Tektronix 7a11 Instruction Manual page 25

Ci
r
cuit
Description-7A1
1
i ng the voltage at the
d
ivider ce nte r tap by a
pp
roxi mately +
or -12.2
V . This change in voltage appears simu ltaneously
at the bases of Q312 and Q314, wh ich act as complemen-
tary emitter followers. Circuit losses re
d
uce the maximu m
p
ossible offset voltage to approximately + or -12 .0 volts at
the R 312- R 314
j
unction .
W
hen
t
he O FF SET switch is ON,
t h is voltage is app lied through R 317 to the base of Q124A
in the In put Am plifier circuit.
The offset voltage a
p
pearing at the C317-13317
j
unction
is also app lied th roug h R 322 to the inve rting input of
am plifier
U 324 . The output of this am p lifier is ap
p
lie
d
to α
circ uit which is i
d
entical with
t he circ uit connected to the
O FF S ET
p
ote n tiometer . T he output of this circuit is
ap
p
lied to R49 in the
P
ro be as
p
reviously ex
p
lained . It
shou ld be
n oticed t hat R322,
U 324, Q332, 0334 and R335
ma ke up an operational amplifier.
F eedbac k resistor R 335
is equal to input resistor R 322, an d U324
h as extremely
h ig h gain. The overall gain of the am
p
lifier circuit is there-
fore equal to ap p roximately one. This ma k es the inverte
d
outp u t equal in amplitu
d
e to the non-inverted output
whic h is applied to the left si d e of R 317 .
General
V
OLTS/DIV
S
W
ITCH
A
ND RE ADO UT
R efer to the
V OLTS/DI V Switc h
a
n
d
Rea dou t sc hemat-
ic in the
b ac k of this manual . The
V
O LTS/DI V switch is of
the cam ty p e. Raise
d
p
ortions on in d
ivid u al sections engage
spring contacts to make switch connectio ns. Dots on the
switch matrix in
d
icate whic h contacts are close
d
in any
given switch position .
F
or exam
p
le, the switch is show n in
the 5 mV positio n. Dots in the 5 mV row in d icate that the
switc h contacts associate
d
with cams 1, 3, 5, 6, 7, 9, an
d
10
are close
d
. All ot her cam o
p
erated switches are show n in
the open
p
ositio n.
T he
V
O LTS/DIV switc h cams can be groupe d un
d
er
t h ree fu n ctions . Cams 1 through 4 co ntrol attenuator
switching in the
P ro be. Cams 5, 6 and 7 control atte nuator
switching in the Inpu t Am plifier . Cams 8 throug h 12 con-
trol rea
d
out circ u itry . The fu nctions associated with cams 1
through 7 are ex plaine d
in the
P ro be and I np ut Am p lifier
circuit
d
escriptions. The functions associated with cams 8
throug h 12 are ex plained
h
ere.
R
ea
d
out Circuitry
Oscilloscopes can
b
e e
q
u ippe d with readout circuits
w hic h are capable of
d isplaying rea
d
out data associated
with
t
he traces being
d
isplayed on the CRT . T his data must
be enco d
e d by the plug-in u nits in use. In the 7A11, the
d
ata is
d
e
p
en dent upon the position of the
V O L TS/DI V
switch, the
P O LA R ITY switch, the CA L 1 Ν switch , and t he
ID EN TI F Y switch .
3-6
E
nco ding informatio n is p rovi
d
e
d
by two currents wh ic h
are referre d
to as "column" current and "row" current.
Any one of eleven discrete current values can be present on
each line at any o n e time, ma king 121
p ossible current
com binations available . Wh en the in
d
icator unit
h as been
set to
d
is
p
lay the 7Α 11 readout
d
ata, α voltage pulse is
sequentially a
pp
lie
d
to enco ding li nes Β 29, Β32, Α32, Α30,
A33, and B33.
F or example, assume that α 15 volt pulse is app lied to
th e Α 32 line an d that the Α37 an d Β37 li nes are at ze ro
p
otential . W
it h
t
he
V O LTS/DI V switch in the
p
osition
s
hown , 400 μΑ will flow through R 451 and 200 μΑ will
flow throug h R453. The combine
d
600 μΑ appears on the
Α37 line . Since no connection exists between A32 and B37,
no current app ears on the Β37 line . The simu ltaneous
occure nce of 600 μΑ on Α37 and zero current on Β37 is
d
eco
d
e
d
as t he character 5
b
y the oscilloscope.
Assume now that t he Α32 line
h
as been returned to zero
p otential and α 15 volt pulse is
b
ei ng ap
p
lied to the Α30
line . The contact associated with cam 9 is closed an
d
100
MA flows throug h R447 an
d a
p
p
ears on the Α37 line .
Simultaneo usly, 300 μΑ flows throug h R446 an d
a
pp ears
on the Β37 line . These two currents are
d
eco
d
e
d
b
y the
oscillosco pe as the c haracter m. Α30 is then retu rned to
zero
p
otential an d
α voltage is ap
p
lie
d
to Β 29 . The Β29 cur-
rent
p
at hs are
n
ot interrupte
d
by switches, and therefore
cause 200 μΑ on the Α37 line and 400 μΑ on the Β37 line
whenever α 15 volt pulse is applied to Β29 . This ca uses t he
character V to be
d
eco
d
ed by the oscillosco pe. Thus, in
t
he
exam ple ju st given, α
d
eflection factor readout of 5 mV
woul
d
b
e printed on oscillosco pes e
q
ui pped with rea
d
out
circuitry.
The other inp ut lines function in α similar man ner to
p
rovi
d
e information such as UN CA L, I NVER TED, or to
add α 0 or decimal
p oint to the 1, 2 or 5 value co de
d
by
Α32 .
N
ote that the CA L/UN CA L an
d
P
O LA R ITY circuits
are controlle
d
by front- panel push switches rather t ha n by
the
V
OLTS/DI V switch. When UN CA L operation is select-
ed, the symbol for "greater than" ap pears immediately pre-
ce d ing the
d eflection factor selecte d by
t
he
V
O LTS/DI V
switch.
Identify Circuit
The ID EN TI FY circuit is normally connecte
d
to ground
in the
P robe. Wh en eit her of the
P
ro be or Plug-in
ID ENTI FY switches a re
d epressed, this groun d circuit be-
comes open an
d
c
urrent throug h R 421 causes Q424 to
saturate . Its collector (w hich had previously
b
een
h
eld at
-15 volts by CR424 and R424) then goes to ap
p
roximately
ground potential, causing sufficie nt current through R 269
to in ject an ap proximately 1/4
d ivision deflectio n signal in-
to the out p ut signal line at connector Al 1 . The Q424 εοΙ-