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Block
diagram
of
oscillator
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5.
In the
final stage,
a low
source impedance and
low distortion output
are provided
by
a
shunt-regulated emitte
r
follower
VTlO and VTll
.
The
two
paths of
a. c
.
feedback
derh·e
their
signals from the output
of this
stage.
Beyond,
the
low output impedance is
pa
dded up
by
resis-
tance to
present
a
constant
and
s uitable
value of source for the
output le \'el
control
circuit.
The
output
s tage
has
its
working
conditions
s
tabilized
by d.
c. negative
feedback
,
returned
from
it
to
the fi
r
st
tra nsistor
of the amplifier,
a
ll
the
transistors
being
d.
c
.
c
oupled.
6.
The
output
level
of the
oscillator
is
determi
ned primarily
by a
GOO
r1
T
network
attenua
tor;
fi ne control
is achieved by
a
s
m
a
ll variabl
e
resistor
in
series with
the attenuator.
Both
controls
a
re continuously
variable
and
un-calibra
ted.
The attenuator
is
connected
to
either
the front
panel terminals
or
the rear
coaxial
socket according to
the
setting
of
S3.
Feedback
7
.
Two a
.
c. feedback
paths
exist between the
output
of the
final
stage
and
the
input
of
the first
stage of
the
amplifier.
One
of these determines
the
frequency
of
oscillation
by
providing
posi-
tive
feedback
at a
chosen frequency through
a variable
parallel
T
ne
twork.
This
ne
twork has
continuously variable
resistors RV6a to RV6c which are
ganged
together and form
a
tuning
con-
trol
over a frequency
band,
a nd switched
capacitors
C17
to
C29
enabling
a
series of
six
bands to
be
co\·ered.
At
freque ncies
above and below the
chosen
frequency
phase
changes in
the
network
cause the
feedback
to
be
negati ve;
this
provides a
considerable
measure
of
harmonic distortion
suppression
.
8
.
The
other
a.
c.
feedback
path
conveys negative
feedba
c
k
at
a
ll
frequencies.
At
the selected
frequency
of oscillation
it
is
overcome by the positive feedback at the
junction
where
the two
feed-
l:k1.ck
paths enter the
amplifier
.
This
wide
-
band
negati,·e
feedback is derived
from
two
s ucccs
-
sh·e
resistive potentiometers at the output
of the
ampl
ifier.
In
the first
potentiometer,
ther-
misto
r THl
occupying the
upper
arm is sensitive
to the
le\·el
of
output and varies its
resistance,
a
nd
therefore
n
tries the
negative
feedba
ck
,
so
that
the
leve
l
of
output
is s tabilized,
and also
so
that the
s
tre ngth of
oscillation
is
always
moderate
(and therefore
generates
a minimum of di
s-
tortion).
In
the second
potentiometer
,
a
physica
lly
larger thermistor TII2
in
t
he
lower arm
is
Chap. 4
Pa~e
2
Nov. 79
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