940
com
p lemented by Q1030 a nd Q1040 . T he co n figuration is
suc h t h at for α positive-going inp u t waveform, t
h
e collec-
tors of Q930 a nd Q1030 both move in αn egative directio
n .
U nder t h e same co n ditions,
t h
e
collectors of Q940 an d
Q1040 both move in α positive direction . T h ese pairs
differentially drive transistor pai r s Q950, Q952, Q1050,
an d 01052 res p ectively . V oltage gain ta
k
es
place i n t h is
stage . T h e outpu t from p air 0950 an
d Q952 is ta ken at t he
collector of Q952, a
n
d
for pair Q1050 and Q1052 at t he
collector of
Q1052 . F
or
α n egative-going i np ut sig n al, t he
above
me n tioned collectors b ot h move in α p ositive
directio n . Com po n ents R 950, C950, C1050, R 1050, C965
an d R 965 provide h ig h fre q ue ncy compensatio n and
R 1015 ad ju sts t he d o bala
n
ce
for t he en tire amplifier .
T
h
e collector of Q952
drives t h e base of out p ut emitter
followers Q970 and Q980
w h ile Q1052 drives Q1070 a nd
Q1080 . T he feedbac k pat h is t hroug h R 1090 and C1090 to
t he bases of Q930 and Q1030 .
Output Attenuator
T h e signal from
t he output amplifier is fedtot h eoutput
atte n uator
circuitry . T he variable am p
litu de control
(R1 100A an d Β ), wit h resistors R 1102
an d R 1104, form α
b
r i d ged Τ atte nu ator wit h α
maximum atte n uation of
approximately 20 dB . T h ic k film atte
nu ator c h i p s R 1110,
R 1112, a nd R 1114 p rovide
α total of 50 d B of ad d itional
attenuation . T h ese
attenuators are com p ensated resistive
divi d ers .
T
h
eory of
O pe r atio n - F G 504 (S N Β 040000 & UP)
Offset Circuitry
O u tput offset is a dju
ste d by R 1130, t he O FF SET
control . S h oul d a dd itional
positive offset be desire d , t he
O FF SE
T
co
n trol is moved in t he n egative direction
. T h
is
causes t h e output of U 1145 to move
i n the negative
direction . T h roug h the action of
Q1150 a nd Q1170, t he
bases of Q1160 an
d Q1180 move i n α negative d irectio n .
T h is causes
t h eir commo n collector circ u it to move in α
positive d irection, i n creasing c u rre n t flowth roug
h R 1122 .
T he do offset generate d by t
h
e
increased curre n t flow is
p assed to t
h
e OU
T PU T terminal t h roug h R 1120 and
L 1120 .
Gati ng and
Triggeri ng Circuitry (Diagram 5)
In t h is circ
u itry, U 1225 B o p erates as an E C L li n e
receiver . Ι η t he FREE RUN mo d e, t h e set in p ut (pin 5) for D
flip flo p U 1215A is connected to
+5 V t h roug h C R 1235 .
T h is p laces α
h ig h
on
t he
Q
o u tput at pin 2 . T h is actio n
lowers
t h
e com
b ined collectors of Q1245 and Q1250 and
red
u ces conduction in Q1254, turni ng Q1262
off and
Q1260
on
. No curren t flows t h roug h diodes C R 1285
and
C R 1286 . T h e tria n gle ge n erator is now
free to ru n . In t he
GAT E mode of ope r atio n ,
t h
e
set input of U 1215A is
connected to t
h
e
output of U 1225B . Wh enth e i nputgate is
h ig h , t h e circuit functions as i n t h e n ormal
mode wit h
Q1262off a nd Q1260on, all owing the triangle generator
to
run . Wh en t he i np ut gate goes low,
t he n ext positive-going
pulse at p in 6 (cloc k ) from
t he triangle ge n erato r causes
t he Q terminal to go low
. T h is actio n resets t h efli p flo p for
t h
e
n ext trigger . H owever, Q1262 does n ot conduct
un til
t he sync pulse at t he base of Q1245 goes low at t
h
e
pea k of
t he triangle ram p . T h is actio n
allows com p letion of t he last
cycle .
W it h Q1262 on, t h e tria n gle ge n erator is preve n ted
from operati ng t h roug h C R 1286 . In t
h
e
triggere d mode,
t he out p ut of t he trigger
shaper is connected to t h e cloc k
in p ut of U 1215A
. T he triggering sig n al causes t h e cloc k
termi n al to go
h ig h . T h is transfers t he level at t h e D i n put
to
t he
Q
input . If t he Q output is h ig h (Q low), t
h
e
tria n gle
gen erator ru ns u n til α p ositive-going
sync h ro n izi ng
sig n al, w h ic h occurs at
t he start of t h e up ramp, forcest he
Q output low .
At t he start of t he down ram p , t h e
sync
h ro n izi ng sig n al goes low, w h ic h causes
Q1262 to
co nd uct an d t u rns Q1260 off .
T
h e
triangle now ramps
down u n til CR 1286 comes
into co n ductio n . T h is poi n t is
d etermine d by
t h
e
setti ng of t h e ph ase control, w h ic h
determines t he voltage at t
h
e o
u tput of U 25A a nd at t h e
cat h ode of C
R
1285 .
The n ext positive-goi ng trigger from
U 1225 B ca
uses p i n 9 of U 1215A to go p ositive . T h is
transferst h e h ig h on the D i n put tot
h
e Q outp
u t, allowi ng
t h e tria n gle ge n erator to
begin t he next cycle .
Ph ase L oc k ed L oop Circuit r y
T h is circ u itry p rovides α d o outp u t voltage proportio n al
to t he differe nce in fre q uency betwee n an exter n al
triggering sig n al a nd α synchron izi ng sig n al from t he
voltage-controlled oscillator .