Wavetek 188, Instruction Manual

Page: 23 / 50

Scans by Artekmedia
=>
2011 SE C ll0N · 4
CIRCUIT DESCRIPTION
This s
e c t
i
o n
describes the fu
n c t i
o
ns o f m
a
j
o
r c i rcuit
elements
and
thei r relationships
to one another
as
shown
in
figure 4-1 ,
fu nctional block
diagram. The
fol­
lowing
sections in
this manua l provide more
detailed
i n formation for
maintaining the
instrument.
As
shown i n
fi
g
u
r
e 4-1 ,
the VCG
(Voltage
Co
n t r
o l
of
Gene r a t or) sums v o l t
ag
e i nputs
from
the frequency
d
ia
l
and
the
VCG I N
connector.
This
sum
voltage
con­
t rols
the magni tude of
a
complementary
cu rrent
sou rce
and
cu rrent sink . This cu rrent
varies
l
i
n e
a
r
i
l
y
f rom approximately
2 mA
to
2
pA over a
1 000:1 (4.0
to
.004) range
or logarithmically from
approximately
2
ma
to 0.2 �
over
a 1 0,000:1
(4.0
to
. 0004) range
of
each
frequency
mu ltiplier. T h e . V C G
also
controls
the
trigger
baseline compensation
c i rcuit,
which consists
of another
c u rrent si nk
at twice the
current
magn itude .
The diode
gate, controlled by the comparator
output,
connects
either
the
c u rrent
source
or
the current
sink
to
the
timing
capacitor selected
by the frequency
m u l tiplier. When
the current
so u r c
e
is switched
in, the
c h a rge
on
the timing.
capaci tor
will
rise
Iinea rily,
pro­
ducing
the positlve-going
triangle
slope. Likewise, the
c u rrent
sink p
rodu
c
e s
the negative-going
triang le
slope .
The
t r i
a
n
g
l
e
amplifier is
a
unity
gain
ampl ifie r
whose
output
is fed to
the
compa rator and to
the
output
c i r­
cu its. The c o m p
a
r a t o
r
operates
as
a wi ndow det
ec
tor
with li mit
points
set
to the t r
i
a n g l e peaks.
The
±
2V
output is
sent
back to the diode gate and
to
the output
c i r cu it s .
When
the
output
is
+
2V, the · triangle
is
positive-going until
the +
1 .25V limit
is
reached and
the
comparator output switches to
-
2V
.
When
the
output is
-
2V, the
t riangle
is negative-going
unti l
the
-
, .25V
l imit
Is
reac hed
and the comparator
output
switches back to
+
2V,
repeating
the
p roce s s . In this
manner,
the basic function
gene rator
loop, the
bold
path in
figure
4-1 , produces simultaneous generation
of
triangle
and
square
waves at
the
same
frequency.
The output frequency
is
determined by the
magnitude
of the
t
i
m i n
g capacitor
selected by
the frequency
mu ltiplier swi tches and by the magn itude of the
cur­
rents
s u
p
p l i e
d
to
and
removed
from it. Since
the cu r­
rents a re lineari ly p
r
op o rt i o n a l to the sum
of the
VCG
inputs,
so
wi l l
be
the outpu t
f r eq u ency .
To extend the lower frequency
capability
of the
generator, a capac itance multipl ier c i rcuit
divides
VCG
currents by 1 0
(effectively
multiplyi ng
the
timing
capac itor
by 1 0) for each of the
lower
3
multiplie r
r anges .
The TIL
square from
the comparator
i s
buffered and
sent to
the
SYNC OUT TIL connector. The other side
is sent
to
the trigger
flip-flop
and to a
leve l sh ifte r to
produce
the ±
2V
bipola r squa re
for the
diode
gate
and
the
square shaper
circuits.
The . sq uare shaper
conve rts
the sauare i
n
t
o
a c u rrent
signal and
appl ies
it
to the
1..;
FU NCTION
switch,
The
buffe red triangle is
applied
to
the
"
"
FUNCTION switch and
to the
sine
conve rte r
input. The
si ne conve rter,
using the
non l i nea r c haracteristics of
its
d iodes, conve rts
the
tria ng le i nto a sinusoida l
current
for the
rv
FUNC­
TION
switc h .
The
selected function
i s
s e n
t to the preamplifier,
w
h
e
r
e it is
inverted
and
buffered. The
preampl ifier out­
put goes to the
output
a mpl i
f
i e r
through
the
AMPLITUDE
control
where
it
is summed with offset
voltage from the DC OFFSET
control. He r e , waveform
and offset are
inve rted and ampl ified
to
a ±
1 0V
peak
signal
w
h
i
ch can dri
v
e
a
50n termination from a 500
sou rce i mpedance .
The output
amplifier d rives
the
500 OUT
HI
connector
and
a
resistor divider
produc­
ing
the 500 OUT LO output.
Noncontinuous
modes of operation
(t rigger
and
gate)
resu lt from a l l ow
in
g or preventing
the VCG
c u rrent
sou rce
from charging the
tim ing capaCitor.
W h en - eve r
the trigger
flip-flop
output is low,
each
of the two
trig­
ger diodes
conduct
a
cu r rent
I,
sou rcing
21
to the
baseline compe nsation ci rcuit.
This
removes
the
cur­
re
n
t
I from the
VCG
current source and forces
a
OV
basel i ne
at the triangle amplifier
input.
4-'