Northrop Grumman's IOT with CEA technology
By Buzz Miklos
Many stations were waiting until the last minute to conclude transmitter deals, hoping that the FCC would delay the digital rollout. Recent changes in the FCC’s position regarding the deadline for stations to begin transmitting high-power digital signals may now give station operators more time to make the right equipment choice, and transmitter OEMs more time to develop and demonstrate the constant efficiency amplifier (CEA) technology before the stations need to commit to an IOT-based system.
Portions of Northrop Grumman’s CEA IOT used for low-power broadcasting do not need to be modified significantly when a broadcaster makes the transition to high power.
One option for broadcasters is Northrop Grumman’s CEA, an IOT with a multistage depressed collector (MSDC), which sorts electrons according to their energy, causing both the beam current and the average electron collection voltage to go up and down with the RF output. In effect, the MSDC is operated at voltages that minimize the input power for a given set of TV signal statistics. As a result, the IOT can operate in digital service at a 50 percent improvement in efficiency compared to standard IOTs, enabling the television station operator to save at least $20,000 per year per tube in power expenses.
Evolution of an IOT
Following its product launch at NAB2001, Northrop Grumman Electron Devices began an intensive program of testing and refining the IOT, including sending it to transmitter OEMs for testing, qualification and system integration. The product now available was developed through a three-phase program. The first phase was the design and testing of a form-fit-and-function IOT that would operate in existing transmitters. In the second phase, RF cavity design, engineers focused on minimizing insulation, RF leakage and mode problems by connecting the input cavity directly to the grid of the IOT. An idler cavity of one-half-wavelength placed between the grid and anode minimizes RF leakage from the high-voltage seal while enhancing efficiency and linearity. The third phase included the development of the MSDC to provide nearly constant efficiency.
Today’s tube will operate at 130kW of peak power or about 30kW average power in 8-VSB service. It meets all FCC requirements for IMD, mask, shoulders and phase noise when operated at full-rated power. It has been designed to fit into a standard L-4480T trolley assembly and uses the same heater, bias and RF drive voltages as a standard IOT. Safe PAO oil cooling allows the tube to operate at a maximum of 30kW average power.
The recent decision by the FCC to allow stations to broadcast digital at reduced power and thereby hold their licenses will have a significant effect on the high-power marketplace. Now a station can use what is essentially the driver section of a high-power transmitter to get on the air. The portions of the system purchased to broadcast at low power do not need to be modified in a significant way and will not become obsolete. The exciter, control circuits, facilities and driver amplifier will all be retained, and when the station is ready to go to high power, the final power amplifier will be added.
Buzz Miklos is director, sales/marketing, for Northrop Grumman Electron Devices.