PL75-9 datasheet(89/141 Pages) ETC

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Trompeter Products Catalog - T21 Military/Aerospace

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Electronic systems wiring & cable

Bonding & Grounding

Good bonding and grounding are absolutely essential if noise pickup

reduction is to be accomplished. Equipment isolation, cabling, bond-

ing, and grounding are all part of the noise pickup and EMI/RFI prob-

lem. The method of equipment interconnecting and grounding is a func-

tion of the signal frequency and no one simple answer can be provided.

The following common conditions require detailed consideration:

• “Earth” grounds: require extensive grids, ground rods, and chemi-

cal preparation to obtain an extremely low resistance and impedance

system ground return.

• Ground isolation: In many instances in low-frequency data systems

where equipment is widely separated, equipment ground “planes” should

be isolated from earth grounds to avoid “noisy” ground loops caused

by power and other equipment in the immediate area.

• Parallel cabling: Space isolate cabling of similar functions ( i.e., RF

from RF, video from video, and cables carrying vastly different voltage

levels) so that they do not have mutual capacitive or inductive cou-

pling.

• Termination impedance: Properly terminate all pulse and high-fre-

quency cables in their characteristic impedance so that the cable reac-

tive components are cancelled out and the voltage standing waves are

reduced to a minimum.

• Cable selection: The higher the frequency, the faster the pulse rise

time, or the longer the cable run; the bigger the cable required to reduce

dielectric losses and lessen the distortion of pulse shapes.

• Grounding location: If “system” ground and “earth” ground must

be connected, it should be done at minimal locations (preferably one)

using extremely low-impedance bonding paths and materials. Other-

wise, RF and high-frequency bonding should be made frequently to

provide the shortest RF path to ground and to prevent the ground return

from acting as an additional length of antenna.

Systems Usage

Most engineers are quite familiar with the longtime use of 75 ohm coax

cable in baseband telephone transmission installations and its univer-

sal use for broadcast and cable TV. Coax and twinax cables are being

used for local dedicated installations with the rapid growth of com-

mercial computer data distribution. The non-dedicated commercial

coax data bus systems (Ethernet) tie many terminals to one high bit-

rate trunk cable. Military aircraft systems call for 78 ohm twinax data

bus distribution for main functions of guidance and control, naviga-

tion, communications, etc., per MIL–STD–1553B (TRS and TRB se-

ries connectors). MIL–STD–1397 specifies 50 ohm triax in naval ship

digital data bus applications which use the TRB and TRC series con-

nectors listed in MIL–C–49142.

Interference is becoming a major problem with the proliferation of digital

computer installations, data transmission systems, and local area net-

works (both baseband and broadband). The main interference encoun-

tered is caused by the high harmonic content of the digital pulses that

fall in the RF region and radiate into free space or cross-talk to other

victim cables.

Choice of Cable Type and Installation:

• Grounded Coax: Can pass the high information rates, but is sub-

ject to ground loops as well as magnetic and radiated noise pickup.

• Ungrounded Coax: Can pass the high information rates, but is

subject to magnetic and radiated noise pickup. Substantially low-

ers the ground loop interference.

• Triax Cable: Can pass the high information rates, but is subject to

magnetic noise pickup. Substantially lowers the ground loop in-

terference and removes the radiated noise.

• Twinax Cable: Passes only medium information rates, but hin-

ders the pickup of magnetic interference due to the “twist” of the

signal pair. (Equipment usually operated with balanced inputs and

outputs.)

• Ungrounded versions of Twinax and Triax: Provide even more

ground loop isolation.

The choice of materials to be used in these cables is of major impor-

tance. For commercial installations in public buildings, the National

Electrical Code, Underwriters Lab and local ordinance requirements

must be complied with. Fire wicking cables routed horizontally

through fire walls and vertically floor-to-floor are required to be metal

ducted (a very expensive and inflexible mode of construction). Ap-

proved cables made of FEP can be routed horizontally and verti-

cally, without ducting, in air plenums with a great reduction in size

and cost and with much greater flexibility for future modifica-

tions. Increasing insurance premiums will almost dictate that flame

retardant nontoxic materials be used in all future public building

cable installations to reduce potential loss of life and property. The

same applies to military aircraft cables which must also be able to

withstand exposure to fuel and cleaning solvents. Connectors are

available for these special cables.

Trompeter Electronics, Inc. has developed coax connec-

tors that are isolated above ground, but have a 6 nanofarad bypass

capacitor, shunting the outer braid to ground for high harmonic at-

tenuation. In many instances, interference can be so severe that twinax

(balanced-shielded) or triax (shielded coax) cables must be used along

with the proper connectors to provide good isolation, bonding and

grounding.

As previously discussed, high bit-rate signals require high-frequency

transmission cables (coax, twinax, triax and even quadrax) to mini-

mize amplitude and frequency distortion and to prevent noise pickup

from external interference.

The design engineer must select the optimum cable and connectors

for maximum reduction of interference due to radiated RF and mag-

netically coupled or direct contact ground loop noise. This can re-

duce the number of noise suppression filters and amplifiers required.

A repeated word of caution is offered for those contemplating "digi-

tal" use. Low-voltage digital lines should not be placed in the near

proximity of high-voltage and high current cables. Nor should a

single multifunction connector be used due to the strong probability

of cross-talk coupled interference. Physical separation of the cables

is the first and best solution, with shielding and isolation above ground

being the second consideration. ■

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