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VCG
IN connector.
The
VCG
Summing
Amp
is
an
in-
verting amplifier
whose
output
voltage
is
used to
control
a positive
current source
and
a
negative current source.
For
symmetrical output waveforms,
the currents
from
the
two
current sources
are
equal
and
directly
proportional to
the voltage of the
VCG
Summing
Amplifier output.
The
Diode
Gate,
which
is
controlled
by
the Hysteresis Switch,
is
used to switch the
positive
current or the negative
current to the timing capacitor
selected
by
the
FREQ
HZ
+2.5V
!
C
-2.5V
I
Figure 3-2
-
Simplified
Timing Diagram
selector.
If
the
positive
current
is
switched
into
the
timing
capacitor, the voltage across
the capacitor
will
rise
linearly
to
generate
the
triangle
rise
transition.
If
the
current
is
negative,
the voltage across the timing capacitor
will
fall
linearly
to
produce
the
fall
transition.
The
Triangle
Amplifier
is
a
unity
gain amplifier
whose
output
is
fed to the Hysteresis
Switch
as well as
to the
Sine Converter.
The
Hysteresis
Switch
has
two
voltage
limit
points
(-1-1.25
V
and —1.25V).
During
the time the
output
voltage of the Triangle
Ampli-
fier
is
rising,
the
output
voltage of the Hysteresis
Switch
is
positive.
But
when
the
output
voltage of the Triangle
Amplifier
reaches
-1-1.25
V,
it
triggers
the
Hysteresis
Switch
causing the
sv.'itch
output
to
go
negative.
Once
the control voltage
into
the
Diode Gate
becomes
negative,
it
will
switch the
positive
current
out and
switch the
negative current
in
to the
timing
capacitor, so that
the
voltage across the capacitor
will
reverse, starting a linear
decrease of the
waveform.
When
the decreasing voltage
reaches
—1.25V,
the
output
of
the
Hysteresis
Switch
will
switch
back
to
positive, reversing
the
process.
This
action generates the
triangle
waveform
as
shown
in
Figure
3-2.
Since the
output
of
the
Hysteresis
Switch
is
a
square
wave,
the
result
is
simultaneous generation of
a
square
wave
and
a triangle
wave
at
the
same
frequency.
The
output frequency
is
determined
by
the
magnitude
of the capacitor
selected
by
the
FREQ HZ
selector
and
the
magnitude
of the
positive
and
negative current sources.
Since the current sources
are linearly
proportional
to
the
control voltage of the
VCG
circuit,
the
output frequency
will
also
be
linearly
proportional to the control
voltage.
If
the current of the negative current source
is
decreased
by
19
times,
the
fall
time
of the
triangle
will
be 19
times
longer
than the
rise
time
of the
triangle;
resulting
in
an
unsymmetrical
waveform and
a division
of
the frequency
by
a
factor
of
10.
Gradually
increasing
the current
from
the negative current source
and
decreasing the current
from
the
positive
current source
in
such
a
way
that the
period
for
the
triangle
to
complete one
cycle
remains
constant, the
symmetry
of the
output
waveform
can be
continuously
varied
while the frequency
is
held constant.
The
inverted
output
of the Hysteresis
Switch
is
fed to
the
Sync
Amplifier
and
also
the
Square
Wave
Amplifier.
The
Square
Wave
Amplifier
consists
of
a
shaping
circuit
which
limits
the
output swing
to
±1.25
volts.
For
positive
pulse outputs,
it
limits
the
output
voltage
swing
from
—1.25
V
to
0
V; and
for negative pulse
outputs,
it
limits
the
output swing from 0
V
to
+1.25
V.
The
output
signal
from
the Triangle Amplifier
is
applied
to the Sine Converter, consisting of a
sine
shaping
cir-
cuit
and
a sine amplifier.
The
sine, triangle,
square, positive
pulse, or
negative pulse
is
fed to the
Output
Amplifier through the
WAVEFORM
SELECTOR
switch
and
the
30
V
P-P
MAX
control.
The
Output
Amplifier
is
an
inverting amplifier
whose
output
is
fed
into
a
step
attenuator
and then
to the
output
BNC
connector.
The
Attenuator
consists
of four
pi
attenuators,
each having
a
50-ohm
input
impedance and
a
50-ohm
output imped-
ance.
There
is
one
—10
dB
attenuator,
one
—20
dB
attenuator,
and
two
—30
dB
attenuators.
By
combining
them
in
different
combinations,
these
four attenuators
provide
10
dB
steps of attenuation;
from
0
dB
to
-60
dB.
3-2
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