PL75-9 datasheet(87/141 Pages) ETC

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Electronic systems wiring & cable

desired signal in the coax. The sophistication of cable and equipment

isolation techniques used is dependent upon the frequency of the

interfering noise and how it enters the cable system. Any additional

measures taken to reduce incoming noise will also reduce outgoing

radiation and cross-talk.

Ground Loops and Common Mode Returns

Coax cable consists of an inner and an outer conductor insulated from

each other. Each conductor carries a desired signal current (source

to load or return). The outer conductor is usually grounded at the

source, load, bulkheads and other intermediate points. “Ground loop”

or “common mode” currents caused by potential differences of ex-

ternal noise sources are also simultaneously carried on the outer con-

ductors and noise is introduced into the system, greatly reducing the

“signal-to-noise ratio.”

Low frequency signals (20 KHz to 6 MHz) are particularly suscep-

tible to both ground loop and common mode interference. Coax cable

is recommended in this case with the complete coax chain having a

minimum-number of outer conductor ground contacts. Reducing the

number of ground connections reduces the number of possible ground

loops. Major equipment, relays, switches, connectors, patch panels,

etc., should be isolated from ground with the ultimate design hav-

ing only one system ground connection at the source.

In the case of nuclear attack, a tremendous electromagnetic pulse

(EMP), transmitted for many miles from ground zero, will induce ex-

tremely high voltages in all unguarded and unprotected cable and

equipment. It is estimated that a vertical electrical field intensity of

50K volts per meter will be present 62.5 miles from a 10–megaton

near-ground explosion. Solid-state transistors and diodes, integrated

circuits, front end RF coils, sensitive relays and reeds are typical of

the sophisticated elements used today for both military and commer-

cial systems that may fail even though far removed from the fireball

area.

The lack of money, time and qualified personnel will not permit com-

plicated and costly post-completion interference cleanup of poorly

engineered systems. The use of good coax, twinax, triax, in isolated

or guarded circuitry, will do much to suppress outgoing EMI and RFI,

reduce incoming noise pickup, and help protect both your system and

the adjacent system from mutual interference. Careful cable to equip-

ment interface planning must be exercised to produce workable com-

patible systems. Design engineers are not free to treat cable installa-

tion as casually as they may have in the past.

Coax Cable

Shielded cable should be used to protect against magnetic and capaci-

tive stray fields in all cases of potential interference (low or high fre-

quency). Grounded coax cable is excellent for use from 20 KHz to 5

GHz for most systems. But even coax, if subjected to very strong

interference, will not completely protect the desired signal. Coax cable

is a two conductor cable. The outer conductor of a coax cable is not a

true shield in that it is also the signal return. A true shield is insulated

from the signal carrying conductors and protects the enclosed coaxial

conductors. However, the outer conductor of a coax cable is an elec-

trostatic shield and thus protects against capacitive interference. When

two conductors at different voltages are placed near one another, a

charge difference exists between them. Since a charge difference ex-

ists between the two conductors, a capacitive effect also exists be-

tween them. Such capacitive effects can give rise to interference sig-

nals in both conductors.

To prevent wires which carry low-level signals from being influenced

by capacitive effects, you need to surround the low-level signal wire

with an electrostatic shield. This type of shielding is effective be-

cause external electric fields cannot penetrate an enclosure surrounded

by an electrical conductor. Coax cable should be used when low-

level signal wire is likely to come in contact with another signal car-

rying wire. This is especially true when the voltage difference be-

tween the signal carrying wire and the source of interference is large.

Any external voltage injected in this braid will act as noise to the

I N

Source

Grounded

Bulkhead

Non-Insulated Feed Thru

Multiple Noise Currents (I )

Source

Load

Insulated at

Bulkhead

Insulated Feed Thru

No Noise Currents (I ) in

Signal Return

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