Paul Thurst – Page 44 – Engineering Radio

Shielded Category Cable

There is some disagreement in the organization that I work with regarding the use of Shielded Cat 5e cable. Is it needed and if so, when and where? Category cables commonly used in Ethernet computer networks and also used for analog audio and other data applications come in a variety of flavors. Shielded (Shielded Twisted Pair or STP) and unshielded (Unshielded Twisted Pair or UTP) Cat 5, 5e, and 6 are the most common in radio broadcast facilities.

The main purpose for using UTP and STP for high-speed data transmission is common-mode rejection. Cables that are installed in office buildings are subject to various electric and electronic noise sources. Properly installed UTP works to reject these unwanted signals by using differential signaling, which is balanced. Differential signaling can best be described as transmitting information using two complementary signals that are opposite from one another.

Noise rejection, differential signaling. "DiffSignaling" by Linear77 - Own work. Licensed under CC BY 3.0 via Wikimedia — Noise rejection, differential signaling. “DiffSignaling” by Linear77 – Own work. Licensed under CC BY 3.0 via Wikimedia

The key performance measurement in category cable is Common Mode rejection. Outside noise will introduce a common mode signal on the cable which will be canceled out by the differential amplifier on the receiving end of the circuit. Proper terminations and good wiring techniques are very important for proper performance.

Using the correct patch panel termination, terminating block or RJ-45 (8P8C) connectors are required to maintain the advertised bandwidth of the cable. There is also a difference in the connector and terminating block designs for solid versus stranded cables. Using improper connectors for the type of cable installed can cause dropouts and loss of data.

When installing category cable, care must be taken not to kink the cable, not to exceed the recommended minimum bending radius, or exceed the maximum pulling force. Each of these will degrade the cable performance by changing the physical characteristics of the cable. Each pair of wires in category cable has a different twist. Altering these twist ratios by stretching the cable or bending it too sharply will increase the NEXT (Near End Cross Talk) and FEXT (far end cross talk) between pairs. In Gigabit networks, this will degrade throughput and create bottlenecks.

Generally speaking, the minimum bending radius is four times the cable diameter, or approximately one inch for Category 6 cable. The maximum pulling tension is not more than 25 ft/lbs or 110 Newtons.

In high EMF environments, shielded cable (STP) can be beneficial in mitigating high electrical noise along with the proper installation techniques noted above. Signaling levels on 100BaseT are +1, 0, and -1 volt (MLT-3 Encoding). On Gigabit Ethernet, the levels are +1, +0.5, 0, −0.5, and −1 Volt (PAM-5 Encoding). Induced voltages on cables from external sources can degrade network performance and create bottlenecks. High EMF environments would include places like transmitter sites and anything on a tower or rooftop. Properly terminated shielded cable is necessary for EMP protection from lightning strikes or other sources. STP has special shielded metal connectors which each category cable class. These connectors supply the path to the ground through the RJ-45 jack.

Ungrounded shields are useless.

RJ-45 or 8P8C shielded plug for Category 6 STP

There are also other cable characteristics to consider such as UV-resistant jacking for outdoor installations or gel-filled (AKA “flooded”) cable for wet locations. Fortunately, there are plenty of sources for these types of cables and they are not terribly expensive.

To answer the question at the beginning of the post; STP can be beneficial at high EMI/EMF or RF sites to mitigate induced voltages on the cable from external sources provided it is properly terminated. In office and studio locations that are not at or next to a transmitter site, UTP is more than adequate provided it is properly installed and terminated.

Be careful where you put your hands!

So, I was working at one of our FM clients in Albany when I decided I had a few moments of spare time, so I could neaten up the remote control rack. I opened the rack door and was staring intently at the remote control interface panel when out of the corner of my eye, I saw something move.

