Klystron High Power Amplifiers for SATCOM Applications

Klystron High Power Amplifiers for SATCOM Applications

Communications & Power Industries (CPI), Satcom Division
Palo Alto, CA

A new family of C-band, Ku-band and direct broadcast satellite (DBS)-band compact GEN IV klystron power amplifiers (KPA) has been introduced. The new amplifiers incorporate a multistage depressed-collector (MSDC) klystron that allows for a smaller, more efficient and cooler running amplifier than other KPAs on the market. The unique design translates into lower power costs and longer lifetime for the klystron and other electronic components. In addition, a built-in power saver function provides even greater power savings. The new amplifier family comprises the C-band series, Astra series (13.25 GHz), K4U64 series (14.5 GHz) and K4D62/4 series (18.4 GHz).

KPA Description
The GEN IV series KPAs feature 2.4 kW of RF output power at the klystron, which is tunable over the 5.850 to 6.725, 12.75 to 13.25, 13.75 to 14.50 or 17.30 to 18.40 GHz frequency ranges, depending on the klystron used. Options exist for other frequency ranges as well. The amplifier’s compact, two-drawer design provides state-of-the-art modularity for ease of use and maintainability.

The GEN IV KPA’s RF chain is housed in the RF drawer and comprises a solid-state integrated power amplifier (IPA), the MSDC klystron, a high power output isolator, and associated couplers and isolators. The klystron blower, front control panel and embedded controller also are contained in this drawer. A mechanical channel tuner mechanism permits the operator to change any of up to 24 preset frequencies. An optional channel selector (< 1 s per change) with remote controllability is also available.

The power supply drawer houses the AC input filtering, monitoring and high voltage power supply. This supply provides regulated beam, heater and collector voltages for the klystron. The GEN IV amplifier is adaptable to all primary three-phase voltages available worldwide. The KPA is protected from damage caused by AC/DC/RF faults and insufficient cooling, and will automatically recycle after a prime power interruption or transient fault.

The control panel contains a large color thin-film technology liquid crystal display (TFT-LCD) with a wide viewing angle, light-emitting diodes and a long-lasting membrane-style keypad. The controls directly interface with the earth station computer system; a remote control panel with identical functions and configuration is available as an option.

The RF Circuit
Figure 1 shows the RF circuit block diagram. The low level input signal is applied through an isolator to the IPA, which includes a voltage-controlled PIN diode attenuator, input and output isolators, and directional couplers. The attenuator permits the operator to quickly adjust the RF drive level within a 25 dB linear range with a resolution of 0.1 dB. An internal memory function returns the amplifier to the previously set level in case of a power outage.

The IPA also includes an RF inhibit function to remove the drive signal during faults, channel changes, transfer switch operations or external interlock actions. The IPA is transparent to most final amplifier parameters and is temperature compensated to minimize drift. As a result, the overall KPA gain is stable to within 1 dB over a 20°C temperature change and within 2.5 dB over the 0° to +50°C operating range. Table 1 lists the KPA’s specifications.

The MSDC Klystron
The klystrons used in the C-, Ku- and DBS-band GEN IV amplifiers incorporate an advanced design using MSPCs that deliver up to 2.4 kW (3.35 kW in C band) of output power across the specified frequency band. The instantaneous (-1 dB) bandwidth is 45 MHz (80 MHz optional) for C band, and 85 MHz (80 MHz for Astra) and 50 MHz (85 MHz optional up to 1.7 kW) in the Ku and DBS bands, respectively. A standard integral mechanical channel tuning mechanism provides precise tuning and retrieval of up to 24 channels and allows for manual frequency changing in the field. Preset frequencies are factory tuned in accordance with a standard frequency plan unless otherwise specified.

The Output Circuit
The output of the klystron is fed to the integrated output waveguide assembly, which comprises two arc detector ports, a circulator and loads, a harmonic filter and a three-port directional coupler. The optical arc sensor is attached to a waveguide bend that is located near the output of the klystron. If an arc occurs in the waveguide, the light is detected and the protection circuit rapidly turns off the IPA to inhibit the RF drive, thereby quenching the arc.

