TWTs, MPMs Meet Broadband, High Frequency Defense Applications Requirements
By Shawn Gentile, Technical Writer, dB Control
While solid state amplifiers are a good choice for low power applications, vacuum electron devices (VEDs) still offer the most power per device for radar, electronic countermeasures, communications, and other high-power applications. At high microwave- and millimeter-wave frequencies, monolithic microwave integrated circuits and RF power transistors can’t match the power/bandwidth of vacuum tubes. While VEDs such as klystrons, crossed-field amplifiers (CFAs), gyrotrons, magnetrons, inductive output tubes (IOTs) and traveling wave tubes (TWTs) may conjure up images of older technology like television tubes, these devices achieve power levels still unmatched by transistors.
For those who still aren’t convinced that TWTs have a place in today’s high-power applications, it helps to go back to the basics (refer to Figure 1). TWTs use a slow-wave structure (either a coupled-cavity circuit or helix) to create interaction between a high-energy electron beam and an RF wave in a vacuum envelope. Electrons are generated by a heated cathode in an electron gun assembly and launched into the interaction region.
The electron beam is controlled by an electrode that switches the beam on and off by changing the control electrode potential (bias) to either positive or negative with respect to the cathode. This switching of bias voltages is performed by the modulator in the transmitter to transition the device from a conduction state to a cutoff state.
The electron beam is focused by magnets along the axis of the TWT and the beam is accelerated by a high potential between the cathode and anode (collector). The RF wave propagates from the input to the output through the slow-wave structure, and the high energy electron beam gives up energy to the RF wave as it travels along the tube, providing amplification before the high-frequency signal reaches the RF output port.
WTs are Core Transmitter Element
TWTs are the most widely used VED for microwave defense, instrumentation and satellite communications because they provide the extremely high output power density required by these applications at microwave frequencies. To produce higher RF output levels, TWTs require proportionally high voltages to be applied to their electrodes. For example, an 8kW, X-band helix TWT requires an input voltage of about 14 kV, while a 100kW coupled-cavity TWT requires about 45 kV input voltage. In a radar transmitter (refer to Figure 2), a pulsed signal from the radar waveform generator is applied to an amplifier that employs RF power transistors to produce an output that drives the TWT. This signal is sent to the TWT input so isolators can ensure proper input matching and inter-stage isolation. The TWT is protected from power overloads by a PIN-diode switch that shuts off the driver’s output when required.
As the core element of a transmitter, the TWT affects nearly every aspect of the transmitter’s performance. While manufacturing processes may differ, the checklist of TWT design considerations includes power supply requirements, operating voltage levels, power consumption, design power dissipation, thermal design, temperature/altitude/vibration performance, size/weight and demonstrated record of reliability. For example, a suite of pulsed power-combined wideband TWT amplifiers from dB Control (Fremont, Calif.) are well suited for test and measurement, RFI/EMI/EMC testing, antenna pattern and radar cross-section measurements, electronic countermeasures (ECM), and electronic warfare (EW) simulation.
These TWTAs use two wideband, periodic permanent magnet (PPM)-focused TWTs to amplify CW, AM, FM, or pulse-modulated signals. Custom designed by dB Control, the TWTAs are manufactured in-house using proprietary transformer fabrication, encapsulation and high-voltage potting techniques developed specifically for stringent military applications.
MPMs Provide Broadband, High Frequency
As applications increase in sophistication, the need for smaller and lighter assemblies is top of mind — especially in instances where cargo loads need to be minimal (i.e., unmanned aerial vehicles like the MQ-9 Reaper that fly at altitudes of up to 50,000 feet). That’s where the mini TWT comes in. Well suited for use in microwave power modules (MPMs), mini TWTs are designed for use with a lower-voltage power supply up to 8 kV. Although it provides a lower power output than its larger counterparts (to about 200 Watts CW, 1 kW peak), the mini TWT retains its broadband, high-frequency capabilities through approximately 40 GHz.
MPMs equipped with mini TWTs are suitable for maturing technologies like active electronically steered array (AESA) radar applications because power output can be increased by one or more orders of magnitude greater than the power achieved with the solid state power amplifier in the transmit/receive module. Additionally, MPMs are useful for synthetic aperture radar (SAR) systems, EW, ECM and other commercial and military satellite communications systems. Specifically, dB Control’s MPMs are employed in many UAVs in which the platforms’ prime power, size, and weight are very limited and long, reliable operation time is essential.
MPMs have wider bandwidths and greater heat tolerance than solid state amplifiers — as evidenced by the suite of MPM-based transmitters for ECM applications developed by dB Control (refer to Figure 3). Covering 2-7 GHz and 6-18 GHz to produce a total 2-18 GHz frequency range, the suite consists of four MPM-based CW and pulsed transmitters operating from aircraft three-phase 115 VAC power. Designed for the harsh conditions encountered in airborne environments, the transmitters can withstand gunfire vibration, operate at +100°C for short periods, and are compliant with MIL-STD461E. The suite delivers RF output power of 250 Watts under CW or pulsed conditions from 2-7 GHz and 100 Watts from 6-18 GHz, 1.5 kW peak (6% duty cycle) from 6-18 GHz, and to 300 Watts under CW or pulsed signal conditions from 6.5-18 GHz from its dual-transmitter “high-band” section. The RF input section includes isolators, directional couplers for sampling at various stages, bandpass filters, switches for selecting specific filters, and a TWT gain equalizer.
A more recent MPM example can be found with the dB-4127 (refer to Figure 4), which provides 200 Watts of continuous wave RF power over a frequency range of 6-18 GHz. With double the power of previous MPMs, this new MPM is one of the most efficient on the market — due to a highly efficient TWT and HVPS. Even with twice as much power, the new dB-4127 MPM has nearly the same heat dissipation as the previous dB-4118 (100 Watt MPM) at 9.5 GHz, and averages only about a 10% increase in heat at other frequencies.
MPMs and TWTs will remain essential sources of power amplification for decades to come. They are still the optimal choice for many defense platforms (like EW, ECM and radar systems) – and some commercial and industrial applications because they provide RF power outputs of up to 2.5 kW CW and 35 kW pulsed at frequencies to 95 GHz. Fortunately for all industry players, the wide variety of defense and commercial applications leaves room for both solid state and vacuum electron technology.
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