Wavetek 188 Instruction Manual page 18

SECTION
T
CIRCUIT
DESCRIPTION
This section
describes the functions ot
majw
circuit
elements and
their
relationships to
one another as
shown
in
figure 4-1.
functional
Block
digram
The
fol-
lowing sections
in
this
manual
provide
more
detailed
information for
maintaining
the
instrument
As shown
in
figure
4-1.
the
VCG (Voltage Control
of
Generator)
sums
voltage inputs
from
the
frequency
dial
and the
VCG
IN
connoctor
This
sum
voltage con-
trots
the
magmtudo
of a
complementary
current
source
and current sink This current
vanes
linearity
from approximately 2
mA
to
2 |iA
over a 1000
1
(4
0
to
004) range or
logarithmically
from approximately
2
ma
to
0 2 »a
over a 10,000 1(4
0
to
0004) range of
each
frequency multiplier
Tho
VCG
also controls the
trigger
Baseline
compensation
circuit,
which
consists
of
another current
sink at
twice the
current
magnitude.
Tho diode
gate, controlled
By
the
comparator
output,
connects either the
current source
or the current sink
to Ihe
timing capacitor selected
By the
frequency
multiplier
When
the
current source
is
switched
in.
the
charge on
tho timing capacitor will
nse imeaniy. pro-
ducing the positive-going
triangle
slope.
l*ew.se. the
current sink
produces the
negative-go^
triangle
slope.
The triangle amplifier
is
a
unity
gam
amplifier
whose
output
is
fed to the
comparator
and
to the
output
cir-
cuits
The comparator operates as a
window
detector
with limit
points set to
the
triangle
peaks The
tZV
output
is
sent
Back
to the
diode gate
and
to
the output
circuits.
When
the
output
is
+2V. the
triangle
is
positive-gong
until
the
+
1
25V
limit
is
reached and
the
comparator output
switches
to
-2V When
the
output
is
-
2V.
the
triangle is
negatrve-gomg
until
the
-
1.25V
limit
is
reached and
the
comparator
output
switches
back
to
*
2V. repeating the
process
in
th«s
manner,
the
basic
function generator
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 timing capacitor
selected
By tho
frequency
multiplier
switches
and
By
the
magnitude
of
the cur-
rents supplied to
and removed from
it.
Since
the cur-
rents
are
imeanly
proportional
to
the
sum
ot
the
VCG
inputs, so
will
Be
the output
frequency.
To
extend
the lower frequency capability
of
the
generator, a
capacitanco
multiplier circuit
divides
VCG
currents
By 10 (effectively multiplying
the timing
capacitor By
10) tor
each
of
the
lower 3
multiplier
ranges
The TTL
squaro from
the
comparator
is
Buftorod
and
sent to the
SYNC OUT TTL
connector
The
other side
is
sent
to the tnggor
flip-flop
and
to
a
level shifter to
produce
the
*2V
bipolar
square
for
the
diodo gale
and
the
square shaper
circuits
Tho
squaro shapor
converts
the souare into a current signal
and
applios
It
to
the
Tj
FUNCTION
switch The Buffered triangle
is
applied
to
the
\
FUNCTION
switch
and
to the sino
converter
input
The
sine
convertor, using the
nonlinear characteristics
of its
diodes, convorts tho
triangle into
a sinusoidal current
for fhe
%
FUNC-
TION
switch
The
selected function
is
sent
to
the preamplifier,
where
it
is
inverted
and
Buffered.
The
preamplifier out-
put
goes
to
the
output amplifier
through the
AMPLITUDE
control
where
it
is
summed
wilh
offset
voltage
from
the
DC OFFSET
control Here,
waveform
and
offset
are inverted
and
amplified
to
a
± 10V peak
Signal
which
can
drive a
500 termination
from a
500
source
'mpedance The
output
amplifier drives the
500
OUT
HI
connector and
a resistor divider
produc-
ing
the 500
OUT
LO
output.
Noncontmuous modes
of
operation (trigger
and
gate)
result
from allowing
or
preventing
the
VCG
current
source from charging the timing capacitor.
Whenever
the
trigger
tiip-flop
output is
low.
each
of the
two
trig-
ger diodes conduct
a current
I,
sourcing
21 to
the
baseline
compensation
circuit
This
removes
the cur-
rent
I
from
the
VCG
current
source
and
forces a OV
Baseline
at the triangle amplifier
input.
4-1