Tektronix PG 508 User Manual

pulse is slow e n oug h so t h at t he D i np ut of fli p flop U 720A
has not reac hed t he h ig h level w hen t he pulse at t he cloc k
te rm i
n
al (p i
n
6) of fli p flo p U 720A goes h ig h (lea d ing
p o r tio n of t he n ext p ulse drivi ng t he tra n
sitio
n circuitry)
t he 0 termi n al, p i n 3, of fli p flo p U 720A goes h ig h . T h is
h ig
h
is connected to t he set termi n al, p i n 12, of U 720 B
causing t h e 1 terminal of U 720 B to go h ig h illuminating
t he CO NTR O L ERR O R lig h t . T he on time an d reset fort h is
error i n
dicati
n g
mode now
p
rocee ds as previously
d escribed .
L evel Co n trol M ulti p lier
T
h
is circuitry p rovides i n de pe
n
den
t
to p a
n
d bottom
level
co
n
trol of t h e out p ut p ulse by
controlling t he
am p litu d e and offset
of
t h e d rive sig n al
to
t h e li near outp u t
am p lifier . Also i n clu d ed is circuitry to accom p lis h t he
n ormal com p leme n t functio n a nd t he p reset fu n ction .
Co
n
t
r
ol voltage clam
p
s to e
n
sure t
h e
out
p
ut
amplifier
is
n
ot over d
rive n
are also p rovi ded
.
Am p litu d e co n trol of t h e sig n al occurs i n t h e a n alog
multi p lier, U 850 . T he p ulse sig n al p rovi des t h e
Χ
i np ut,
and th e level control voltages p rovide t he Υ in p ut
.
T he
Χ -
Υ
p roduct of t h ese i np uts is co n verte d to α d rive curren t for
t he outp ut am p lifier .
I np ut and com p lement p ulses from t he varia b le tra n si-
tion
time gen erator are a pp lied to t h e bases of Q825 and
Q840 . T h ese tra n sistors form α d iffere n tial am p lifie r ,
su pp lied by co n stant curren t source U 800 B an d Q820 . Α
p ositive-going sig n al at t he base of Q825, wit h t he
complementary (negative-goi n g) sig n al at t h e base of
Q840 causes t he sig n al curren tat p i n 11 of t he multiplierto
go negative an d t h e signal current at pin 12
to go
positive .
Wh en t h e pulse p olarity reverses, at t h e b ases of 0825 an d
Q840, t he sig n al curre n t also reverses polarity at pins 11
a
n
d 12 . T h e difference between t h e currents at p ins 11 an d
12 corres
p o
nd
s to
t
h
e Χ sig
n al i
np
ut for t h e multi p lier
.
T he total curren t flowing from pins 2 an d 3 of U 850 is
essentially eq ual to t he current req u ire d by t h e constant
currentsource, U 895A an d Q900 . H owever, t he d ifference
in currents between t hese pins
corresponds to
t he Υ input
signal for t h e multiplier .
T he am p litu d e differe nce of t h ese curren ts is con t r olle d
by U 895 B . This is α do differential am p lifier w h ic h
am p
lifies t
h
e differe n ce betwee n t h e h ig h & low level
control voltages to prod uce t h eΥ i np ut sig n al . Gain
ad j ust m e n t for t h eΥ input sig n al is provide d b yR 885 .
T he h ig h a nd low level co n trol voltages a r e d etermine d
by t h eir respective front panel co n trols, R 770 B an d R 770A .
If t he preset function is selecte d , t he preset h ig h an d
preset low potentio m eters, R 775 and R 785, p rovi de t he
T h eory of Operation--PG 508
co
n trol
voltages .
T h esevoltages are buffere d by u n ity gai n
am p lifiers U 780A a nd U 780 B . B ot h co n trol voltages ra n ge
betwee n 0 a nd - Ι --5 .2 V . Wh en t h e co n trol voltages a r e
eq ual, t he Υ i nput is zero an d t he m ultiplier sig n al out p ut
(X-Y) e qu als zero . Α differe n ce of -42 .6 V b etwee n t heh ig h
a
nd
low
level
co
n
trol voltages correspo
n
ds to
maximum
out p ut am p litu de from t h e p ulse gen erator .
T he n ormal complement switc h inverts t he level co n trol
voltage i n p uts to differential am p lifier U 895 B . H owever,
si nce t he differe nce betwee n t h e voltages is un c h ange d ,
t he control voltage
i
np ut sig n al
h
as co
n
sta n
t amplitu d e,
but reverses
polarity . T h
is
com p lements t h e pulse
generator outpu t . T h e normal com p leme n t bala nce ad -
j ustment, R 910, ensures t h at t he Υ multiplier inp u t
respon ds equally to ch anges in eit h
er
t
h
e
h ig h
or low
level
control voltages .
T h e signal current at p ins 5 and 6 of U 850 is t he pu lse
signal . Since Q845, in con j un ctio n wit h U 800 B , p rovides α
co n sta n t current si nk , t he cur r e
n t
t
hr
oug
h R
954,
from t h e
emitter of commo
n
base stage Q954, also contains
signal
cu r
re
n t variations
.
T h
e cur
r en t
d
rive r
for
t he outp ut
amplifier is Q954 . T h e sig n al curren ts i n to p i n s 8a nd 9 of
U 850 also con tai n t h e p ulse signal . H owever con sta n t
curren t si nk R 847, an d commo n b ase stage Q950 a r e
inclu d e d only as α bala ncing t h ermal loa d for t h e mul-
ti p lier .
To obtai n i n depe ndent co n trol of t h e output p ulse h ig h
an d low levels, t h e co n trol voltages are average d by
resistor n etwor k R 914,
R
915,
R
918 a nd R 920 . H ig h a nd
L ow trac k i ng potentimeters, R 915 an d R 920, are a d -
j ustable to provi d e minimum interaction betwee n p ulse
levels . T he d o voltage from t h is n etwor k , along wit h t he
voltage from t h e offset ad j ustment R 925, is summe d an d
inverte d by U 930A . U 930 B proves furt h er gain an d level
s h ifting an d , i n con j unction wit h Q945, serves
as
α level
controlle d offset generator . Α d o current sou r ce to t he
collector of 0954 is provi de d by Q945 . T h e collector of
Q954 is t he virt u al ground inp u t to t h eo u tput amplifier .
Wh en t he h ig h a nd
low
level control potentiometers
are
both at mid ra n ge (-42 .6 V zero out pu t) Q945 sources
all
t h e quiescent bias current required b y Q954, w h ic h is
a
pp
roximately 15 m A . T h erefore, t here is no curre n t d rive
to
t
h
e
outp ut am
p
lifier t h roug h R 975 or R 1055 . If t he h ig h
level control
is
t urned fully
cw(maximum o u t put), t h e low
level control voltage
remains
at +2
.6
V . T h
e
h ig
h level
control voltage i n creases to +-5 .2 V . T h is causes t h e
voltage output of U 930A to decrease, causi ng t he offset
generator U 930 B a nd Q945 to sou r ce ap p roximately 20
mA . T h
is is an i ncrease of 5 mA . T h is d iffere nce i n co n trol
voltage setti ngs
also causes maximum d ifference in t he
con trol voltage i np ut sig n al to t he multiplier .
This
action
also causes 10 mA p ea k to peak sig n al curren t variations in
t he collector current of Q954 . Si nce t he sig n al curren t

Tektronix PG 508 Specifications

General

Output Waveform	Pulse
Duty Cycle	Variable
Output Impedance	50 Ω
Modes	Continuous, Triggered, Gated
Trigger Output	Yes
Polarity	Positive or Negative