PL75-9 datasheet(82/141 Pages) ETC

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signal transmission in

high frequency/high density applications

Impedance

An inherent characteristic for transmission lines is impedance,

which is measured in ohms. Impedance is the ratio of voltage to

current in a traveling wave. For a "loss-less" cable, it can be

defined in terms of the ratio of inductance to capacitance. In a

coaxial construction, it can be alternatively defined as a function

of geometry and material properties of the dielectric material

between the concentric conductors. This equation can be stated

as :

character impedance

ε =

dielectric constant of the insulation material

D =

inside diameter of the outer conductor

outside diameter of the inner conductor

Physically, impedance can be thought of as the value of resis-

tance that, when connected between the center conductor and

shield at the end of the cable, makes the cable appear infinitely

long. The most efficient transmission of electrical power, hence

maximum signal transfer, occurs when the distributed compo-

nent impedance matches that of the system.

Impedance mismatch creates a reflected wave of energy, which

can be measured and stated in different forms. By defining a

reflection coefficient (

ρ), which is the reflected voltage divided

by incident voltage, the measure of reflection losses can be

stated as:

Reflection Loss

(dB) = 10 Log

10 (1 / 1-ρ

2) dB

Voltage Standing Wave Ratio

(VSWR) = (1+ρ) / (1-ρ)

Return loss is the decibel loss due to reflection, and VSWR is a

unitless ratio relating incident voltage to reflected voltage.

These commonly used terms that quantify the loss associated

with reflection are two scales for the same characteristic. They

can be equated as follows:

Return Loss (dB) = 20 Log

10 {(VSWR + 1) /

(VSWR -1)}

Reflection losses are a function of frequency - increasing with

increasing frequency - and impedance mismatch. It is helpful to

note that VSWR is the ratio of the maximum standing wave

voltage to the minimum standing wave voltage. VSWR, there-

fore, is infinite for total reflection (i.e. 0 minimum voltage) and

is 1.0 for perfect transmission (no reflection). The closer to

unity, the better the impedance match and the more efficient the

transmission system.

Three Examples

Figure 1 (on page 81) shows comparison return loss data from

1 MHz through 2 GHz for the following three scenarios:

1. 75

Ω system (includes cable and test set); 50Ω BNC

connector;

2. 75

Ω system; approximate 75Ω BNC connector; and

3. 75

Ω system; "true" 75Ω BNC connector.

From this, it is clear that the return loss degrades as a function

of the frequency and also as a function of the extent of the

impedance mismatch between the connector and cable. At

higher frequencies the impedance match becomes more critical.

Other factors influence the reflection loss as well.

These

include: the homogeneity of a composite dielectric material; the

even dispersion of air within and around a dielectric; and the

"smoothing" of the mechanical transmissions between the im-

pedance mismatches within the connector. This includes the

tandem increase or decrease in sizing of the diameters for the

inner and outer conductor, as well as the effect of chamfers/

breaks at sharp transitions.

Other Considerations

Aside from the electrical and electromagnetic performance

issues, connectors that disconnect should be capable of tough-

ness, durability and robust in-field use with high counts of repeat

mating cycles without failure or interruption. Other nonelectri-

cal performance measures of importance are insertion force and

retention force, particularly after repeated matings.

Conclusion

Market Trends,

Technical Considerations and Practical Examples for

Shielded Disconnectable Interconnects

Market needs are impacting electrical and electromagnetic de-

mands on connector performance. A basic understanding of the

physics involved in electrical circuit design and transition lines

is necessary to appreciate the impact of losses associated with

the connector. This understanding includes resistance, capaci-

tance, inductance, efficiency, reflection losses associated with

impedance mismatch, and dielectric losses associated with the

materials used for connector dielectrics.

Impedance mismatch between the source, cable and connector

is less significant at lower than at higher frequencies. However,

as system content and speed increases, the permissible fault

tolerances goes down dramatically. In other words, one impact

of high technology is greater dependance on tighter tolerance

quality and part-to-part repeatability. These discoveries have

caused shifts in sourcing, as buying companies mature in their

appreciation for robust design engineering and fabrication ex-

cellence. The connector industry is not immune from this

development. ■

138

√ε

(Log D/d)

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