Television Transmission Technology
A spinoff system that offers a substantial improvement in
UHF-TV transmission efficiency is the VKP-7990 MDC Klystron,
which is manufactured by the Microwave Power Tube Division of
Communications and Power Industries, (CPI), Palo Alto, California.
There are 90 CPI klystrons in operational service at 36 UHF-TV
stations.
The MDC klystron is the result of a multiyear cooperative
development program by a group that included Lewis Research Center,
the National Association of Broadcasters, the Public Broadcasting
System, TV transmitter manufacturers, and Varian Associates,
Inc., also of Palo Alto. Throughout the klystron development
period, the Microwave Power Tube Division was part of Varian,
but Varian's Electron Devices Group has since separated from
Varian to become CPI.
The CPI MDC klystron cuts the electric power consumption of
UHF-TV transmitters by half.
The program was initiated to address a problem experienced
by the UHF segment of the TV industry: for adequate reception,
UHF stations need greater transmitter power than their VHF counterparts
and, additionally, UHF transmitters are inherently less efficient;
thus, UHF station operators had to pay substantially higher electric
utility costs, a significant competitive disadvantage.
The development group sought a way to make available to UHF
operators power amplifying devices with efficiencies comparable
to VHF. The indicated line of approach was to incorporate into
UHF transmitters a power amplifying device known as the Multistage
Depressed Collector (MDC) developed a decade earlier at Lewis
Research Center to enhance the efficiency of communications satellite
transmissions; the MDC allowed satellites to transmit more powerful
signals, thus enabling the use of smaller, less costly Earth
stations for signal reception.
The klystron is a vacuum tube used to generate and amplify
ultrahigh frequencies. It draws radio frequency energy from a
high voltage electron beam but does so at very low efficiency
levels; most of the energy is dissipated as waste heat. The concept
behind the Lewis/Varian development was that the MDC could recover
much of the wasted energy by recycling a large part of the electron
beam energy, in effect doubling the amount of the beam energy
being converted to radio frequency energy.
The program began in 1984. Within the initially targeted three
years, a successful MDC klystron had been produced by Varian
and thoroughly tested. It took another two years to integrate
the klystron into a complete transmitter for field operation.
Varian started commercial production of the tubes in 1990 and
CPI took it over in August 1995.
CPI continues to monitor the klystron's performance. According
to CPI senior scientist Earl W. McCune, the total operating time
for the tubes in service has exceeded three million hours; projected
Mean Time Between Failures is 60,000 hours; the klystrons show
no significant adverse effects due to incorporation of the MDC
feature; and the initial project goal of cutting electric power
consumption by half in UHF-TV transmitters has been realized.
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