Motorola R-2400 Operator's Manual page 35

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SECTION
5
APPTICATIONS
5.tr.0
SERVICE SHOP SETUPS
This
section
of
the
manual guides
troubleshooting,
diagnosing, and adjusting communications equipment
ranging
from
pocket pagers to
repeaters.
It
contains in-
formation on how to
connect equipment under test
to
the
R-2200
lR-2400
service
monitor
and
how
to
set the
controls and indicators
to
obtain the anticipated LCD
display. Maintain a permanent written log of all
meas-
urements and
information important to your operation.
CAUTION:
These are
typical applications and
are
not all
inclusive.
Not
every test
works with every
transceiver.
Motorola
takes no responsibility
for their
accuracy, ap-
plicability or
safety. Always refer
to your own
trans-
ceiver's service
manual
for
recommended test
methods
and specifications.
The service
monitor's DVM input
is unbalanced
(ground
referenced). use an appropriate interface
to
measure
balanced
circuits,
such
as
certain
receiver
audio outputs
or
telephone
lines.
FIGURE
5.1 -
SPECTRUM
ANALYZER DISPLAY
5.1..I
SPECTRUM ANALYZER
The
spectrum analyzer
, figure
5.
I
, displays the
RF
spec-
trum while
center frequency
is shown on the LCD
dis-
play.
Use
the keyboard to enter the channel center fre-
quency
directly.
Use
the spectrum analyzer
to
identify
interference, trace RF and
IF
signals, observe transmit-
ter
harmonics, check spurious
responses
and
receiver
local oscillator radiation. The
LCD
displays
the
ap-
propriate frequency and
changes
the
system
operating
frequency
as
required. The CRT displays a window
of
the RF
spectrum
at
either
the
"Antenna"
or
"RF
InlOut" port
in a
range
selected
from
I
MHz
to
999.9999
MHz programmable
in
100
Hz
increments.
The observed
window for the
analyzer can
be
controlled
with
the dispersion
control.
The spectrum analyzer has
a
dynamic
range
of
at least
55
dB
with
the step attenuator
in
the 0 dB position. With
the attenuator, the
useable
range can
be
extended up
to
the
maximum allowable in-
put level. When
using the attenuator, add
l0
dB
per
step
attenuation
to
the actual
dB reading on the
scope.
5.I.2
OPERATING NOTES FOR THE
R.24OO
SERVICE MONITOR
WITH
SPECTRUM
ANALYZER
In
the normal
use
of
a
spectrum analyzer,
the
user
should
keep
in mind
certain characteristics common to
this instrument.
l.
Noise
Floor
(grass)
-
caused
by the
basic sensitivity
of
the spectrum
analyzer receiver.
When
the disper-
sion switch is
set
to it's widest
sweep
(l
MHz/div.),
the receiver's
IF
is necessarily
set
to it's
widest band-
width
(approximately 40
kHz) allowing
more noise
through the
IF to
be displayed.
At
the narrower
dispersions,
0.1
MHz/div. and
.01
MHz/div.,
the
IF
bandwidth is reduced
to
15
kHz
and
6 kHz
re-
spectively, causing the noise
floor
displayed
to vary.
2.
In
the medium and narrow dispersions, the
shape
of
an unmodulated carrier displayed is due
partially to
the characteristics
of
the
IF
filters. The filters
used
in
the
R-2400
give the best compromise
between
shape, attenuation, sweep speed,
and RF
sweep
direction (Im
age
switch).
3.
The
noise
that
appears near a strong signal is called
Sideband
Noise (SBN).
It
is a
measure
of
the im-
purity
of
the
signals presented
to
the
spectrum
analyzer receiver, either the signal applied
from
an
external
RF source (i.e. transmitter) or the
service
monitor's own lst mixer's injectioil,
and
these
noise
components
will
add
in
the
mixer
and could both
be
observed
on the CRT.
It
is important
to
note that
when attempting
to judge the quality
of
an RF car-
rier that the RF
synthesizer
in
the
service
monitor
could contribute to the SBN of the
observed signal.
4. As
in
any
spectrum analyzer, excessively strong
signals, even those
at a
frequency
not
displayed,
could overload the
I
st mixer circuit
in
the
service
monitor and
produce
excessive
SBN
or
discrete
spurious
signals
that
are
not
actually
present in
the
applied signal. Therefore,
it
is always
good practice
to
change
the step attenuator
in l0
dB
steps while
observing
a
suspected spurious signal on the CRT
display;
if
the signal
rises
(or
falls) more than (or
less
than)
l0
dB
for
a
l0
dB
step
of
the attenuator,
the mixer is probably being overloaded. The
solu-
tion
is
to
add enough attenuation to
insure
a
dB
for
dB
change
on the display.
If
an off-screen signal
is
still too strong to allow observation of
the signal
of
interest, a "suck-out" circuit (either
a cavity
or
dis-
crete component
filter)
must be inserted to
reduce
this
signal.
5. When a "spurious" signal
appears above
or
below
the center frequency displayed,
it
could
be an "im-
age"
signal
caused
by the
service
monitor's
mix
er
/
local oscillator. To determine
if
this spurious signal
is being generated internally, change the "Image"
switch and
observe
if
the spur
moves
to
the other
side
of
the display
or
goes away altogether:
if
it
does, then
it
is being generated internally and may
be ignored.
5-l
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