Tektronix FG 504 - Page 78

T h eo r y of Ope r atio n - F G 504 (S NΒ 040000
& UP)
co n ductio n until t he p air switc h es . P i n 3 of U 375 now
goes
from abo u t -2 .8 V to about -3 .2
V
and
pin 9 goes to abo u t
-2 .8 V . T h is action d isa b les t he lower p ai r of tra n sistors i n
U 375 an d en ables t he u pp er p air . Wh en Q80 ceases
co n duction, Q90 comes i n to conductio
n
an
d Q100 ceases
co
n
dution
.
Curre
n t
now
flows t h roug h Q90, diode C R 190,
an d t he p ositive curre
n t source, Q55 . B ridge dio des
C R 188 and C R 186 are bac k -biase d . No current flows
t h roug h C R 180 as Q100 is off . C u rre n t from t
h
e
negative
cu rre n t source now
flows t h roug h d iodes C R 182 an d
C R 184 . T h is actio n charges t he timi ng cap acito r s i n t he
n egative direction, causi n g α negative-going ramp at
t he
outp ut . As t he ramp goes n egative,
t h
e o
u t p ut
of
U 375 at
p i n 14 gradually goes p
ositive . P i n 13 of U 375 gra d ually
goes
negative . Wh en t h e ram p reaches α voltage su f-
ficie n tly negative, Q65 turns off, Q80 comes i n to co nd u c-
tion, t he p ositive cu rre n t source is co nn ecte d to t he timi ng
cap acitors, and t h e p reviously describe d cycle repeats .
T he bases of emitter
followers Q410 and Q395 are
co
nn ected to t he non-inverting o u tputs of U 375 . T he
outp uts from t h ese emitter followers d rive t he bases of
common emitter amplifiers Q415 a
n
d Q406,
w h ic h
gene r ate t he
T R
IG O U
T a nd synch ro n izing sig n als .
S q uare W ave Generator
Switch i n g t r ansistor 065 ge
n
erates
t he square wave .
Wh en t
h
e
ram p is rising in t he p ositive directio n , t he
collector of Q65 is i n its most negative state . T
h
is
sig n al is
p assed to t h e base of 0450 .
T h
e
emitter of Q450 d rives t he
base of Q456 . T he emitter of Q456
d rives Q462 and Q470 .
Wh en 0456 is co
n
ducti
n g, Q462 is off ; Q470 is co n ducti n g
wit
h
C
R 470 b ac k-biased . Curre n t flows t h roug h C R 462
an d t he square wave moves i n t he p ositive d irectio
n . Wh
en
t h e switc h i n g waveform chan ges state,
Q462comes i n to
co n dutio n , bac k -biasi
n
g C R 462 .
T h
e square
wave now
moves i n t h e
n egative d irectio n by curren t t hr oug h C R 470
as Q470 is not co nducting .
V ariable R ise and F all Circuitry (Diagram 2)
Th is circuitry consists of variable p ositive and n egative
sources, α curren t switc h wit h i n tegrati
ng capacitors, and
an output buffer
. Tra n sistor Q540 serves as t h e positive
curren t source w h ile Q580 serves as t he n egative cu rre
n
t
source . T h e rise and fall times are ch an ged by varyi
n g t he
curren t t h roug h t he constant
curre n t sources and by
ch a n gi n g t he values of
t he integrating capacitors .
T he curre n t available to charge t h e i n tegrati ng
cap acito r s, C592 t hroug h C598, is varied by
switch i ng
R 536, R 538,
R
584,
a nd R 586 i n to t h e emitter circuit of
0540 an d Q580 as n ecessary . T h e cu r rent is also varied by
t h eV A R control t h roug h U 530 and Q530 . Wh en t he V A R
co n trol is i n t h e calibrated positio n , t he wiper is at
t h e
grou n de d end of t h e con trol
. T h
is
actio n i n creases t h e
curre n t t h roug
h
t
h
e
emitter resistances of t he curre n t
source
tra n sistors . An increase i n ch arging curren tfort he
integrati ng capacitors re d uces t he rise an d falltimes of t he
waveforms .
