Skjønner ikke at noen faktisk ikke ønsker å måle kablene sine for å finne ut sammenheng med lyd og egenskaper på kablene.
Koster jo hele svimlende 600,- for et slikt apparat:
Men det er vel mer "nyttig" for de fleste å heller snakke om merker og følelser istedet for å finne ut hva virkeligheten faktisk innebærer.
Om en kabel er god, sjekk kapasitans og induktans. Om man finner en annen kabel med samme verdier, ta en blindtest imellom de to og se om det har en sammenheng. Kanskje man lærer noe nytt av det også, men det er vel heller ikke av interesse for de fleste troende.
Er egentlig ikke veldig opptatt av spec på kabel, men lest litt av nysgjerrighet. Her fra en normal kabel leverandør.
Impedance, Velocity of Propagation, Attenuation, Rise Time Degradation
These four parameters have a less direct bearing on data communications applications. Sometimes they are specified, but after the basic cable dimensions are given, these param- eters essentially depend on the Capacitance and DC Resis- tance.
The various lumped circuit element parameters; capaci- tance, inductance, resistance, and conductance of a trans- mission line, are all interrelated in a single parameter known as the characteristic impedance of a transmission line. This is the impedance that if used to terminate a transmission line, no signal will be reflected back to the source. If there is a mismatch, the bigger it is, the bigger the reflections will be. Impedance matched conditions are sought for all system de- signs, particularly at high data rates, because the reflections affect the performance of the generators and prevent some of the signal from ever reaching the receiver. The generator and cable termination load of EIA-422-A and EIA-485 are specified in such a way as to closely match the impedance of typical “low capacitance” cables having about 12 to 16 pF/ft mutual capacitance.
Velocity of propagation, the speed at which a signal (an elec- tromagnetic wave) will travel along a cable (a transmission line) is dependent on the properties of the insulation. The di- electric constant of a plastic is actually the inverse of the square root of the velocity of propagation; the speed that electromagnetic radiation will travel through a dielectric com- pared to the speed of light in a vacuum. The velocity of propagation is normally expressed as a fraction of the speed of light. The actual velocity of propagation is complicated somewhat by the fact that the signal normally travels through a combination of air and plastic, but the result is to make it a little faster than the theoretic speed derived from the insula- tion dielectric constant alone. The velocity of propagation de- termines the impedance relative to the capacitance.
The impedance and resistance determine the attenuation vs frequency. This parameter is normally expressed in dB/1000 ft at a given frequency. This is a measure of the amount of signal loss that occurs from the cable. More signal is lost at higher frequencies than at low frequencies. Re- member that the rise time of the pulse, not the data rate, de- termines the frequency range covered by the signal.
Since there is more attenuation at higher frequencies than at lower frequencies, signal pulses are dispersed as they travel down the cable. This property is measured as rise time deg- radation. Rise time degradation is roughly proportional to cable length. System designers are constantly balancing rise time effects. On one hand, fast rise times produce more crosstalk, that will, if great enough, result in errors. On the other hand, slow rise times that get further degraded will cause receiver errors.
5.0 CABLE APPLICATIONS
Lowering capacitance improves the performance of cables used for both unbalanced and balanced transmission. Unbalanced transmission is limited by near-end-crosstalk. The unbalanced lines interfere with each other primarily through capacitive coupling between the lines. Lowering any capacitance parameter of a multi-conductor or twisted pair cable will result in proportionally lowering all of the various capacitances within the cable. In the case of unbalanced lines, coupling capacitance, and therefore crosstalk, is low- ered proportionally. The mutual capacitance of a paired cable used for unbalanced transmission does not directly in- dicate the coupling capacitance between lines, but compar- ing the mutual capacitance of two cables is generally a good indication of their relative crosstalk performance.
Balanced transmission is primarily limited by rise time degra- dation. The primary cable capacitance of concern, in this case, is the shunt capacitance across the signal generator’s two output terminals. Keeping everything else equal, lower- ing capacitance results in a decrease in attenuation vs fre- quency proportional to the square root of the capacitance re- duction. In the general domain of data rate and distance for current applications, the resulting change in rise time degra- dation is nearly proportional to the square of the change in attenuation vs frequency. This gives a relation, similar to the unbalanced case, where a comparison of the mutual capaci- tance of two cables is generally a good indication of their re- spective proportional rise time performances.
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