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4.1
VOLTAGE
CONTROLLED GENERATOR
The heart
of
the generator
consists
of
the positive and
nega-
tive current
sources,
the current switch, timing
capacitors,
triangle amplif
ier,
and
hysteresis
switch
(figure
41ir
The
positive
and
negative
current
sources generate
equal
but
opposite
polarity
currents
which
charqe
and
discharge
the timing
capacitor
selected
by
the
range selector.
The
current
switch,
which
is
controlled by
the
hysteresis
switch,
selects
either
tlre
positive
or
the
negative
cu
rrent
as
the
input
to
the
capacitor.
Sirrce
the
capacitor
is
being clrarged
by
a
current
source
which
changes
polarity periodically,
tl-re
voltage
across the
capacitor forms
a trianglle
waveform.
This
wavef
orm
is
f
ed
through the triangle
br,rffer
amplif ier
to
the
tiysteresis
switch.
The
hysteresis
switclt
determines
when
the triangle waveform
reaches
predetermined positive
ancl
negative peak values. When
this
occuis,
the
output
of
the
hysteresis changes state
ancJ caLrses
the
curren
t
switch
to
select
the
opposite
polarity
current.
Tire
ourtput
of
tlie
hysteresis
switch
is a
square
wave whose
edges correspond
to
the triangle
peak
valries.
The
rrlagnitude
of
tlre current
proriuceci
by
the
currrent
sources
is
dependent upon the
output of
the VCG amplif
ier'.
By
var.ying
the
output
of tlre
VCG
amplitier,
the
f
requency
of
the
trian-qle
and
square waveforms
may be
controlled.
ln
order
to
generate sine
ln;aves,
the
triangle waveform
is
sine
shaped
in
the
sine
converter
circu
it with
nonlinear
elernents"
The
waveforrn
switch
selects
the
waveform
of
interest and a
portion of
the
signal
is
selected
by
the
ampli-
tude
potentiometer and applied
to
the oirtput
amplifier.
The
output
amplifier
is capable
of
driving
a
50S) load
and
may be
dc offset.
The amplif
ier
output
is
routed to
a
50S)
attenuator which can
provicle
60 dB
of attenuation in
20
dB
steps.
An
additional 20 dB
of
attenuation
can be obtained
from
the amplitude control.
The
square
wave
from
the
hysteresis
switch
is
also applied
to
the
TTL
sync
pulse
generator,
whose square wave
output
is
TT
L compatible.
To
change
f
requency
ranges,
d if
f
erent
tim
ing
capacitors
may
be selected
by
the frequency
range
switch. On the
very
slow
f
requencies
the
capacitance
multiplier
becomes active.
SECTION
CIRCUIT DESCHIPTION
This
circuit
senses
the
capacitor charging
current
and
then
subtracts a
certain
percentage
of it
from
the capacitor.
As
a
result,
the
capacitor
does
not
charge as
fast,.and the
fre-
quency,
as
a
result,
is
lower.
Several
things can
affect the
frequency
of
the generator
by
varying the
output
of
the
VCG amplifier. One is the
fre-
quency
dial
of
the
function
generator
which
feeds
a
voltage
to
the VCG
amplifier. ln addition to
the frequency dial,
the
frequency
vernier feeds
in
a voltage
to
the
VCG amplifier.
The
ran-qe
of
the
vernier is approximately
1%
of
the
fuli
scale
frequency. Finally,
an external voltage applied
to
the
VCG
input
can
control
the frequency
of
the generator
loop.
The
VCG
input
allows frequency modulation
of
the
gen-
erator by
an
external
signal.
Under normal conditions the
generator
loop
runs
with
the
positive
and
negative
current
sorirces balarrced.
This
results
in
symmetrical
sine,
triangle and
squar-e
waveforms,
or
in
the
case
of
the
square
waveform, a
duty
cycle
of
509ir. By
varying tlre
syr-nrnetry
control, the
cLrrrent sources
may
be
urrbalanced
which
results in the generation
of
asymmetrical
,ruavef
orrns. This allows the generation
of
pulses, ramps,
and
otl-rer
waveshapes.
ln ihe
trigger mode,
tlre
generator
is
stopped
by
the
trigEer
amplif
ier. This
amplif
ier compares
the
output
of
the triangle
amplif
ier
to
ground. lts output
draws
just
the
right
amoLint
of
current
away
frorn
the
capacitor
to
keep
it
at
zero
volts.
This
level is
known
as
the trigger
baseline. When an
external
signal
is
applied
to
the trigger input,
it
is
shaped
into
a
fast
rise
time
pulse
by
the
squaring
circuit
arrd
is
applied
to
the
trigqer logic
circuit.
This
circuit
in
turn
shuts
off
the
trigger
amplif
ier
for
one cycle
of
the output
waveform.
Trigger
input
may
also
be
made
manually
by
the
manual
trigger
switch.
The
trigger logic
circuit
also allows
the
generator
to
run
in
the
gated mode.
ln
this
mode
the generator
will
run
as
long
as
the trigger input
signal
is
positive. When
it
goes negative,
the
generator
will
continue
to
run
until
the
last cycle
is
complete
and
then remain at the trigger
baseline level.
The GCV
output
is
an analog
output
voltage proportional
to
the
instantaneous
output
frequency
of
the
generator.
4-1
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