Eddystone 990R - Circuit Description; The RF Section

C
I
R
C U
IT
D E
SCRIPTION
THE RE
SECTION
General
.
This portion of
the
receiver
employs
a high-gain RF Amplifier
(TRl),
a
low-noise
Mixer
(TR2)
and a
comprehensive
Local Oscillator
system
utilising
a
total
of
five
transistors
(TR5-TR7).
An associated
circuit
is
the
Crystal Calibrator
which
uses
two
transistors
(TR8 & TR9)
.
The
whole
Section
is sub-divided
into
three
parts
;
TR1-TR4
being
housed
in
the
RF
Tuner Unit and
TR5-TR7 in
a
separate
screened
enclosure . TR8
and TR9
are located
in
a
further screened
housing
mounted
on
top of
the
RF
Tuner Unit.
The five
stages
in
the Local Oscillator system
provide
for
continuous
tuning
over
the whole
coverage,
spot frequency
working
on
up
to
eight
switch-selected
crystal-
controlled
channels, or
(semi-)
continuous
coverage
using external
synthesised
drive
wh
en
high-stability
operation with
flexibility of
frequency
selection
is
a
requirement.
The method
of
oscillator
control is
selected by the
SYSTEM SWITCH
(SJA-U)
which
also
serves
as
"crystal-selector
15
.
Local Oscillator injection
is always
higher
than
the
received signal
frequency
(10.7
MHz
i*f»).
The
RF
and
Mixer
Stages
.
(TR1
&
TR2)
Both the
RF Stage
(TRl)
and the Mixer
Stage (TR2)
employ AF239
germanium
mesa
trans-
istors.
TR1
is
operated
as a
grounded-base amplifier
with
manual
or
automatic
gain
control selected
by one section of
the
MANUAL/
AGC SWITCH
(S6A)
.
Gain
control
is not
applied
to
the Mixer
Stage but
is
applied
to
the IF
Pre-amplifier
TRIO
which
follows
the Mixer.
Control is
from the
same
line
as the
RF
Stage,
a
separate
circuit
being
used
for
the
later
IF
Stages.
Aerial
input is
taken via
SIB
to
low-
impedance
taps on
the
input
circuits
LI-L4. D1
(MYJU
silicon
diode)
is wired
directly
across the
input to
provide
some
measure
of
protection
for
TRl when
using the receiver in
close
proximity
to
a
high-power
trans-
mitter. The
emitter
of
TRl
is
fed via
SID
and
CIO from
separate
low-impedance
windings
on
the
input
tuned
circuits.
All unselected
main windings
are
shorted
to
ground by SIC
which
also
connects
the
aerial
section
of
the
4-gang
tuning
capacitor
to the
selected
circuit.
Separate
trimming
capacitors are
provided
for all
circuits,
those
for
Range 1
being
directly across
the tuning capacitor
and
therefore in circuit
on
all
ranges.
Coupling to
the
Mixer
Stage
(TR2)
is
by
a
tuned
bandpass
circuit
with
low-impedance
link coupling
between primary and
secondary. The collector
of TRl
is
matched
to
the
primary by
tapping
down
the appropriate coils (L5-L8),
while
separate
coupling
windings
are
provided
on
LI0—D12
to
match
the
base
of
the
Mixer Stage.
Unused
circuits
are
shorted
as
on
the
input stage and
the two
sections
of
the tuning
capacitor
(
C
1 3
6c
C23)
are
switched
from
range
to
range
by
S1F and
S1G.
Oscillator
injection is
to the
emitter
of
TR2
via
C45,
regardless
of
the
oscillator
arrangements
in
use.
A two-stage
low-pass
filter
is included
in the IF
feed to
T1
which has
a
low-
impedance
secondary for
connection to
the
IF
Pre-amplifier.
Connection
is
to
SK5?
the primary
of
T1
being timed to
10*7
MHz by
the
trimmer C30.
S2 provides a
means
of disabling
the Mixer Stage
to
prevent
noise
and
signal
break-
through
when
using
SK10
as
an
IF
input connection.
S1A
switches a
set of
four
indicator
lamps
(lLPl-4)
to
show
which
range
is in use.
The
lamps
are
situated
at the
left-hand end of the
appropriate
range
marking
on
the
scale
plate.
All
RANGE SWITCH
wafers
have five
positions but
are
fitted
with
a
mech-
anical
stop
to
prevent
selection of
the
fifth
position.