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4
SECTION
CIRCUIT
DESCRIPTION
4.1
FUNCTIONAL BLOCK DIAGRAM ANALYSIS
This section
describes the functions
of
major
circuit
elements and
their
relationships
to
one
another as
shown
in
figure
4-1
,
functional
block diagram.
The
fol-
lowing sections
in
this
manual
provide
more
detailed
information
for
maintaining the instrument.
As shown
in
figure
4-1
,
the
VCG
sums
voltage
inputs
from
the
frequency
dial
and
the
VCG
IN
connector.
This
sum
voltage controls the
magnitude
of
a comple-
mentary
current
source
and
current
sink.
This current
varies
linearily
from approximately 2
mA
to
2
/«A
and
over
the 1000:1
(4.0 to
.004)
range
of
each
frequency
multiplier.
The
VCG
also controls the
trigger
baseline
compensation
circuit,
which
consists
of
another
cur-
rent sink at
twice the current
magnitude.
The
diode
gate, controlled
by
the
comparator
output,
connects
either
the current
source
or the current sink
to
the timing capacitor selected
by
the
frequency
multiplier.
When
the current
source
is
switched
in,
the
charge on
the timing capacitor
will
rise
linearily,
pro-
ducing
the
positive-going triangle slope. Likewise, the
current sink
produces
the negative-going
triangle
slope.
The
triangle amplifier
is
a
unity
gain 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
±
2V
output
is
sent
back
to
the diode
gate
and
to
the output
circuits.
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
triangle
is
negative-going
until
the
-
1
,25V
limit
is
reached and
the
comparator
output
switches
back
to
+2V,
repeating the process.
In
this
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
the
frequency
multiplier
switches
and
by
the
magnitude
of
the
cur-
rents
supplied
to
and removed from
it.
Since
the
cur-
rents
are
linearily
proportional to the
sum
of
the
VCG
inputs,
so
will
be
the
output frequency.
To
extend
the lower
frequency
capability
of
the
generator,
a capacitance
multiplier circuit
divides
VCG
currents
by 10
(effectively
multiplying the timing
capacitor
by
10)
for
each
of
the
lower 3
multiplier
ranges.
The TTL
square from
one
side
of
the
comparator
is
buffered
and
sent
to
the
SYNC OUT
TTL
connector.
The
other side
is
sent
to
the
trigger
flip-flop
and
to
a
level shifter to
produce
the
±2V
bipolar
square
for
the
diode gate
and
the
square shaper
circuits.
The
square shaper converts
the
square
into
a
current
signal
and
applies
it
to
the
Tj
FUNCTION
switch.
The
buffered
triangle
is
applied
to
the
\
FUNCTION
switch
and
to
the sine converter
input.
The
sine
con-
verter,
using
the
nonlinear characteristics
of
its
diodes, converts the
triangle
into
a
sinusoidal current
for
the
%
FUNCTION
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
with
offset
voltage
from
the
DC
OFFSET
control.
Here,
waveform
and
offset
are
inverted
and
amplified
to
a
1
0V
peak
signal
which can
drive
a
50fl
termination
from a
50Q
source impedance.
The
output
amplifier drives
the
50Q
OUT
HI
connector and a
resistor divider
produc-
ing
the
500
OUT
LO
output.
Noncontinuous
modes
of
operation
(trigger
and
gate)
result
from
allowing or preventing the
VCG
current
source from charging
the timing capacitor.
Whenever
the
trigger
flip-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
0V
baseline
at
the
triangle amplifier
input.
When
the
CONT
switch
is
released, trigger
logic
is
in-
hibited
from passing any
trigger
signals
and
the
trig-
ger
flip-flop
output
is
held
high.
This prevents the
trig-
ger diodes
from conducting
and
the generator loop
operates
continuously.
When
the
CONT
switch
is
pressed, the generator loop
is
held
at
the
0V
baseline.
Pressing the
TRIG/GATE
TRIG/GATE
switch puts the instrument
in
triggered
mode
and any
external or
manual
trigger
signals
at
the
trigger logic input
will
be
transformed
into
a
nar-
row
pulse corresponding
to
the
low-to-high
transition
of
the
trigger input.
This pulse sets the
trigger
flip-flop
high
and
allows the generator loop
to run.
When
the
triangle
negative
peak
is
reached, the
comparator
low-to-high transition
clocks the
trigger
flip-flop
low
4-1
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