3- 2
T
h
e curre
n t switc h determines w h ic h current so urce is
co
n
necte
d to t he integrati ng capacitors . Transistors
Q495, Q500, a nd Q510 form α clip p
ing a nd speed-up
circuit for t h e sq uare
wave comi ng from t h e M ai n L oop .
Wh en
t he s q uare-wave driving t h is circuitry is n egative
wit h respect to grou nd , 0570 is con d ucti ng
.
Cu r rent
f r
om
t
h
e
n egative curren t sou r ce p asses t h roug h C R 566,
Q570,
and VR 570 to t h e +15 V sup p ly . Dio des C
R 562 an d C R 564
a r e bac k -biased, p r eve n ti
n g
curre
n t flow to t he i n tegrating
capacitors . Tran sistor
Q550 is off, causi ng curren t f r om
t
h
e
positive co n sta n t curre n t source to p ass t h roug
h
dio d es C R 558 and C R 560 .
T
h is curre
n t charges t he
i n tegrati n g cap acitor, causing
t h egate of fet Q600to move
at α con trolled rate
i n t he p ositive d irectio n . In t h e F IX E D
p
osition of t he R IS E an d F A LL TI ME switch , t
h
e
current
switc h a nd t h e i n tegrati n g ca p acitors are b ypasse
d
t h roug h switc h S590-7 . F et Q600 serves
as α fet follower
wit h 0610 su pp lyi n g consta
n t cu rrent . t he source of Q600
drives
t h e b ase of Q615, w h ic h forms α differe n tial
pair
wit h Q620 . Clip p ing ta kes place
i n t h is differe n tial cir-
cuitry . Α differentially
driven amplifier, Q640, restores to
t he out p ut
sig n al t he amplitu de lost i n t he cli pp i ng
process
.
Sine S hape r,
B uffe r , an d M od ulati ng Circuitry
(Diagram
3)
T h is circ u itry generates t he sine wave
from t he basic
triangle waveform by means of
α four ste p app roximatio n .
T he sine wave is also
amplitud e modulate d , if desire d , by
t h is
circuitry and buffered for d elivery to t h e outp ut
am p lifier .
T he si ne s
h
aper co
n sists of four diode bri d ges, CR 745-
C
R 748, C R 750-C R 753, C R 755-C R 758, an d C R 762-C
R
765 .
T h ese bridges a r e conn ecte d b etwee
n t he +15 V a nd
-15 V sup p lies .
T h
e
voltages from t h e common an o des
a
n
d
cat h odes of eac h b r idge are equal i n value .
T h
e
resista
nce values for eac h bri d ge are, however,
differe n t
from t he ot h er bri d ges . T h is ca
u
ses eq u al
p ositive and
n egative break points for an i nd ivi d ual b
r i d ge, but
d ifferent amplitude voltage brea
k p oi n ts for t he d iffere n t
bri d ges . As
t h e tria n gle voltage rises to +1 V, it
progressively exceeds t he brea k p oi n t of eac
h
b
r i d ge . T he
non-linear c h aracteristics of t h e diodes p
revent sh ar p
transitio ns at t he b rea
k poi n ts . T he op p osite action occurs
as t he i np ut tria
n gle d rives to -1 V . T h eo u tput currents
from
t h
e bri
d ges are summed at t he input to U 770, a
n
d
t he
result is α close ap p roximatio n of α pu re
si ne wave .
T h
e outp
u t from t h e sine s haper is fed into U 770, α
tra
n
sco
n
ducta
n
ce
multi p lier . T h is multi p lier is used to
amplitude mo d ulate t he si n e wave . T h e outp ut from t
h
e
multi p lier is ta k en at p i n s 8 and 9 ;
t h
e
amplitude
modulating sig
n
al in
p
uts
at p i n 12 . Gain a dju stme n t R 855
sets t
h
e si
n
e
wave gai n for U 770, a nd R 870 s h ifts d o levels
soth at t
h
e sine
wave is modulate d eq ually i n bot hp ositive
and n egative directio ns . Centeri ng of t he si ne wave wit h
respect to grou n d is accomplis h ed wit
h
offset a
dju stme n t