Russell Brown /
12.07.2008 12:15 PM
Converting to digital II

Note: This tutorial continues where Converting to digital left off in covering KMTP-TV and its plans for conversion to digital broadcasting.

By running two transmitters at once, each supplying half the power, a station can be sure that no single transmitter failure will take it off the air. But this scenario does not work for every station, because it raises the electric bill, and if the required output power is too low, the inductive output tube (IOT) transmitters will not run efficiently and raise the electric bill even more than it would otherwise. An alternative is to run one transmitter at a time and switch to the standby when needed, thus reducing the power bill. (See Figure 1.)

An important issue for those stations that will be switching between full-power transmitters, such as KMTP plans to, is that IOTs are not fond of being off for any length of time. Using two transmitters like this requires that each be placed on the air every other week to keep excessive gas from building up as well as other changes that take place within an IOT when it is left off.

When an IOT has not been in operation, for even a short time, it will tend to trip the crowbar protection circuit due to internal arching when high voltage is applied. Many times, this is just the nature of the beast, but sometimes, it is caused by vacuum ion overcurrent triggers. The vacuum ion circuit is used to remove ion molecules that the getters cannot capture. The voltage applied varies but usually runs about 3.5kV. The current drawn from this supply is in direct proportion to the amount of gas being drawn off, so that it is an indication of the amount of pressure in the tube. The typical vacuum ion current is less than 1µA.

During an emergency, when the standby transmitter must be started up remotely, the possibility exists that several crowbar trips, some caused by vacuum ion overcurrent and others by an internal arc, can require an engineer to travel to the transmitter site to get it back on the air. There are a couple of ways to address this problem, as addressed below.

By using a mode called “black heat,” a transmitter can maintain an IOT in standby mode for up to two weeks. In this mode, high voltage is off and heater voltage is reduced, while vacuum ion voltage and cooling air flow is maintained. Black heat will allow a rapid switch on because it removes any gas buildup within the tube; although, not all transmitters are designed with a black heat operating mode.

Another option is to run the standby transmitter in “beam” mode, where high voltage is applied but no RF. An IOT can be operated in this mode for more than a week without negatively affecting the tube. The problem in this case is with the electric bill — with a beam voltage of 34kV and beam current at 500mA, the transmitter is still drawing more than 17kW per hour. But by running both tubes all the time, the standby transmitter will be there as soon as the RF switch is thrown.

Another factor that Michael Boyle, engineering manager at L-3 Communications, brought up was drive power: The analog tube’s intermediate power amplifiers (IPAs) are Class A amplifiers and will not produce the peak power or bandwidth needed for digital. A new set of Class A/B IPAs will be needed to complete the conversion.

Changing channels

The differences between the analog channel and the digital one is another factor in how much equipment can be reused in the conversion process. For KMTP, the analog and digital transmitters operate on Channel 32 and 33, respectively, so the conversion should not be too complex. Much of the IOT amplifier is frequency independent, so only the cavities around the IOT need to be adjusted. With such close channels, the cavities will only require a slight retuning, which will be accomplished using a spectrum analyzer and tracking generator.

After tuning the IOT directly into the station load, without the mask filter, it must be switched into the mask filter, and the tuning has to be rechecked at low power levels before going to full power. Although it is not common, care should be taken when tuning an IOT into the mask filter to keep the output power low (below 50W is good), because it is possible to heat and damage the reject loads. The reject loads are usually rated for only 500W and connected with flexible coax or helix; if the output bandwidth is too wide, the sideband power will be directed into these relatively small loads and can easily overload and damage them.

One problem with single IOT transmitters is that some are not configured to be able to direct the output into a station load before the mask filter — without this, some tuning problems can be difficult to diagnose because the mask filter will “mask” the true output of the IOT. It’s always better to be able to look at the output of an IOT directly into a station load to examine the true output of the tube.

Magic-T combiners are always built for a certain channel and are not capable of being used on a different frequency — not even a single channel change. The paddles within the magic-T, as well as the cavities, are shaped and sized to shift the phase of the signal passing through it. If the frequency is different than what it was designed for, the phase shift will be incorrect and the combiner will not work.

Other frequency-dependent components found in the transmitter’s high-power amplifiers (HPAs) are driver amplifiers, circulators and even some directional couplers. These must all be examined to ensure they will work at the new frequency. The sampling probes for forward and reverse power use low-pass filters (because they are inserted at the tube’s output before any other filtering) and should not be an issue with a single channel change.

Of course, the analog exciter will be removed, the 8-VSB exciter will need to have its output split and one of the outputs will need a variable attenuator placed between it and one of the amplifier cabinets. This will allow for balancing of the two amplifiers, so they will put out the same amount of power. If the two were being combined, a tuner (trombone) would also be added to match the phase of the two amplifiers and gain maximum power output by reducing the reject power on the magic-T combiner.

Control

When combining any high-power systems to work together, controlling them is of prime importance. By combining the control systems, a single button can take care of an emergency changeover, making it much simpler for operators as well as sleepy engineers in the middle of the night. Sometimes this is not possible, such as when the two systems are not from the same manufacturer or one system is much older than the other, but even this can be overcome to some degree with certain transmitter remote control systems that allow for complex operations to be carried out.

KMTP’s DTV transmitter uses a central GUI controller that’s capable of controlling several HPA cabinets, but the analog TX is manually controlled. (See Figure 2, above.)

The analog amplifier cabinet will need to be upgraded to enable it to work with the DTV GUI controller; this mainly involves adding an I/O interface module and connecting it to the amplifier’s control and status lines. The I/O interface module is then connected to the GUI controller’s communications network. The transmitter manufacturer will then need to reprogram the GUI controller to work with the added cabinet. The DTV GUI controller already controls the common cooling system for both cabinets as well as the RF switching for digital (antenna or dummy load). There are several advantages to creating a unified system, but the station budget may not allow it, in which case the added cabinet will be treated as a separate unit. (See Figure 3.)

Acknowledgments

Todd Loney of Richland Tower, Michael Boyle at L-3 Communications and Fred Stefanik and Ted Karam of Thomson contributed to this tutorial.

Next time

The next “Transition to Digital” will continue with digital conversion regarding the RF system.

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