When you wandered through the halls of NAB2006, the transmitter offerings seemed to be the same old things. There were small solid-state ones, ones with a big tube-type device, ones with several big tube-type devices and so on. However, when you investigated more thoroughly, it became apparent that there were some significant changes at the show.
The big news, at least to me, was the power levels available in solid-state systems. Until recently, it seemed that solid-state systems were available up to 10kW or so, which was nice for the DTV systems with ERP values of 50kW to 200kW. If a station was going to go the whole way to an ERP of 1MW, it really had no option but to pick out the klystron type it wanted. While the tube transmitters may still be the system of choice, based on economic factors such as utility bills and replacement costs, solid-state systems are now available for high-power levels.
In addition to the high-power level stuff, several manufacturers showed new models of low- to medium-power solid-state systems that offered improved features for applications ranging from translators to scattered site systems and the majority of DTV stations.
One of those systems was Harris' new Atlas transmitter. It provides up to 13.5kW average power when using multiple cabinets or up to 4.5kW from a single cabinet. The series uses liquid-cooled amplifiers and power supplies. The exciter — the APEX ATSC system, which includes Harris' real-time adaptive correction — has been around long enough to get the bugs out. Add a color touch screen for troubleshooting, and it's a very usable box.
The Atlas is also available for analog systems for stations whose old transmitter won't make it another couple of years. In analog configuration, the maximum available power is 30kW. Of course, the analog version can be converted over to digital by changing the exciter, filters, etc.
Axcera showed two levels of digital transmitters — that's power, not quality levels. The Innovator LX is a small unit designed to operate as a translator, booster, driver or standalone transmitter. The available power levels range from 5W to 3kW for digital systems or from 10W to 6kW for analog. The transmitter's function can be changed by inserting different slide-in modules or rack-mounted components. For example, the available components would include various digital modulators, an analog modulator or a receiver for translator use. The equipment is air-cooled and frequency agile.
The higher-powered box from Axcera is the Innovator HX. The available power ranges from 2kW to 60kW digital and from 5kW to 120kW for analog service. Again, the equipment is air-cooled. With available dual exciters and automatic switchover on failure, it is designed for long-term unattended operation. The exciter system contains all the necessary correction circuits to maintain the proper digital output signal.
To look at the power level more carefully, let's assume that a station has an authorization for 1MW. For this example, the antenna has a medium gain of 25 and a 1000ft run of 6 1/8in 75Ω line on channel 25. The result would be a transmitter power output of 52.2kW. That means that the system would be doable with the Axcera box. We won't discuss cost and power use. It is important to recognize simply that a box out there is capable of working with big DTV stations. This is a big step up from solid-state transmitters five years ago, and it shows the continuing design of more powerful transmitters.
Another company showing high-powered transmitters at the show was Acrodyne (Ai). Ai's Depressed Collector Quantum transmitters feature power levels up to 120kW average ATSC 8VSB and DVB-T COFDM. The transmitters use the e2v ESCIOT and operate efficiently in either digital or analog service.
DMT's transmitters and repeaters include digital and digital-ready analog models in VHF and UHF bands, with air or liquid cooling. They offer 0.1W to 40kW output power and are compatible with all types of digital terrestrial TV networks.
Thales' Optimum and Ultimate range of solid-state TV transmitters range in power from 2kW to 60kW for analog and 800Wrms to 22kWrms for digital.
LARCAN has a new solid-state series known as the Eclipse. This system uses air-cooling with hot-pluggable modules for power amplifiers and power supplies. For DTV service, it ranges from 5kW to 40kW, with higher power available if needed. The unit is compact. A 20kW system can fit into less than 25sq ft. The system includes extensive monitoring and diagnostics to aid the station technicians. It is worth remembering that modern systems are sophisticated to repair. Therefore, any help that the manufacturer can build in is a huge bonus to the operator.
Air vs. liquid cooling
Going back to the liquid side, Rohde & Schwarz has NV 7000 digital transmitters available with power levels to more than 5kW. The modules are hot-pluggable without the loss of coolant. Each amplifier module has its own power supply cooled by the same liquid. The transmitters are available with dual exciters with automatic changeover. Both exciters fit in a standard 19in rack in only seven units of vertical space. That particular statistic should cause the old-timers reading this article to shake their heads. I remember when an old RCA sync generator, two units with power supply, would fill an entire rack. Adding color took another half rack. Of course, the signal generator has shrunk to a chip, including color, and has a stability never dreamed of in the old days.
The NV 7000 can be operated by a PC with Windows or by the built-in display. The entire construction of the units is high quality.
The one remaining discussion point is whether air or liquid cooling is more desirable. The liquid cooling is cool (pun intended), with little tubes running all over the place. It also is good for keeping the transmitter clean as the amount of air involved is limited. Liquid cooling does require an external heat exchanger to remove the heat from the liquid. Rohde & Schwarz uses a coolant called AntifrogenN.
Air cooling, on the other hand, is simpler. It is only necessary to keep a large amount of filtered air directed at the necessary heat producers. More maintenance is required to change filters regularly and to clean the equipment more often. No great determination is offered here — just a couple of things to ponder when shopping for one of those new and significantly improved digital systems.
Don Markley is president of D.L. Markley and Associates.