Wavetek 142 Instruction Manual page 19

voltage followers
whose output
is
fed
back to
Q12
and
Q13
through R46, R47,
R54, and
R55 forming
the
flip-
flop configuration.
Q9
is
connected
as a
7-volt
zener
reference for IC1
and
the flip-flop.
In
the state
where
Q13
is
conducting and
Q12
is
cut
off,
the voltage at
the emitter of
Q16
is
high
(-t2.5
V) and
the voltage
at
the emitter of
Q11
is
low (—2.5 V).
Once
tunnel diode
CR13
is
turned
on by
IC1,
the
V^e of
Q15
will
increase until
Q15
turns
on and
in
turn
Q13
is
turned
off.
This causes
the emitter voltage of
Q16
to
drop to
—2.5
V
which, through feedback
resistors
R47 and R46,
keeps
Q12
in
tne
on
state.
Thus
the emitter voltage of
Q11
will rise
to -i-2.5 volts.
Although the tunnel diode
CR13
is
turned
on only
a
very short time during the
peak
of the triangle
waveform,
Q13
will
remain
in
the off state
because
of the feedback through
R46, R47, R54, and R55.
The output
of
Q11
is
used
to drive the diode
gate.
3.2.4
Diode Gate
As shown
in
Figure 3-4,
the
Diode Gate consists of four
diodes,
CR7
through
GRID. Assume
that the voltage at
point
C
is
-1-2.5
V.
Diodes
CR7
and
CR9
are
then forward
biased,
and
CR8
and
CR10
are reverse biased.
The
negative
current source (I—) will
draw
current
from
the Hysteresis
Switch through
CR9, but the
positive
current source
(l-t)
will
charge
up the timing capacitor
"C" through
CR7. The
Figure 3-4
-
Simplified
Diode Gate Diagram
result
is
a linear
rise
of voltage across the timing capacitor.
When
this
voitage reaches
-H.25
volts,
the
output of the
Hysteresis
Switch will
change to
—2.5
volts. In this state,
CR8
and
CR10
are
forward biased
and
CR7
and
CR9
are
reverse biased.
The
positive current will
be diverted to
the
Hysteresis
Switch through
GRID
and
the negative
current source
will
draw
current
from the timing capacitor
through
GR8.
This action causes a linear
discharge of the
timing
capacitor.
The
process
is
repeated, thus a triangle
waveform
is
generated.
3.2.5 Sine
Gonverter and Amplifier
The Sine Gonverter
is
a
sealed
subassembly containing
matched
diodes and precise current-biasing
resistive
net-
works.
When
a
precise triangle
wave
is
fed into
the shaping
network, through
voltage divider
R149
and
RMS,
the
triangle
wave
is
shaped into a sine wave. Potentiometers
R159 and
R168
adjust the voltage supplied to the sine
converter to obtain
minimum
sine distortion.
The
ampli-
tude of the
sine
converter
output
is
approximately
150
mV
p-p.
This voltage
is
amplified to approximately
2.5
V
p-p
by the Sine Amplifier.
The
Sine Amplifier consists
of
transistors
Q30
through
Q35
connected
as a differential
feedback
amplifier.
The
gain of
the amplifier
is
controlled
by R165, R164, and R158.
R145 and R146 compensate the source current into
Q30
to
minimize the distortion of the
sine
converter output.
3.2.6
Square Amplifier
The
inverted
output from
Hysteresis
Switch
Q16
(±2.5
volt
square wave)
is
fed to the
Square
Wave
Shaper
com-
prised of
GR14
through
GR19. For
a
square
wave
output,
GR14
and
GR19
are reverse biased.
In
one
state,
if
the
cathode voltage of
GR15
is
+2.5
volts,
GR15
and
GR17
will
be reverse biased
and
GR16
and
GR18
will
be forward
biased.
The output
at
the
cathode of
GR18
is
limited to
+1.25
volts
by the current
through
R63
and
R64
into
R69.
When
the input voltage
is
switched to
—2.5
volts,
GR15
and
GR17
will
be forward
biased
and
GR16
and
GR18
will
be
reverse biased, limiting the
output
at
the
cathode of
GR18
to
—1.25
volts
by
the current
through
R66
and
R67
into
R69.
When
the positive pulse
output
is
selected
by
the
WAVE-
FORM
SELEGTOR
switch, the
cathode
of
GR14
is
con-
nected to
ground through R61.
In this
configuration, the
output
at
the
anode
of
GR18
will
be limited to a voltage
swing
of
—1.25 and
zero.
Because the output amplifier
is
an inverting amplifier, the signal at
the 50fi
OUT
con-
nector
will
be
a positive pulse. In
a similar
manner,
when
the
anode
of
GR19
is
connected
to
ground through R61,
the
output swing
at
the
cathode
of
GR18
will
be
limited to
zero
and +1.25
volts,
resulting
in
a
negative pulse at
the
50J2
OUT
connector.
G26
is
a
high
frequency
filter
to
filter
the high
frequency
noise
on the square
wave,
resulting in a
cleaner signal.
At
this
point,
the
output
signal
is
buffered
by
a
double
emitter follower
stage consisting of
017 and 018, and
the
output
at
the
common
emitters
is
applied to the
output
amplifier
through the contacts of the
WAVEFORM
SE-
LEGTOR
switch.
3-5
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