由 kingcable 發表於 週六 7月 17, 2004 11:20 pm
crosstalk----Star Quad結構就是將串音解決之最好方式.所以是更好.而不是嚴重.
參考一下
Crosstalk
Cables with twisted pairs are used where balanced signal transmission is required, together
with low crosstalk. Crosstalk refers to interference on one channel caused by a signal in
another channel and occurs between cable pairs carrying separate signals. This definition
embraces both near end crosstalk and far end crosstalk. Near end crosstalk is the crosstalk
which is propagated in a disturbed channel in the opposite direction to the current in the
disturbing channel, and far end crosstalk is propagated in a disturbed channel in the same
direction as the current in the disturbing channel.
Crosstalk is fundamentally the resultant of two factors, or couplings: capacitance
unbalance and mutual inductance. Capacitance unbalance gives rise to electrostatically
induced potential differences between the wires of the disturbed pair. This coupling is
important at all frequencies. When the capacitance of one conductor against the sheath and
all the other conductors is not the same as the capacitance of its mate (similarly measured)
the pair is said to have a capacitance unbalance, which is measured by the difference
between these two capacitances. When the capacitances are equal, the pair is said to be
balanced. It is generally desirable to keep the capacitance values as low as economically
practicable by using low permittivity insulations.
Mutual inductance gives rise to electromagnetically induced potentials along the wires of
the disturbed pair. This coupling exerts little influence at audio frequencies but becomes
increasingly important as the frequency increases.
Both couplings are due to departures from balance between the circuits concerned. In
design and manufacture, attempts are made to ensure that the distances between the wires of
one pair and the wires of another are equal, as well as the distances between each wire and
earth. This is done by careful choice and control of materials, insulation thicknesses and
twinning lays and tension, but unavoidably during manufacture small differences occur
which result in residual values of the couplings.
For unscreened pair cables at audio frequencies(本站不支援羅馬字.有些符號有誤.看看就好)
crosstalk = 20 log [8/(ωκ|Z|)] (dB/length)
where
w = 2πf
κ = capacitance unbalance between the two pairs for the relevant length(farad)
|Z| = modulus of the characteristic impedance of the pairs at frequency f Hz
Since capacitance unbalance is an electrostatic coupling, it can be reduced by enclosing either
or both of the interacting pairs in a screen. The screen need only be thin, and laminated foil
screens are generally used.
At higher frequencies, mutual inductance becomes important and differential twinning lays
in adjacent pairs are used to achieve the best results.
Design features
A variety of twisted pair designs is used. Conductors are generally in the range 18-24 AWG
solid or stranded, and tinned for ease of termination. PVC insulations are used for short
distances, and low speed data, but for higher speeds and longer distances polyethylene and
polypropylene in solid or cellular form are used. Individual pair screens to reduce crosstalk
are generally laminated foils and incorporate a drain wire to achieve satisfactory electrical
connections. Overall screens are used as protection from oufside interference and can be
laminated foils with or without an overall braid.
Overall sheaths are generally PVC formulated to suit the particular application, including
high and low service temperatures, and to impart flame retardance.
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