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HP 651B - General Description; Circuit Description; Oscillator Circuit

HP 651B
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Model
651B
Section
IV
4-1.
GENERAL
DESCRIPTION.
4-2.
The
Model
651B
Test
Oscillator
includes
an
oscillator,
power
amplifier,
peak
detector,
attenua¬
tor,
and
monitor
circuit.
A
block
diagram
of
the
in¬
strument
is
shown
in
Figure
6-1.
The
oscillator
circuit
uses
a
modified
Wein
bridge
network
to
gen¬
erate
a
stable,
distortionless
sine
wave
signal
which
is
applied
to
the
power
amplifier
circuit.
The
peak
detector
circuit
provides
a
degenerative
feedback
voltage
to
the
oscillator
circuit
to
stabilize
the
signal
applied
to
the
power
amplifier.
The
power
amplifier
circuit
is
used
to
increase
the
output
power
available
at
the
50
ohm
and
600
ohm
output
connectors
and
to
improve
the
frequency
stability
of
the
output
signal
with
changing
output
loads.
The
output
attenuator
provides
a
means
of
attenuating
the
signal
at
the
out¬
put
connectors
in
nine
steps
of
10
dB
each.
The
mon¬
itor
circuit
continuously
monitors
the
signal
level
at
the
input
to
the
attenim.tor.
The
regulated
power
supply
provides
all
voltages
required
by
the
651B
circuits.
4-3.
CIRCUIT
DESCRIPTION.
4-4.
Refer
to
Figures
6-2
and
6-3
for
the
follow¬
ing
discussion.
4-5.
OSCILLATOR
CIRCUIT
.
4-6.
The
oscillator
circuit
generates
a
sinusoidal
signal
at
the
frequency
selected
by
the
RANGE
switch
and
FREQUENCY
Dial
located
on
the
front
panel.
The
RC
bridge
network
is
a
modified
Wein
bridge
circuit,
consisting
of
an
RC
frequency
selective
network
and
a
resistive
voltage
divider
network.
The
Weinbridge
in
the
Model
651B
Test
Oscillator
differs
from
the
conventional
Wein
bridge
circuit
in
the
design
of
the
resistive
voltage
divider
network.
The
resistor
in
the
conventiorml
Wein
bridge
is
replaced
with
imped¬
ance
Zl,
which
consists
of
A2CR6
and
A2CR7.
4-7.
Oscillation
at
the
selected
frequency
is
made
possible
by
the
use
of
both
positive
and
negative
feed¬
back.
Positive
feedlmck
is
provided
through
a
fre¬
quency
sensitive
RC
network
to
the
differential
amp¬
lifier
A2Q2
and
A2Q3;
negative
feedback
is
provided
to
the
differential
amplifier
through
a
network
insen¬
sitive
to
frequency.
Only
at
the
selected
frequency
will
the
positive
feedback
exceed
the
negative
feedback
voltage
to
sustain
oscillation.
4-8.
The
RANGE
switch,
SI,
selects
combinations
of
resistors
and
capacitors
(SlRl
through
S1R24,
and
SlCl
through
S1C14)
to
establish
the
frequency
sen¬
sitive
RC
networks
for
the
she
frequency
ranges
of
the
instrument.
The
FREQUENCY
Dial
varies
the
main
frequency
tuning
elements
CIA,
CIB,
and
CIC.
The
RC
components
maintain
the
proper
phase
rela¬
tionship
of
the
positive
feedback
voltage.
At
frequen¬
cies
where
Xc
=
R,
the
positive
feedte-ck
voltage
is
in
phase
with
the
oscillator
output
voltage
(refer
to
Figure
4-l)and
exceeds
the
negativefeedbackvoltage.
At
frequencies
other
than
where
Xq=
R,
the
positive
feedback
voltage
is
neither
of
the
right
phase
nor
of
sufficient
amplitude
to
maintain
oscillations.
4-9.
A
field
effect
transistor,
A2Q1,
is
used
as
the
impedance
converter
because
of
its
extremely
high
input
impedance
and
low
noise
characteristics.
It
provides
a
high
impedance
in
series
with
the
input
impedance
of
the
differential
amplifier
on
the
lower
four
frequency
ranges
(XIO
-
XlOK).
The
high
im¬
pedance
added
prevents
the
RC
bridge
circuit
from
being
loaded
by
the
low
input
impedance
of
the
differ¬
ential
amplifier,
A2Q2
and
A2Q3,
on
the
lower
fre¬
quency
ranges.
The
impedance
converter
is
bypassed
on
the
XIOOK
and
XIM
ranges
due
to
lower
resistor
values
in
the
RC
bridge.
4-10.
The
difference
between
the
feedback
voltages
from
the
bridge
circuit
is
amplified
by
differential
amplifier
A2Q2
and
A2Q3,
and
Is
applied
to
the
com¬
plementary
symmetry
circuit
A2Q5
and
A2Q6,
throi^h
emitter
follower
A2Q4.
A
positive
feedback
voltage
from
the
output
of
the
complementary
symmetry
cir¬
cuit
is
applied
between
resistors
A2R8
and
A2R9,
in
the
collector
circuit
of
A2Q2,
on
the
first
four
fre¬
quency
ranges.
The
application
of
the
feedback
volt¬
age
at
this
point
is
used
to
make
the
effective
resis¬
tance
of
the
collector
l<»d
higher
tte.n
the
input
impedance
of
the
emitter
follower
A2Q4,
forcing
the
collector
current
into
the
base
of
the
emitter
follower.
The
increase
in
the
Imse
current
results
inan
increase
in
the
loop
gain
of
the
oscil^tor
circuit.
The
feed¬
back
voltage
is
removed
on
the
XIOOK
and
XIM
fre¬
quency
ranges
due
to
the
value
of
resistors
A2R8
and
A2R9
exceeding
the
input
impedance
of
the
emitter
follower
at
the
higher
frequencies.
01810-2
4-1

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