fm microwave radio systems
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AM are more sensitive to amplitude
nonlinearities inherent in wideband microwave
amplifiers.
FM are insensitive to this type of non linear
distortion and can be transmitted through
amplifiers that have compression or amplitude
nonlinearity with little penalty.
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FM signals are less sensitive to random noise
and can be propagated with lower transmit
powers.
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In AM systems, inter-modulation noise is caused by
repeater amplitude nonlinearity, and it is a function of
signal amplitude.
In FM systems, inter-modulation noise is caused primarily
by transmission gain and delay distortion, and it is a
function of signal amplitude and the magnitude of the
frequency deviation.
Thus, FM are more suitable than AM for microwave
transmission.
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Provides flexible, reliable, and economical
point-to-point communications
Carries a few narrowband voice circuits up to
thousands of voice and data circuits
Easily expandable
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Microwave radios can also be configured tocarry high speed data, facsimile, broadcast-
quality audio, and commercial television
signals.
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Basebandis the composite signal that modulates the
FM carrier and may comprise one or more of the
following:
1. Frequency-division-multiplexed (FDM) voice-band
channels
2. Time-division-multiplexed (TDM) voice-band
channels
3. Broadcast-quality composite video or picturephone
4. Wideband data
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Pre-emphasis network provides an artificial
boost in amplitude to the higher baseband
frequencies.
This allows the lower baseband frequencies
to frequency modulate the Intermediate
Frequency ( IF ) carrier and the higher
baseband frequencies to phase modulate it.
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FM deviator provides the modulation of the
IF carrier that eventually becomes the main
microwave carrier.
IF carrier frequencies 60 MHz to 80 MHz
IF carrier frequencies 70 MHz most
common
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Low-index frequency modulation are kept
between 0.5 and 1 producing a narrowband
FM signal.
IF bandwidth resembles a conventional AM
and is approximately equal to twice the
highest baseband frequency.
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IF and sidebands are up-converted by the mixer,
microwave oscillator and band-pass filter.
Microwave generators consist of a crystal
oscillator followed by a series of frequency
multipliers.
The channel-combining network provides a
means of connecting more than one microwave
transmitter to a single transmission line feeding
the antenna.
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The channel separation network provides the
isolation and filtering necessary to separate
individual microwave channels and directthem to their respective receivers.
The band-pass filter, AM mixer, and
microwave oscillator down-convert the RFmicrowave frequencies to IF frequencies and
pass them on to the FM demodulator.
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The FM demodulator is a conventional, non-
coherent FM detector (i.e., a discriminator
or a PLL demodulator ).
At the output of the FM detector, a de-
emphasis network restores the baseband
signal to its original amplitude-versus-
frequency characteristics.
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Permissible distance between transmitter
and receiver depends on the following
system variables:
1. Transmitter output power2. Receiver noise threshold
3. Terrain
4. Atmospheric conditions
5. System capacity
6. Reliability objectives
7. Performance expectations
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Typical distance is between 15 and 40 miles.
With systems longer than 40 miles or when
geographical obstructions, such as a
mountain, block the transmission path,
repeaters are needed.
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A microwave repeater is a receiver and a
transmitter placed back to back or in tandem
with the system.
The repeater station receives a signal,
amplifies and reshapes it, and then
retransmits the signal to the next repeater
or terminal station down line from it.
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The location of intermediate repeater sites is
greatly influenced by the nature of the
terrain between and surrounding the sites.
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Preliminary route planning generally assumes
relatively flat areas, and path (hop) lengths
will average between 25 miles and 35 milesbetween stations.
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In relatively flat terrain, increasing path
length will dictate increasing the antenna
tower heights.
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Transmitter output power and antenna gain
will similarly enter into the selection
process.
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The exact distance is determined primarily
by line-of-sight path clearance and received
signal strength.
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For frequencies above 10 GHz, local rainfall
patterns could also have a large bearing on
path length.
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In all cases, however, paths should be as
level as possible. In addition, the possibility
of interference, either internal or external,must be considered.
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Intermediate Frequency ( IF )
BASEBAND
Radio Frequency ( RF )
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- also called heterodynerepeaters
Received RF carrier is down-converted to an
IF frequency, amplified, reshaped, up-
converted to an RF frequency, and then
retransmitted.Consequently, the baseband intelligence is
unmodifiedby the repeater.
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received RF carrier is down-convertedto an
IF frequency, amplified, filtered, and then
further demodulated to baseband.
The baseband signal, which is typically
frequency-division-multiplexed voice-band
channels, is further demodulated to a master
group, super group, group or even channel
level.
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This allows the baseband signal to be
reconfigured to meet the routing needs of
the overall communications network.
Once the baseband signal has been
reconfigured, it FM modulate an IF carrier,
which is up-converted to an RF carrier and
then retransmitted.
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Repeater demodulates the RF to baseband,
amplifies and reshapes it, and then
modulates the FM carrier. With this
technique, the baseband is not
reconfigured.
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Essentially, this configuration accomplishes
the same thing that an IF repeater
accomplishes.
The difference is that in a baseband
configuration, the amplifier and equalizer
act on baseband frequencies rather than IF
frequencies.
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The baseband frequencies are generally less
than 9 MHz, whereas the IF frequencies are
in the range 60 MHz to 80 Mhz.
Consequently, the filters and amplifiers
necessary for baseband repeaters are simpler
to design and less expensive than the ones
required for IF repeaters.
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The disadvantage of a baseband
configuration is the addition of the FM
terminal equipment.
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The received microwave signal is not down-
converted to IF or baseband; it is simply
mixed (heterodyned) with a local oscillator
frequency in a nonlinear mixer.
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The output of the mixer is tuned to either
the sum or the difference between the
incoming RF and the local oscillator
frequency, depending on whether frequency
up- or down-conversion is desired.
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The local oscillator is sometimes called a
shift oscillator and is considerably lower in
frequency than either the received or
transmitted radio frequencies.
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Example:
RFin = 6.2 GHz
LO = 0.2 GHz
RFout = 6.4 GHz (sum) RFout = 6.0 GHz (difference)
For UP-conversion:
RFout = 6.4 GHz
For DOWN-conversion:
RFout = 6.0 GHz
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The radio signal is simply converted in
frequency and then re-amplified and
transmitted to the next down-line repeateror terminal station.
Reconfiguring and reshaping are not possible
with RF-to-RF repeaters.