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SECTION 5
APPLICATIONS
5.1.0 SERV.ICE 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/R-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.
HI
FIGURE
5.1-SPECTRUM
ANALYZER
DISPLAY
5.1.1 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
In/Out"
port in a range selected from 1 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 10
dB
per
step
attenuation to the actual
dB reading on
the scope.
5-I
5.1.2 OPERATING NOTES FOR THE R-2400
SERVICE
MONITOR
WITH SI'ECTRUM
ANALYZER
In
the normal
use
of a spectrum
analyzer, the user
should
keep
in
mind
certain
characteristics
common
to
this
instrument.
I.
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
(I
MHz/div.),
the receiver's
IF
is
necessarily set to it's widest
band-
width
(approximately 40kHz) 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,
atlenuation,
sweep speed,
and RF sweep
direction
(Image switch).
3.
The
noise
that appears near a
strong signal
is called
Sideband
Noise (SBN).
It
is
a measure of
the
im-
purity of
tbe
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
injection,
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 10 dB
steps while
observing a suspected
spurious
signal
on the CRT
display; if the signal rises (or falls) more
than
(or
less than)
lO
dB for a
10
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.
lf
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
fi
lter) must be insened 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
mixer I
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.
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