Eddystone 990R - Local Oscillator Circuits

Local
Oscillator
Circuits
.
(TR3-TR7
)
Germanium
epitaxial
mesa
transistors
(GM290) are
used for
all
stages
except
TR3
which
employs
a
further
AF239.
TR3 &
TR
4
are
located
in
the RF
Tuner
Unit.
TR3
is
the
normal
Local
Oscillator
for
continuous
tuning
and
is tuned
by
the
fourth
section of
the
tunihg
gang
(C
40
),
Output voltage
is
developed
across
the
earth
return
inductance
from
the
tuned
circuits
which
is
marked "Z" on
the
circuit diagram.
TR3
is
functional with
the
SYSTEM
SWITCH
S3
at
"NORMAL"
and is
disabled
in the other
positions
by removal of
the
emitter
supply
voltage.
Base
voltage is
retained at
all times.
The remaining
transistor
located
in the
RF Tuner Unit is
TR4.
Its function
is
to
facilitate
connection
to
the
normal
oscillator
tuned circuits
(via
the
isolating
resistor
R17) from
the
alternative
oscillator
input
at
SK
6
.
Operating conditions
are
such
that
TR
4
will
function
as
a
frequency
multiplier
when
required,
multiplication
by
factors
of
two
or
three
being
possible
with normal
drive
levels.
Oscillator
circuits
L13-L16 offer
some
measure
of
protection
against
spurious
drive signals especially
when
using
the
receiver
with
external
synthesised
drive.
TR
4
is
switched
by
interrupting
the
emitter supply
at
S3 when this
is
set
to
the
"NORMAL"
position.
Base
voltage
is
retained
as
on
TR3
.
The
miniature
socket
SK
6
located
on
the
RF
Tuner
Unit is
fed via PL3
which
termin-
ates a
short
coaxial
lead
from
TR6 and
TR7 in
the separate
Crystal Oscillator Unit.
Both
these
transistors
are
used
as
emitter
followers
and serve to isolate
the
separate
feeds
from the
internal
crystal
oscillator
and
the
external oscillator drive
source.
The
Crystal
Oscillator
(TR5) employs
a
series-mode
circuit
with
output taken
from
the
collector
via
C
56
.
Up
to
eight
crystals
can be
fitted at any
one
time, separate
tuned
circuits
being
provided
to
permit
accurate
alignment
of the oscillator circuit
to
suit
the
crystal
in
use.
The
tuned
circuits
are
adjusted
when installing the
crystals
and
allow
slight
pulling
of
the
crystal
to
bring the
appropriate signal channel
into
the
centre
of
the
i.f.
passband.
All
circuits are
identical and have
values
to
suit
crystals
in
the
basic
range
37-88
MHz.
Actual
crystal
frequencies
are
calculated
from
(fs
+
10.
7
),
l/2(fs
+
10.
7
)
or
l/3(fs
+
10.7)
MH
z
depending
on
the
part of
the overall
range in which the
selected
signal
frequency
falls.
Detailed
information
on
calculation
of crystal frequencies
for
specific signal
channels
will be
found
on page
22
in
the
Section on
"Operation".
Oscillator
drive
from
the
Crystal
Oscillator is
applied
to the
base of
TR6
which
provides a
low-impedance
output
to
feed the
alternative
oscillator socket SK6 via PL
3
.
External
oscillator
drive is
introduced
to the
Crystal
Oscillator Unit via SK7 which
is
linked
to
a
BNC socket
at
the
rear
of
the set
by
a
short
length
of coaxial cable
terminating
in
PL4.
Input
level
should
be of
the order
500mV, isolation being
provided
by
the
other
emitter
follower
TR7
which
feeds
SK6
via
PL3 and C60.
Switching
of
TR5,
TH6
and
TR7
is
by S3C &
S3D. Base
and emitter
supplies are re-
moved
from
TR5
except
when
S3
is
set
to one
of the
eight
"CRYSTAL"
positions.
Base
voltage
remains
on
the
two
emitter
followers
(TR6
&
TR7) except
at
"NORMAL"
when
all
supplies
are
removed
from
the Crystal
Oscillator
Unit.
The
emitter
voltages
for
TR6
and
TR7
are
applied
only in
the
appropriate
positions,
i.e. to
TR6
at
"CRYSTAL" and
to
TR7 at
"EXT
(OSC)".
Crystal
Calibrator
.
(TR8 &
TR9)
The
crystal-controlled
calibration
oscillator
(TR8
s GM
378
)
and
its
associated
Tone
Oscillator/Modulator
(TR9
sGET880)
provide
crystal
check-points
at
10
MHz
intervals
throughout
the
tuning
range
of
the
receiver.
Injection is
to the primary
of
the
inter-
stage
coupling
between
RF
Amplifier
and Mixer
by a
small
probe
attached
to SK4.
This
is
fed
from
the
Calibrator
via a
short
length of coaxial
cable terminating
in
PL5.
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6
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