Transmitter technology review

Other than a possible new tower, the single biggest item at the transmitting plant is probably the transmitter.
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Other than a possible new tower, the single biggest item at the transmitting plant is probably the transmitter. As such, it behooves the chief engineer to plan carefully before buying a new box and to carefully evaluate the current crop of devices on the market.

Transmitters are like cars in a way. Every year, manufacturers will come out with one or more new models. One can expect some performance changes, hopefully for the better, accompanied by a price increase. In some cases, the changes will introduce a whole new level in technology. More often, the changes resemble the automobile model that has simply had the chrome rearranged.

The final amplifier stage du jour is still the inductive output tube. This technology has evolved from four external cavity klystrons, five internal cavity klystrons, MSDC devices (still klystron type) and various other definitions, to the IOT, which is still a klystron type of device. The IOT itself has been fairly constant over the past few years and nothing seems to be on the horizon that will replace it for high-power UHF. That is not to say that there have been no improvements. Such a statement would bring down the wrath of the manufacturers, and rightly so. Work is being done every day to improve the performance of the output stages to the benefit of the industry. It's just that one does not see a giant leap into a new technology.

Solid-state transmitters are becoming more popular for the obvious reasons of reliability and ease of operation. However, their cost is still prohibitive for most stations at high power levels. Hopefully, that will change in the future as more and more manufacturers look for the magic step to reduce the number of solid-state devices needed without making the linearity corrections impossible or impractical.

The most conspicuous changes have been occurring in the associated areas of improved exciters, new designs for driver stages and in the control of the exciters and transmitters themselves. In addition, it seems to be fashionable these days to change one's name. For example, ADC Broadcast Systems is now Axcera and Thomcast Communications has become Thales Broadcast & Multimedia. It seems that keeping track of the name of the manufacturers is almost as hard as keeping track of where your favorite salesman is working this year. It seems that Ma Bell started all of this, and it progressed through Hewlett-Packard down to television transmitters.

In any case, Axcera has developed a new line of LDMOS driver amplifiers for their IOT transmitters. They put forth the argument that these new devices require less correction than earlier systems, permitting more correction to be available for the IOT. The new amplifiers are claimed to run cooler than MOSFETS, reducing junction temperature and increasing reliability. It is such small changes as improving the driver amplifiers that seem to be the rule this year.

Axcera joins the other major manufacturers in offering a sophisticated system of correction for distortions in the amplifier system. At least two such systems are now fully usable from any remote location with a PC. The Harris system of control has been around for a couple of years now. Thales (see above) now has what they call their “thin client” feature, which is a complete control system for the transmitter. This system allows for the normal control functions such as on/off/power adjust and the like. It also permits changing the system mode of operation, such as which IOTs are switched to the antenna and/or the dummy load. While this is supposed to be automatic in case of failures, the control system backs up those features.

The Thales system and the Harris system are both capable of making changes to the correction circuits in the exciter. Both systems also allow monitoring of the necessary wave-forms to determine if further correction is necessary and to confirm that the system is providing the best possible signal. These two systems do vary somewhat in that the Harris system is only accessible by direct contact through a dial-up modem. The Thales system is also accessible by telephone call, but can be configured to present a Web page and be contacted via the Internet.

Obviously, arguments can be made about the advisability of opening such access, but the truth is that a modern firewall can provide more than adequate protection.

One change of note is the power levels available. Up to 35kW of average power is now available per high-power amplifier. That offers both a disadvantage and an advantage and should be considered carefully before a purchase. It is now possible to build the complete transmitting plant around a transmitter with a single IOT. This offers the obvious advantage of less space required in the building, a single power supply and heat exchanger, and considerably less initial cost than buying a two-tube unit for the same required power. Those same advantages are the disadvantage, other than the price. The station now has no backup for anything. This is probably unacceptable in a major market and provides food for thought in smaller markets.

With two amplifiers, the failure of a power supply, heat exchanger or IOT makes very little difference in the transmitted signal. Obviously, after switching, the output signal drops by 3dB. That isn't really the end of the world except in the far out boonies where the signal is becoming marginal. In the main market, it probably won't be noticed by anyone. That gives the station staff a reasonable period of time to make the necessary repairs in a pleasant working atmosphere. Otherwise, the staff will find themselves scrambling frantically to get the station back on the air while enduring screams and moans from the front office. As a rule, this will always occur during sweeps or while some major sports event is taking place.

The point is that the redundancy offered by having more than one output device is highly desirable — maybe even necessary. Today's transmitters do provide an excellent level of reliability, but they will still break down occasionally.

In summary, there hasn't been any big, earthshaking development in television transmitters for the last couple of years, at least. However, there has been a steady improvement in corrective circuitry, control systems, driver stages, power output capability per device and, thankfully, efficiency. All of this work is continuing and will show up this year at the big show as there are more improvements in the products being offered. While the changes aren't mind-boggling, they represent a lot of hard work and a gradual, but steady, upgrade of today's crop of transmitters. Now, if we could just get them to quit confusing us by changing their names all the time.

Don Markley is president of D.L. Markley and Associates, Peoria, IL.

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