PL75-9 datasheet(84/141 Pages) ETC

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Electronic systems wiring & cable

The 93 ohm cable was developed to address the need for a low-ca-

pacitance instrumentation coax cable. By removing some of the coax

dielectric and substituting air in the RG59 cable, the distributed ca-

pacitance was lowered, thereby creating a lower voltage loss transmis-

sion medium (RG62 cable).

More sophisticated cables and connectors of twinax, triax and quadrax

are now available. These newer components improve external noise

rejection and containment of classified signals.

Signal Degradation

Signal degradation in any transmission medium usually consists of

voltage amplitude reductions, wave shape changes, phase or delay

changes, or power losses where power is transmitted. In most systems

the interconnecting cable is the longest transmission path and its se-

lection, manufacture, testing and installation should be carefully con-

sidered. Cable should not be randomly selected and installed.

When selecting cable from specifications, always consider the length

of the cable run, heat exposure, frequency and power to be transmitted

vs. the acceptable losses inherent in the cable, the external noise fields

and frequencies to be anticipated or encountered. Also consider the

availability of connectors to terminate the cable. Too small a cable

will always be cause for excessive losses. Fast rise time digital pulses

will have the leading edge distorted due to the high resistance “skin”

effect of small coax cables. When selecting a cable for a long run,

observe the insertion loss to assure that your signal gets to its destina-

tion without too much loss.

When in doubt select a larger cable! Incomplete copper coverage in

the outer braid over the dielectric will cause transmission line losses

and affect the cable's susceptibility to signal leakage or noise pickup.

A copper braid coverage 90% is preferred. Skimping on the quality of

the copper braid will affect the cable's loop resistance and its "line"

and "transfer" impedance. As a good rule of thumb, if the dielectric is

visible through the braid (without bending the cable) then the cable

should not be used.

Unseen manufacturing faults produce signal path impedance changes

or discontinuity which can only be detected by “frequency sweeping”

the cable. TV broadcasters have encountered as high as 60 dB losses

in short runs due to periodicity and other manufacturing faults that are

only detectable by swept frequency testing techniques. RG59, RG62,

and other coax cables use copper weld wire for the center conductor.

Copperweld is a high resistance steel wire with a copper cladding on

the outside and was originally intended to give strength to TV cables

when suspended from poles or pulled through ducts. However this

steel wire will increase the cable attenuation on a long run (particu-

larly at the lower frequencies) due to the high resistance of the copper

clad steel. RG59 and RG62 have approximately 44 ohms per 1000

loop feet as compared to 17 ohms when using pure copper for the

same size center conductor. It is also difficult to effectively crimp a

center contact pin on the hard steel copperweld wire.

After buying the most expensive “end” equipment, many systems

engineers have difficulty in determining the best methods of cabling

and interconnecting for the routing of signals with minimal loss,

degradation and noise pickup. Simple solutions such as selecting the

correct cable, eliminating common mode grounds or physically sepa-

rating long runs of parallel cables will help make the difference be-

tween a good or bad system. It must be emphasized that these good

practices must be implemented at the time of initial design and are

almost impossible to implement after the system is built. This “inter-

face” engineering will be discussed in following sections titled "Sig-

nal Degradation" and "Noise".

The systems engineer should study his system parameters and noise

environment before selecting his cabling and connector components.

Consideration must be given to:

•

Signal frequencies

•Voltage and power levels

•Tolerable losses and degradation

•

Reflected signals due to discontinuity

•

Noise from direct contact common mode and ground loop

returns, or radiated stray magnetic and capacitive fields.

Circuit wiring and cabling are susceptible to the pickup of noise, and

all low voltage signal and data wiring should be shielded irrespective

of the frequency to be transmitted. Coax cable (primarily designed to

carry RF) is excellent for digital data transmission. It is relatively

inexpensive (RG58, RG122, etc.) and many complete series of coax

connectors are readily available.

High speed and broadband digital data must use RF (radio frequency)

coax transmission cable since the fast rise time narrow pulse has har-

monics reaching into the RF region. For higher frequencies, coax

cable must be used for point-to-point wiring since it has the trans-

mission characteristics, flexibility and economy necessary for most

systems.

Impedance Standards of Coax Cables

The most efficient impedance to use when transmitting a signal (con-

sidering only the voltages, currents and powers to be transmitted) is

75 ohm. The telephone industry, followed by the TV industry, uses

75 ohm almost exclusively for the transmission of voice, video, and

data.

The military services were faced with a differing need for low radia-

tion angle omnidirectional antennas during the period 1920 through

1940 for broadcasting ship to ship, airport tower to low flying air-

craft, and base station to ground troops. The only antenna that would

fill that need was the vertical ground plane antenna (in its many

forms) with transmissions at 50 ohm. The military standardized on

50 ohm impedance and spent vast sums of money developing cables

and connectors for all of their coax systems. The current general

connector standard is MIL–C–39012 (primarily for 50 ohm usage).

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