Module 21—Test Methods and Practices
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to AI-3, Index
Attenuation in a
coaxial line in terms of decibels per foot can be determined by the following formula:
As a technician, you need not be concerned with designing coaxial transmission lines. It is, however,
our feeling that you should be familiar with the parameters that go into making a transmission line. It can
readily be seen by the above formulas that transmission lines cannot be randomly selected without consideration of
system requirements. NAVSHIPS 0967-000-0140, EIMB, Reference Data, section 3, lists the characteristics of most
common transmission lines.
Q-5. What factor has the greatest effect on the physical size of a coaxial
Q-6. Is the attenuation of a coaxial cable independent of frequency?
INTERMODULATION DISTORTION MEASUREMENTS
Intermodulation distortion occurs when two or
more frequencies become mixed across a nonlinear device. The resultants are the difference frequency and the sum
frequency, both components of the originals. Undesirable frequencies can be generated by a mixing of two discrete
frequencies. Spurious radiation, arising from close spacing of transmitter and receiver, is a prime source of an
undesirable frequency that can cause intermodulation distortion in an electronic circuit. This is particularly the
case when antenna couplers are employed. Cross modulation and parasitic generation (described in the next section)
are two other sources of undesirable frequencies that may cause intermodulation distortion.
is the main cause of intermodulation distortion?
CROSS MODULATION AND PARASITIC GENERATION
CROSS MODULATION occurs when a signal from an adjacent channel crosses over into a second channel and modulates
the frequency of the second channel. PARASITIC GENERATION occurs when regenerative feedback is sufficient to cause
a circuit to oscillate, even though it is not designed to oscillate. Both types of distortion are common to
systems that are misaligned.
INTERMODULATION DISTORTION DETECTION
The presence of
intermodulation distortion is determined by a two-tone test method. Two sinusoidal frequencies of equal amplitude
are introduced into the system under test. The two frequencies are spaced
close together with reference to the unit under test. The output of the system under test (an
amplifier, receiver, or transmitter) is monitored on a spectrum analyzer that is comparable in characteristics to
the suspect system. The resultant display should be an exact reproduction of the input frequencies. If not, some
form of intermodulation distortion is present. To determine if external sources are causing the intermodulation
distortion, you can use a single-frequency signal. If the display on the spectrum analyzer does not show the
single frequency, then intermodulation distortion is present.
Intermodulation distortion cannot be
entirely suppressed, but it can be minimized by shielding components and circuitry, parasitic suppression
circuitry, and antenna spacing. These factors are incorporated in the design of the system and are tested during
production. Any shields or parasitic suppressors that are removed by the technician must be replaced before
troubleshooting and/or repair can be effective. Antenna locations also pose a consideration when installing a new
system. Ship alteration specifications must be observed when new antenna systems are being installed.
Q-8. When you are testing a piece of equipment for intermodulation distortion, what should the output of the
equipment look like?
The important points of this chapter are summarized in the following paragraphs:
STANDING WAVES are the result of an impedance mismatch between a transmission line and its load.
If a transmission line is not properly terminated, it will cause a percentage of the transmitter power to be
reflected back to the source. The reflected wave or standing wave will increase in magnitude as the mismatch
VSWR refers to the voltage ratio of the incident wave (that which is transmitted to the load) and
the reflected wave (that which is reflected by the load back to the transmitter). An ideal vswr is considered to
be 1 to 1.
Standing waves that are present on a transmission line can be used to determine the
TRANSMITTER FREQUENCY. Voltage or current peaks are present at half-wavelength intervals. By measuring
the distance between peaks, you can compute frequency mathematically.
are usually tested for standing waves with test devices that are inductively coupled to the line. These test
devices vary greatly in their complexity, ranging from bridge circuits to simple neon lamps.
INSERTION LOSS MEASUREMENTS are performed by injecting a signal of a known amplitude into a transmission
line and then monitoring the signal at the far end of the cable with a power meter. Loss measurements must be
taken at various frequencies to determine if the transmission line is good across its frequency range.
The most common cause of INTERMODULATION DISTORTION is improper spacing of transmitters
and receivers. CROSS MODULATION is common to equipment that is misaligned. Intermodulation distortion can be
tested by injecting two signals (different frequencies) into a piece of equipment and then monitoring its output
for distortion using a spectrum analyzer. Intermodulation distortion is usually caused by improper antenna spacing
or by poorly shielded components or circuits.
EIMB, Test Methods and Practices Handbook, NAVSEA 0967-LP-000-0130, Naval Sea Systems Command,
Washington, D.C., 1980.
NEETS, Module 10, Wave Propagation, Transmission Lines, and Antennas, NAVEDTRA
172-10-00-83, Naval Education Training and Program Development Center, Pensacola, Fla., 1983.
415E, NAVSHIPS 0969-139-2010, Hewlett-Packard Co., Palo Alto, Calif. 1968.
ANSWERS TO QUESTIONS Q1. THROUGH Q8.
A-1. At standing-wave voltage peaks.
1 to 1.
A-3. Corrosive effects of salt water and weather extremes.
A-4. Yes, it is quite
A-5. The dielectric constant of the insulating material.
A-7. Close spacing of
transmitters and receivers.
A-8. An exact reproduction of the input.
Introduction to Matter, Energy, and Direct Current,
to Alternating Current and Transformers, Introduction to Circuit Protection,
Control, and Measurement
, Introduction to Electrical Conductors, Wiring Techniques,
and Schematic Reading
, Introduction to Generators and Motors
Introduction to Electronic Emission, Tubes, and Power Supplies,
Introduction to Solid-State Devices and Power Supplies
Introduction to Amplifiers, Introduction to
Wave-Generation and Wave-Shaping Circuits
, Introduction to Wave Propagation, Transmission
Lines, and Antennas
, Microwave Principles,
, Introduction to Number Systems and Logic Circuits, Introduction
to Microelectronics, Principles of Synchros, Servos, and Gyros
Introduction to Test Equipment
, Radar Principles,
The Technician's Handbook,
Master Glossary, Test Methods and Practices,
Introduction to Digital Computers,
Magnetic Recording, Introduction to Fiber Optics