Tektronix 2213 - Page 43

T
h
eory of O p
eratio n -2213 Se r vice
cap acitor at α constant value . Wh en α ca p acitor is c
h
arged
i n t h is ma n ner, t h e rise of voltage across t h e capacitor is
linea r rat h er t h an exponential .
F ield-effect transistors Q630A a nd Q630 B are matc he d
d evices . As such , t h e
IDSS
(
d
rai n cu r re nt wit
h
gate-to-
sou rce s horted ) ch aracteristics of eac h are nearly i d entical
.
FE T Q630 B acts as α source-current su pp ly for Q630A
and h ol d s t he gate-to-so urce voltage of Q630 B at
zero volts .
Α TRIGGER
U 603,
P I N
S
4
&
11
S WP GAT E
U 620,
PIN 6
CO LLE CTO R
EN DO F
S WEEP Q640
CO
LLE CTO R
U
6408,
PIN 9
H O L DO FF
U 603,
PINS 1&13
U
D L Y EN D
U 620
P I N
S 3 & 13
TR IGG ER
P OI N TS
STA RT O F H O L DOFFN

j ENDO F H O L DO FF
S WP GAT E
U 603,
PIN
9
H O L DO FF
TI M I N G
U640B ,
PIN 14
S WP DUTY
U620,
PIN 8
A L T SY N C
C
R 644,
A N
OD E
L O F O R
D
3827-15
3-16
Fig ur e 3-5 . Sweep ti m i n g diag r am . .
Before α swee p starts, pin 6 of U 620 (t he Sweep L ogic
Gate) is ΗΙ , and bot h d isco nnect d io d es (C R 626 an d
C R 630) are forwar d b iased . T he c h arge on t
h
e
selecte d
timing cap acitor will be zero volts .
Wh en U
620 p
i n
6
goes
L O, the d isconnect d io
d
es
become reverse b iase d a nd t he
timing cap acitor b
egins
c
h arging t h roug h t he timin g resistor
to sta
r
t t
he sweep .
T
he ove r all gai n of the amplifie r com posed of source-
followe r Q630A and common-emitter am p lifier Q631 is
very h ig h . As t he timi ng capacito r ch arges, Q631 s u pp lies
fee d b ac k to t he gate of Q630A to h ol d t he gate voltage
nearly constant . V oltage across t h e timing resistor is there-
fore constant, and t he c h arging current to t h e timi ng
capacitor is co n sta n t . T he resulti ng voltage waveform
p r od uce d at t he collector of Q631 is α linear ram p .
Wh en t h e swee p wavefo r m am p litu de reac h es a b out
+13
V,
t
he End -of-Sweep Com p arator (0640) is biase d
on an d t
he Sweep Logic circuit resets . P in 6 of U 620 goes
ΗΙ
to forward bias d isconnect d io de CR 626, and t he
current
t hrough t he timing cap acitor reve r ses d irection .
T he swee p out pu t waveform drop s ra p i d ly until d isconnect
diode C
R630 also becomes forwar d b iased . At t h is p oi n t,
t h e Swee p Generato r is rea dy to start anot h er sweep .
Swee p L ogic
F ollowing t he sweep com p
letion,
α
fi n ite ti m e is
re qu ire d to d isc ha r
ge
t he timi
n
g cap acito
r
. T
h
e
Swee p
L ogic ci r cuit is prevente d from res p ond ing to α
trigger
signal d u ring t
h
is time by t
h e
H
ol doff circ u it .
T
he e nd of
sweep (and start
of t he holdoff p eriod) is d etermi n ed by
t
h e En d -of-Swee p Com p a r ator (Q640) .
T he Sweep ram p waveform is applie d to t
h
e b ase
of
Q640 t hr oug
h bot h
α
voltage d ivi d er an d
α
b iasing networ k
com p osed of R637,R638, a nd C637 . Wh en t h e ram p
amplitu de reach es t he t h res h ol d level of Q640, t h e collecto r
of Q640 goes L O, an d α L O is placed on bot h U640B
pin 11 an d U 607C pin 10 . T he ou tput of U 607C goes ΗΙ ,
an d t he positive fee db ac k supplie d to t h e base of Q640
t h roug h R639 s p ee d s up t h e ch ange of state of Q640 . By
reinforci ng t he switc h ing actio n of Q640 i n t
h
is ma nn er,
noise occurring at the t hr es hold level of Q640 is over r id d
en
.
T he swee p h ol doff p eriod co m mences
w h en
t
h e
L O
from Q640 is applied to pin 11 of monostable multivibrato r
U640B . T he Q out put on pi n 9 goes L O a nd remains L O
for α le n gt h of time d etermi n e d by t he RC timing
com p onents con necte d between p ins 14 and 15 of U 640 B .
H ol d off time can be va r ie d from t h e normal period b y
usi n g VA R H O L DO FF control R557 . Poten tiometer R557