Eddystone 990R - Page 30

Care
must
be
taken
not to
use
too
high
a
level
of input
signal when
checking
the
nose response
since
overload
effects
will
tend to
round off the
individual
peaks
to
present
an
incorrect,
picture
of
the
actual
shape.
It
is essential
that
a
generator
with
slow tuning
rate or
preferably
a
separate
incremental
tuning
control
be
used
in
this
application.
Two trimming
adjustments are
provided
for
correcting the
filter
response,
these
merely
adjusting
the
input
and output
terminations
to provide
a
correct
match
to
the
filter
proper.
Both
trimmers
are
accessible
with
the
side cover
removed from
the
right
hand
side-plate.
Greatest
variation
within
the
pass
band
will
be effected
by adjustment of the
output
(shunt)
trimmer
C79»
The
input (series) trimmer C78
produces only minor changes
in
the
nose shape
but
affects
very
considerably the
amplitude of the
response and
to
a minor
degree the
skirt symmetry.
In
practice, C78
should
be
set for greatest
amplitude,
adjustment
being
quite
broad
and
therefore
permitting
further minor
adjustment
to
assist
C79
in
levelling
out
the
nose
response.
The setting
of
C79 is quite
critical
and
adjustment must
be performed
carefully
with
frequent
examination
of
the
whole
nose
shape.
Correct
skirt response
with extremely
rapid
fall-off below about
lOdB
will
normally
be achieved
automatically
when
the troughs in
the
nose response
are
reduced
to minimum
proportions.
If
a
sweep
generator
is
available,
this
will reduce
considerably the
time
taken
to
correctly
align
the
"NARROW"
filter.
The
swept signal
should
be
introduced
at
the
IP
INPUT
socket.
Drive
to
the
"Y" Amplifier
can
be taken
from
the
AM
Detector
load
or via
the high-level
IP
OUTPUT
using an
external
probe for detection
of
the IP
signal.
As
with
a
normal
generator,
signal
input
should
be
kept at
a fairly low level
to
prevent
distortion of
the
c.r.t. display.
Scanning
should
preferably
not exceed
some 10
sweeps
per second with a
sweep
width
of
the
order
40-50
kHz.
Reduced
sweep width
should
be
used
when
the
receiver is
fitted
with
a narrower
non-standard
filter.
Overall
IP
sensitivity should
be checked
on
completion
of
filter
alignment
(signal
applied
to
IP INPUT
socket,
receiver mode at "AM"), Results
should
approximate
to
those
given
belows-
SEL2CTIVITY
"WIDE"
. .
.
. llpV
for
50mW
output.
SELECTIVITY
HARROW” . .
.
.
9\iV
for
50mW
output.
Standardisation of the
Crystal Calibrator
.
It
is
convenient
at
this stage
in
the
alignment procedure to
check the
accuracy
of
the internal
crystal
calibrator
so
that it
can be used in the
next
phase of alignment
for
checking
the
overall scale
accuracy.
Set
the
IP INPUT
SWITCH
to "OFF"
(upper
position) before
proceeding.
Any
close-tolerance
frequency sub-standard providing
10
MHz
markers
from
50
MHz
up
can
be employed
to
standardise
the
internal
crystal. Connect
the
external
standard
at
the aerial
input
and tune the
receiver
to
the selected
10 MHz point.
Select
"PM"
mode
and
adjust
TUNING CONTROL
very
carefully for
centre-reading
on
the
tuning
meter
with
SELECTIVITY at
"NARROW".
Maintain
TUNING
CONTROL
at
this setting.
Switch
off
the
external standard
and
put
receiver
CALIBRATOR
SWITCH
to
"ON".
Adjust
L25
through
aperture
in
top
of
calibrator
housing
to
bring
meter
needle to
mid- scale
position.
Some
care
is
necessary
in
adjusting
L25
to
ensure
positive
firing
of
the
crystal
and
consistent opera,
tion
of
the calibrator
circuit.
Several
on/off
switching
cycles
should
therefore
be
carried
out
whilst observing
the
tuning
meter
with
receiver
switched
to
"AM".
Meter
indication
should be constant
each time
the
calibrator
is
turned
on.
Frequency
stability can
also
be checked
with
repeated
switching,
by
using
the
"FM"
mode
with
receiver
slightly
off-tuned. Constant
off-centre
deflection
should
be
achieved
with
calibrator on.
~
29
~