- What is Intermodulation Interference?
- What is Transmitter Harmonic Interference?
- What are Transmitter Spurious Output Interference?
- What is Receiver Desensitization Interference?
- What is Transmitter Noise Interference?
- What will LBA's IM analysis service provide me?
What
is Intermodulation Interference?
There are three basic categories
of Intermodulation (IM) interference. They are receiver produced,
transmitter produced, and "other" radiated IM. Transmitter
produced IM is the result of one or more transmitters impressing a signal
in the non-linear final output stage circuitry of another transmitter, usually
via antenna coupling. The IM product frequency is then re-radiated
from the transmitter's antenna. Receiver produced IM is the result
of two or more transmitter signals mixing in a receiver RF amplifier or
mixer stage when operating in a non-linear range.
"Other" radiated IM is the result
of transmitter signals mixing in other non-linear junctions. These
junctions are usually metallic, such as rusty bolts on a tower, dissimilar
metallic junctions, or other non-linear metallic junctions in the area.
IM products can also be caused by non-linearity in the transmission
system such as antenna, transmission line, or connectors.
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What
is Transmitter Harmonic Interference?
Transmitter harmonic interference
is due to non-linear characteristics in a transmitter. The harmonics
are always fundamental frequency multiples and the non-linear design
of the final output stage of the transmitter. If the harmonic signal
falls within the passband of a nearby receiver and the signal level is of
sufficient amplitude, it can degrade the performance of the receiver.
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What
is Transmitter Spurious Output Interference?
Transmitter spurious output interference
can be attributed to many different factors in a transmitter. The
generation of spurious frequencies could be due to non-linear characteristics
in a transmitter or possibly the physical placement of components and unwanted
coupling. If a spurious signal falls within the passband of a nearby receiver
and the signal level is of sufficient amplitude, it can degrade the performance
of the receiver.
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What is Receiver Desensitization
Interference?
Receiver desensitization interference
occurs when an undesired signal from a nearby "off-frequency"
transmitter is sufficiently close to a receiver's operating frequency. The
signal may get through the RF selectivity of the receiver. If this
undesired signal is of sufficient amplitude, the receiver's critical voltage
and current levels are altered and the performance of the receiver is degraded
at its operating frequency. The gain of the receiver is reduced, thereby
reducing the performance of the receiver. A high-power transmitter can be operating
several megahertz away from the receiver frequency and/or its antenna can
be located several thousand feet from the receiver's antenna and still cause
"desense" interference.
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What is Transmitter Noise
Interference?
Transmitter noise interference
occurs because a transmitter radiates energy on its operating frequency
as well as frequencies above and below the assigned frequency. The
energy that is radiated above and below the assigned frequency is known
as sideband noise energy and extends for several megahertz on either side
of the operating frequency. This undesired noise energy can fall within
the passband of a nearby receiver even if the receiver's operating frequency
is several megahertz away. The transmitter noise appears as "on-channel"
noise interference and cannot be filtered out at the receiver. It
is on the receiver's operating frequency and competes with the desired signal,
which in effect, degrades the operational performance.
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What will
LBA's IM analysis service provide me?
The analysis calculates all possible IM product
frequencies that could potentially interfere with receivers at the communications
site based on each receiver’s individual bandwidth. It then predicts
each IM signal level present at the input of each affected receiver. For
each IM frequency, the analysis considers all possible sources of IM generation
in the transmitters. The analysis takes into account the transmitter’s
power output, modulation bandwidth, and conversion losses.
The analysis also takes into account the transmitter’s harmonic characteristics and output level, predicts each transmitter’s signal level and each transmitter’s noise signal level present at the input of each receiver.
It also takes into account the transmission line losses, filters, duplexers, combiners, isolators, multi-couplers and other RF devices that are present in each system. Additionally, the analysis considers the antenna separation space loss, horizontal and vertical gain components of the antennas as well as how they are mounted on the structure. The gain components are derived from antenna pattern data published by each manufacturer.
Additionally, the analysis determines how much isolation, if any, is
required to prevent receiver performance degradation for each IM interference
signal that occurs, any harmonics that fall within a receiver’s passband,
receiver desensitization and transmitter noise interference.
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P.O. Box 8026
Greenville, North Carolina 27835
800-522-4464 / 252-757-0279 / Fax 252-752-9155
Email: Lbagrp@Lbagroup.com
Copyright 2003. LBA Group, Inc. All Rights Reserved.