Most of these standards specifically identify cable constructions and electrical performance necessary to meet the system data speeds and transmission distances. If a certain cable design is specified by a system or standard, it is done so with a reason: any cable with less than the specified performance criteria will degrade the system or not transmit at the specified maximum distance.

Impedance (Ohms) represents the total resistance that the cable presents to the electrical current passing through it. At low frequencies the impedance is largely a function of the conductor size, but at high frequencies, conductor size, insulation material and insulation thickness all affect the cable's impedance. Matching impedance is very important. If the system is designed to be 100 Ohms, then the cable should match that impedance, otherwise error-producing reflections are created at the impedance mismatch (See related articles in this section on Return Loss ).

Attenuation is a ratio comparing power input to output. It is measured in decibels per unit length (db/ft), and provides an indication of the signal loss through the cable. Attenuation is very dependent on signal frequency. A cable that works very well with low frequency data may do very poorly at higher data rates. Cables with lower attenuation are better.

Shielding is normally specified as a cable construction detail. For example, the cable may be unshielded, contain shielded pairs, have an overall aluminum/mylar tape and drain wire or even a double shield. Cable shields usually have two functions: the first to act as a barrier to keep external signal from getting in and internal signals from getting out and the second to be a part of the electrical circuit. Shielding effectiveness is very complex to measure and depends on the data frequency within the cable and the precise shield design. A shield may be very effective in one frequency range, but a different frequency may require a completely different design. System designers often test complete cable assemblies or interconnected hardware for shielding effectiveness to prove their system complies with FCC electromagnetic emission requirements.

Capacitance in cable is usually measured as picofarads per foot (pf/ft). It indicates how much charge the cable can store within itself. If a voltage signal is being transmitted by a twisted pair or coaxial cable, the insulation on the individual wires becomes charged by the voltage within the circuit. Since it takes a certain amount of time for the cable to reach its charged level, this slows down and interferes with the signal being transmitted. Digital data pulses are a string of voltage variations that can be represented by square waves with near-vertical rise and fall transitions. A cable with a high capacitance slows down these voltage transitions so that they come out of the cable looking more like "saw-teeth", rather than square waves, and the circuitry may not recognize the pulse. The lower the capacitance of the cable, the better it performs at higher frequencies.

• Increasing the insulation wall thickness
• Decreasing the conductor diameter
• Using an insulation with a lower dielectric constant.

The size of the conductor is usually determined by the electrical requirements of the circuit that the cable interconnects. If the circuit has been designed to require a 22 AWG wire, you cannot reduce it to 28 AWG just to reduce the capacitance. Also, the insulation wall thickness cannot be increased beyond reason since this increases the diameter of the cable, increasing costs and affecting terminations. Thus, the insulation chosen for the cable often becomes the critical variable.

All dielectric constants are compared to air or vacuum, which is given a value of 1.0. A poor quality PVC insulation may have a dielectric constant of 5.0 to 6.0 or higher. Polyethylene has a much better dielectric constant of approximately 2.0. Foamed polypropylene or polyethylene insulations have constants as low as 1.6.

By balancing conductor size, insulation material and insulation wall thickness, the cable designer can produce electronic cables that are tailor made to transmit high frequency digital data pulses over a maximum distance. The signal output from these cables maintains the required wave form definition and minimizes possible data errors.

Today Quabbin Wire & Cable Co., Inc. manufactures and stocks a family of low cap cables, from 2 pair thru 18 1/2 pair constructions. These cables meet most system requirements that require true low capacitance. They utilize polyethylene insulation with a flexible PVC jacket that provides an ease of termination and installation unmatched in competitive products. These cables do not use band-marked color coding because many customers find this system very difficult to read. Instead, Quabbin Wire provides an extruded, integral color stripe to identify leads. This stripe is impossible to wipe off and is much easier to see. Part number 8508 listed below is a 4-pair member of that family.

To compare the dimensions, insulation systems, capacitance and other electrical characteristics of several cables, refer to the following cables found in Quabbin Wire's catalog and on this web site.