Now, the top of the rack is a little bit dark and I was not sure what I was looking at. At first, I thought somebody had stuffed a rag in the top of the rack. But, I could not figure out why anyone would do such a thing. Then I thought it was some cardboard. I almost reached up and grabbed it, but something was amiss. Then I saw the tough flick out and smell the air:

At this point, I think I may have said something like “Oh, shit!” and took several steps back. Those colors and patterns have two possibilities; Copperhead or Grey ratsnake. Since I could not really get a good look at its head, I could not tell which it was. I went and got a work light to see better with.

A copperhead is a pit viper, which has a triangular-shaped head and a small indentation or pit under each eye. This snake has neither, so it is fairly harmless. Actually, the rat snakes are beneficial because they eat the mice and other pests around the transmitter building. There are several versions of these in the northeast, including a black rat snake which happens to look just like a piece of 7/8 coax laying across the pathway to the door, until it moves that is…

This species can get to be about 6 feet long (1.8 meters) and the larger ones can draw blood when they bite. Even though he looked to be on the small side (approximately 30 inches or 76 cm), I decided that discretion is the better part of valor, closed the door on the rack, and did something else for a while.

The Sportable 3306LED02 Baseball Scoreboard

This post has nothing to do with radio engineering but is full of geeky goodness, nonetheless. My son is playing Little League again this year. This is his first year in the majors division, and I have to say, I have been thoroughly enjoying watching his games. There is, of course, one minor glitch in the matrix; the scoreboard, which occasionally looks like this:

Little League Scoreboard, missing LED segments.

Now, that is more of an annoyance than anything else. I know what inning it is and what the score is. Truth be told, most of the time the scoreboard is being run by one of the parents (read: a mom) and they can become distracted at times. Very often, the ball/strike/out count is not correct, which in turn causes the home plate umpire to angrily stare up and the scorekeeper’s window.

Anyway…

As I was saying, more of an annoyance…

Regardless, I thought to myself; jeez, I fix things, perhaps I should have a go at that sign. So I spoke to one of the Little League board members who were more than grateful for any assistance I could render.

Thus, one afternoon, after work, I got the ladder out and started poking around to see what I could learn. These signs are relatively simple. Each digit on the sign has one circuit board. Each circuit board has seven segments. Each segment has fourteen LEDs in series. There is a Toshiba ULN2803APG, which is a 16-pin darlington driver, and an LM 317 voltage regulator which is fixed with a 62-ohm resistor.

Scoreboard single digit circuit board — Scoreboard single-digit circuit board

Approximate schematic scoreboard circuit board segment

After poking around with the DVM for a while, I determined that the bad segments were due to open LEDs. I measured the working LEDs and determined that each LED was dropping about 1.7 volts. I took a board home with me and rummaged around in the parts bin until I found some orange 5MM LEDs that matched the voltage drop of the ones on the board. I confirmed my ladder-top troubleshooting findings on the workbench using the DVM in diode mode. I also noticed that the Fluke DVM had enough current to light the LED, thus making troubleshooting much easier. There were three bad circuit boards with various segments out.

A few minutes with the soldering iron and presto:

Sign repaired. I little further research and I found that an Everlight MV8104 LED (Mouser part number 638-MV8104) is a near-perfect replacement. Literally, a 23.3 cent (US) part.

In all fairness to the company that makes the scoreboard, this unit was new in 2003 or 2004. It has spent at least 11 years outside in upstate NY, which is not a tender climate. They will replace the digit circuit boards for 175.00 each, plus $25.00 shipping. My repair work used 9 LEDs ($2.10) plus about two hours of troubleshooting and repairing vs. $600.00 plus perhaps an hour to replace the boards.

Meltdown

After one of our clients had an FM station go off the air over the weekend, I investigated and found this:

Looks like something one might find in the reactor room at Chernobyl or Fukushima.

This is at one of those sites with three phase open delta power. Needless to say, the transformer is toast, perhaps the entire transmitter too. This will be another fun transmitter-scrapping project. I was thinking about this; over the last five years, I have scrapped at least ten to fifteen old tube transmitters. The old tube types are going away fast, as are those that can still work on them.