tech stuff – Page 15 – 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.

Upgrading the firmware

The original V series Nautel transmitters have required a couple of firmware upgrades in some cases.

Upgrading the power module firmware, WDVT, Rutland, VT — Upgrading the PA module firmware on Nautel V-5D transmitter, WDVT, Rutland, VT

The first was for the controller to add a little bit of bias to the PAs during analog operation. The second one I have had to do is to the PA modules themselves which were to keep the power supplies from shutting off during re-transfer from Generator power to commercial power.

I have done several of these and once you get the hang of it, it only takes a few minutes to complete. Still, I remember when transmitters didn’t have firmware. The low voltage control circuits were either 120 or 240 VAC with big relays and contactors that loudly confirmed their closure before any meters began to move.

Regarding Nautel transmitters in general; the newer models are not the same rugged, reliable designs that were common in the past. We have AM ND series transmitters that have been on the air for 20 years without a single failure. The models rolling out of the factory these days often have switching power supplies fail without reason or warning and RF pallets that are fragile things. Ah well, I suppose all things are cyclical.

MPX over IP

In the progression from Circuit Switched Data to Packet Switched Data, I can think of many different applications for something like this:

The FMC01 MPX to IP encoder can be used for multi-point distribution (multi-frequency or same-frequency network) of FM Composite audio, or as a backup solution over a LAN bridge, LAN extension, or public network. I can think of several advantages of using this for a backup when composite analog STLs are in use. There are many compelling reasons to extend the LAN to the transmitter site these days; Transmitter control and monitoring, security cameras, office phone system extensions, internet access, backup audio, etc. I would think, any type of critical infrastructure (e.g. STL) over a wireless IP LAN extension should be over a licensed system. In the United States, the 3.6 GHz WLAN (802.11y) requires coordination and licensing, however, the way the rules are set up, the licensing process is greatly simplified over FCC Part 74 or 101 applications.

Another similar CODEC is the Sigmacom Broadcast EtherMPX.

Features include:
• Transparent Analog or Digital MPX (MPX over AES), or two discrete L/R channels (analog or AES).
• Built-in MPX SFN support with PTP sync (up to 6.000km in the basic version). No GPS receivers!
• Unicast or Multicast operation to feed an unlimited number of FM transmitters with MPX from one encoder.
• Linear uncompressed PCM 24-bit audio.
• Very low audio latency: 2,5mS in MPX mode.
• Perfect match with Sigmacom DDS-30 Exciter with Digital MPX input.
• Can be used with high-quality 802.11a/n Ethernet links.
• DC coupled, balanced Analog inputs & outputs with -130dBc noise floor.
• No modulation overshoots due to compression or AC capacitor coupling.
• Decoder provides simultaneous Analog & Digital output for transmitter redundancy.
• Aux RS232 serial transparent link, Studio to Transmitter.
• Auto switchover to Analog input when Digital signal is lost.
• Centralized remote control & management software

One last thought; separating the CODEC from the radio seems to be a good idea. It allows for greater flexibility and redundancy. Using an MPX-type STL allows sensitive air chain processing equipment to be installed at the studio instead of the transmitter site.

Transmitter Haiku

I wrote a little Haiku about Thanksgiving dinner:

Telephone rings
Old transmitter beckons
Dishes get cold

Not exactly the 5/7/5 of a traditional haiku, but close enough. This year, it was the nearly 30-year-old Broadcast Electronics FM35A at WEBE. A set of readings from the remote control reveal; zero forward power, zero plate current, and 12.8 KV plate voltage. My first assumption was some sort of drive issue; a failed exciter or IPA driver. After starting the backup transmitter and making sure that it was running stably, I spoke with the program director and told him we would be out the next morning.

Upon arriving at the transmitter site, I found the BE transmitter had no filament voltage. An obvious clue, I began working backward from the tube socket until I found this:

Broadcast Electronics FM35A filament voltage regulating transformer

This is the auto-transformer that regulates the filament voltage. Schematically, it is noted as T204 and it is in series with one side of the filament transformer. This one is burned open. The bad news; Broadcast Electronics does not stock this part, it is a special order item, the replacement part costs $2,800 dollars and it will take a few weeks to get here. The good news, after digging through our stock of old transmitter parts, I found an exact replacement:

Replacement part name plate, T204, BE FM35A — Replacement part nameplate, T204, BE FM35A

We will be installing it on Monday morning.