For several years I've noticed the output power of solid state TV transmitters on display at NAB has slowly increased. This year, the top power was around 10 kW per cabinet, about the same as last year, but efficiency continued to improve. I didn't see any TV transmitters on display with a tube amplifier. I'm sure one reason for this is that the amount of exhibit space on the show floor devoted to displaying TV transmitters of any type has been decreasing.
People are still buying IOT or higher efficiency MSDC IOT based transmitters for power levels above 20 kW, but with improvements in solid state transmitter efficiency, it is now practical to use two combined 10 kW range solid state transmitters instead of an IOT. While the initial cost is higher, there is no high voltage to worry about and maintenance and installation of the solid state transmitters is simpler.
The efficiency improvement started with Harris' Maxine transmitter using 50 volt LDMOS output devices. Other manufacturers have adopted similar devices, but system efficiency remained close to, but not equal to, the conventional (non-MSDC) IOT amplifier. That changed this year with transmitters from Screen Services (SDT 403-ARK 6) and Rohde & Schwarz using Doherty amplifiers and Thomson's new Futhura Plus transmitter using drain (envelope) modulation. The reported efficiencies are as good or better than IOT transmitters.
Screen Services and Rohde & Schwarz had working Doherty amplifiers on display. Thomson had an amplifier with the cover off behind a transparent cover in a rack but I didn't notice any indication it was actually on when I saw it. Other transmitter manufacturers told me they were looking at Doherty modulation for improved solid state efficiency, but wondered whether their customers would be willing to pay the price of the extra complexity.
All three amplifiers on display were different. The Screen Services amplifier appears to be the simplest in design and will probably be the lowest priced option because the amplifiers are not broadband. This may limit its appeal in some countries, but for U.S. broadcasters, I don't see the bandwidth limitation on the amplifier (about 6 channels total) being an issue. According to Screen Services, the frequency can be changed in the field by a factory technician. A projected lifetime use scenario would likely have the transmitter channel being changed once when the FCC completes its repacking and one more time when the transmitter is removed from primary service and perhaps shipped to another location.
The Rohde & Schwarz Doherty amplifier will be broadband so changing channels or swapping amplifiers is not a problem. The amplifiers in its new THU9 series solid state transmitter shown in the U.S. for the first time at this year's NAB Show will be able to be upgraded to Doherty amplifiers for higher efficiency in the future. While the amplifier is still in beta testing, they had a working display at NAB.
The Thomson drain modulation is more complex, but it is also broadband. It too is still in development. The final efficiency should be as good or better than the Doherty amplifiers.
Other transmitter manufacturers I spoke with are looking at these technologies. Doherty is the most popular, but they are concerned customers may not be willing to pay a higher price for these complicated, high-efficiency amplifiers. It would be tough to justify their use in transmitters under 5 kW output or in backup service where power consumption isn't a major issue. At power levels around 20 kW, solid state transmitters are much more expensive than IOT transmitters. The price gap has narrowed, but if the price of Doherty amplifiers or amplifiers with drain-modulation is much higher than today's conventional solid state transmitter, many customers are likely to stay with tubes. While maintenance can be a pain, the tubes often last 8 years or more. I know of many DTV transmitters still running with their original tubes installed that long ago.
We'll have a better idea of the chance of these new amplifier technologies becoming popular after we see how their prices will compare with IOT amplifiers and conventional high power solid state transmitters.
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Doug Lung is one of America's foremost authorities on broadcast RF technology. He has been with NBC since 1985 and is currently vice president of broadcast technology for NBC/Telemundo stations.
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