Triplett Low Voltage Pro - Manual (Page 22)

20
GLOSSARY
"OHMS" Cable impedance
The high-frequency nature of 21st century CATV and data signaling
requires cabling that preserves the integrity of the signal between trans-
mitter and receiver. This requires a good impedance match along the
transmission route. Standard Cat5, Cat5e and Cat6 data cables employ
twisted pair transmission lines, with a specified 100-ohm characteristic
impedance on each pair. Any deviation from this impedance, or imped-
ance "mismatch", along the transmission medium causes some portion
of the transmitted signal energy to be reflected back to the transmitter,
and resulting in distortion at the receiver. The LVPro impedance mea-
surement reports the near-end impedance of the attached cable - the
characteristic impedance as seen by the transmitter. ACCEPTABLE: +/-
10% deviation from a cable's ohms rating.
"RLQ"
RLQ is a measurement of the impedance match along the entire length
of cable - including the receive-end termination - without regard for the
specific cable impedance. That is, RLQ is a representation of the "flat-
ness" of the impedance along the entire transmission line. A higher RLQ
number indicates a good match, and a lower number indicates a poorer
match. For example, a 100-ohm data cable with a good quality 100-ohm
Cat5e transmission cable and a solid 100-ohm termination at the receive
end will report an RLQ of 2. The same system with an unterminated
cable will report an RLQ of 0. Quality issues can arise in a transmis-
sion cable (Cat5/6 or coax) for a variety of reasons, including physical
damage caused by crushing or bending the cable too sharply, improper
termination, moisture intrusion, etc. See "OHMS" above for the problems
impedance mismatches can cause. ACCEPTABLE: Ratings of 1 and 2.
"DELAY" Propagation Delay
Propagation delay is the travel time for a signal applied at the transmit
end of a transmission line or system to arrive at the receive end. For an
individual cable, propagation delay is determined mainly by the velocity
of propagation (VOP), which is a characteristic value of the particular
transmission medium. VOP is typically represented as a percentage of
the speed of light in a vacuum (nominally 300 x 106 m/s). E.g., a VOP
of 66% indicates a propagation velocity in that transmission line of 66%
of the speed of light, or 198x106 m/s, or approximately 5nS per meter.
ACCEPTABLE: 550ns is the maximum allowed delay.
"SKEW"
On a multi-pair transmission line, skew is the difference in signal
propagation delay between pairs. It is caused mainly by differences in
the physical lengths of the cable pairs. This physical difference, in turn,
results from the fact that the pairs are twisted at different twist rates
(number of twists per foot of cable) to minimize crosstalk between pairs.
Skew is important because modern data transmission systems transmit
symbol data coincidentally on multiple pairs, and expect to receive that
data more or less coincidentally at the receiver. While data systems can
accommodate some amount of skew, if the skew is too large, it results in
unreliable data transfer. The Cat5e standard specifies a maximum skew
of 55ns over the length of the transmission cable. This indicates that
worst case difference that can be tolerated between the shortest (fastest)
pair and the longest (slowest) pair.
ACCEPTABLE: 50ns is the maximum allowed skew.