The high power isolator provides a low SWR source to the external waveguide and antenna feed and protects the klystron from excessive reflected power resulting from external faults. The isolator will continuously dissipate the reflected power from a 2:1 load (11 percent of the rated power), and safely withstands reflected power equal to the full rated KPA output until the protection circuits shut off the RF signal. The optionally removable harmonic filter attenuates the second harmonic to levels at least 80 dB below the fundamental output.

The output directional coupler includes two forward ports and one reflected port. One forward port provides an RF sample at a nominal -50 dBc. The second forward port and the reflected power port are connected through the detector assembly to the controller. The metering signals are also used to activate the high/low RF power alarm and the high reflected power fault-protection circuit. The standard output RF interface is CPR-137F (C band), WR-75 (Ku band) or WR-62 (DBS band) waveguide with a grooved flange.

The Power Supply Assembly
The AC input power is routed through an EMI filter and main circuit breaker and passes through the step start circuit that is used to limit in-rush current to < 180 percent of steady state during high voltage turn on. The AC power is then converted to DC and passed to the power processor, which converts the DC voltage to a 50 kHz AC voltage that is applied to the high voltage transformer and rectifier. The rectified output is filtered and provides the cathode, heater and collector voltages for the klystron. Samples of the cathode and heater voltages are fed to the power processor as part of the regulation scheme. The embedded control system monitors cathode voltage, beam current, body current, heater voltage and heater current.

The cathode and heater voltages are microprocessor controlled and set for the particular klystron. The power saver function uses this flexibility to set the beam voltage when less than rated RF power is required. In addition, the klystron heater voltage is set accordingly to extend the klystron’s life. A standby mode is available to allow extended klystron life when the amplifier is not used for long periods of time but is required to be available at a moment’s notice.

Monitors and Controls
The control and monitor system is designed to assure correct amplifier operation and easy maintenance with minimal operator training and activity. A system of embedded controllers provides automatic sequencing and continuous monitoring of critical parameters. All operational information is clearly displayed on the large color TFT-LCD panel. The membrane keypad provides convenient control from the front panel.

Upon power on, the embedded controller self-tests the internal functions and begins heater warm-up. Once warm-up is complete, the transmit key initiates normal operation. Automatic leveling then can be keyed to maintain a set power output level against RF input variations. The amplifier’s fault handling circuit is tolerant of transient faults, returning the amplifier to the state it was in when the fault occurred (if appropriate). In the event of a fault warranting a permanent shutdown, the fault will be clearly displayed with time and date stamps. The fault then must be cleared and the cancel key depressed in order to reset the amplifier. The standard KPA permits computer control and monitoring through an RS-422/485 (four-wire) serial data bus. This capability provides the operator with a convenient and reliable means to monitor and control the power amplifiers at a remote, unattended earth station.

Mechanical Design
The GEN IV KPA is packaged in two sturdy aluminum enclosures that offer convenient front access for service and maintenance. Both power supply and RF boxes are designed to be mounted in standard 19-inch racks using chassis slides. Within the RF box the klystron, RF components and RF control parts are mounted on a slide-out drawer.

Measuring 26.25" high (15 RU), the KPA is designed for space-constrained locations. All electrical interface connections and exhaust ducting for the klystron cooling air are located at the rear of the unit. Nearly all maintenance activities can be accomplished from the front of the equipment. Two independent cooling circuits are used in the KPA, one for the klystron collector and one for the power supply.

Options and Accessories
Among the available options and accessories are a digital fast-tuning system that provides an automatic channel selector comprising a motor drive and control unit as an integral part of the klystron. This motorized electronic channel changer achieves a maximum 1 s channel changing time. In addition, a peak power meter is available for use when the KPA is used in a TDMA system. The circuit responds to burst signals of 5 ms (min) pulse width and a maximum repetition of 100 ms. With this option the meter reads either peak or CW power in accordance with the signal being transmitted.

A remote control panel is available that duplicates all of the controls and indicators of the main panel and communicates with the KPA via the serial interface. Also available is a 1:1 switching subsystem that comprises an output waveguide switch and dummy load. The circuit provides 1:1 redundant protection with automatic transfer or manual operation. A power phase combiner consisting of a coaxial input divider network, phase shifter and output waveguide combining system provides four operating modes when two KPAs are used in combination. A linearizer is also available that provides an improvement in AM/PM, third-order intermodulation spectral regrowth and noise power ration performance.

CPI, Satcom Division, Palo Alto, CA (650) 846-3700.