The Mismatch Patch
Question from the Aug. 22 issue
(Exam level: CBNE)
A cabling impedance mismatch can be caused by which of the following?
a. Using RJ-45 connectors on Cat-6 cable
b. Running cable in an overhead cable tray near electrical conductors
c. Nicking cable conductors when stripping
d. Mixing shielded and unshielded twisted pair cable in the same segment
e. Not following ITE/EIA/IEEE standard VUFB-101 regards wire twist format (i.e. left overhand)
Note, this question was incorrectly listed as a CBNT level question in the last issue of Engineering Extra. The correct level for this question should be for Certified Broadcast Networking Engineer, or CBNE.
SBE certification is the mark of professionalism in broadcast
engineering. To help you get in the exam frame of mind, Radio World Engineering
Extra poses typical questions in this column. Although similar in style and
content to the exam questions, these are not from past exams nor will they be
on future exams in this exact form. Today’s question is at right.
Normally our questions arise from the combined genius of your
erudite RW Engineering Extra editor and your humble author, mirroring readers’
requests to sojourn into certification topic areas that interest or perplex
Tools of the trade.
This month is unique in that we asked the SBE certification
folks for a typical CBNE question in honor of
this new level of engineering certification. The Certified Broadcast
Networking Engineer level exam focuses on computer networking skills, now an
essential component of broadcast engineering.
At first blush I chose one of the wrong answers, (c).
Obviously a nick in one of the lines is not good and jeopardizes the
reliability of the cable and connection but it is not the most correct answer.
CLASSIC BUC BOGUS
Let’s move expeditiously through the other wrong answers.
As anyone who has installed a computer to a network knows,
the RJ-45 connector mentioned in answer (a) is the most ordinary eight-pin
connector used on Cat-6 cable and thus should not be source of impedance
mismatch. Technically, a poor installation of the RJ-45 could cause this
problem but that isn’t what was written.
Data cable is optimized for common mode rejection; if
properly made up and connected, it is just about impervious to induction fields
caused by nearby electrical cables. The fields might mess with the data, but I
cannot see any connection to an impedance mismatch. Endless miles of this cable
coexist in cable trays throughout the world so (b) is not our answer.
Although it reads with the patina of technical veracity,
answer (e) is a classic Buc Bogus answer. “VUFB” is an inside joke amongst my
confreres, an acronym for Victor’s Ultra-luxurious Flat Black
spray paint, which we use liberally around here to cover rust and wear
This makes our most correct answer (d), mixing shielded and
unshielded twisted pair cable in the same segment.
By way of reminder, actual SBE exams usually have only four
choices. We use five to allow an enlarged discussion and makes it a little more
challenging (as well as allowing us an opportunity for a little humor on
A HISTORY OF REACTANCE
Our devoted RWEE readers know that a large part of our work
is researching technical points for our articles, sometimes very fine ones. In
doing that, I often am amazed at how early in history certain aspects of our
For a moment, let’s look back to circa 1880 and Alexander
Graham Bell’s seminal work on the telephone.
Voice information is transferred to a varying current and
then reconverted to voice again at the end of the circuit. Not exactly digital
data, but nevertheless the effort to send this information a great distance
brought forward the problem of reactance on the lines. The voice varying
current, even though very low frequency, behaved over long distances as
alternating current and reactance began to affect the signal.
Shunt capacitance is a major attenuator of varying current,
and one early solution was to cancel that with peaking coils. If there is
enough interest, we can return to this telco phenomenon in a future column, but
we mention this now to point out that when conveying audio or digital
information over long distances, reactance must be dealt with.
Cable reactance sets the ultimate bandwidth (highest
frequency of data possible and the maximum distances this bandwidth can be
maintained). As capacitive reactance increases, it will tend to shunt
high-frequency signals between conductors or to ground. This ultimately limits
the highest frequency data pulse that can be transmitted. Capacitance between
conductors also creates what is known as crosstalk, wherein data pulses moving
in opposite directions mix together and become unusable due to corruption.
Category cables (e.g., Category 5 or 6) are designed to minimize these effects.
In this day and age of advanced electronics, it is
interesting that the simple mechanical structure of category wiring is its
power. By running digital signals on individual, balanced transmit and receive
pairs, the cable connects easily between different areas in a building without
creating ground loops. And by the simple addition of twisting the paired wires,
category cables reduce the potential for inductive coupling or crosstalk. The
more twists, the better the crosstalk performance.
Category 5e wire stripped to show
4 individual pairs with tight twist.
Current wiring standards call for a minimum of Cat-5e or
Cat-6 cabling. Each of these is capable of passing data at a rate of 1 gigabit
per second when installed properly.
Cat-6 cabling has a nominal design impedance of 100 ohms.
Mixing cables of different impedances will cause a mismatch and increase return
loss (essentially data VSWR). This, in turn, will create a higher bit error
rate (BER), which will result in endless data re-sends, lower throughput and a
tendency for networked systems to crash.
Data cabling should be viewed as a channel, a conduit of
information. In the parlance of the IEEE, the organization that formalizes the
standards for these systems, what we have is a “link segment” — the physical
components of the electrical data connection between local and the remote
equipment. Those components include the main cable runs themselves, compatible
jumper/connection cables, and connectors.
Today’s radio stations have extensive data cable systems,
quite often totaling more wire than the analog connections. The important work
of installation and maintenance of these data wiring systems is part of the job
description of station and contract engineers, so knowledge in this area is
important. Once again, recognition of that knowledge is most often indicated in
our industry by the CBNT and the new CBNE SBE certification.
Among many Web resources addressing
Ethernet/data cabling, an easy comprehensive backgrounder on cable installs is
a white paper
from Wheatstone, “The Role of Wiring Integrity …” available at audioartsengineering.com. Look under Support, then All Downloads.
The next SBE certification exams will be given in the local
SBE chapters in February 2013. Closing date for signing up for the exams is
Dec. 31, 2012. If you are interested and ready to take the exams, we strongly
suggest that you sign up soon, as the next following exams are scheduled April
9, 2013 at NAB.
SBE certification is a peer process. The 14-person committee
is always open to question suggestions as well as constructive feedback on exam
questions you have encountered.
Certified already? Remember that nine-tenths of education is
encouragement, so motivate your own confreres to become certified or advance a
Charles “Buc” Fitch, P.E., CPBE, AMD, is a frequent
contributor to Radio World.
Missed some Certification Corners or
want to review them for your next exam? Find past Certification Corner articles
under the Columns tab at radioworld.com.
A Question of Peak Performance
Question for next time
(Exam level: CRBE)
What is the mathematical relationship of the peak voltage and the RMS voltage value of a uniform triangular waveform?
a. The peak value and the RMS value in volts are numerically
where n is the number of cycles considered (normally based on a nominal 1
second or the waveform frequency) and k is Boozeman’s constant (after Victor
Boozeman) normally rounded to 1.57 or π/2
b. The peak value is the sine value and the RMS value is the
cosine value of the square root of 12 times Vp to p in volts
c. The RMS voltage value is the peak voltage value divided by
the square root of 3
e. RMSE = Vp times
the square root t divided by T where T is the duration of the waveform
(nominally ¼ of the cycle) and T is the total count of cycles or the frequency
of the waveform