Solid State and Tube
Solid-state devices have long been the preferred choice over vacuum devices. Vacuum tubes, which use high-voltage supplies (typically in the range of several kilovolts), are much less reliable than solid-state devices that use voltage supplies (e.g., less than 50 V). Vacuum tube manufacturers and users are facing dwindling supply sources and material shortages.
Solid-state devices generate lower noise and better linearity than vacuum tubes. For example, a solid-state device in “standby mode” (i.e., with DC bias voltage and no RF input signal) generates much lower noise power across the entire spectrum. The noise figure for a medium-power TWT is around 30 dB, while the noise figure for a solid-state GaN MMIC PA is around 10 dB. In an ECM system, this is a significant difference because with lower noise, the input stage of the transmitter can remain in standby mode when not transmitting. The overall switching time is reduced because the main DC power supply to the PA does not need to be turned on and off.
Figure 1: Output power, frequency, and temperature for the Qorvo QPA1003P GaN MMIC with 15 dBm CW input power, 28 V bias voltage, and 650 mA power consumption.
Figure 2: Architecture of the Spatium amplifier.
Figure 3: Measured output power of a Spatium amplifier integrating 16 QPA1003P MMICs.
Solid-state transmitters have another advantage: they can reduce harmonic content in the output signal. For solid-state PAs operating at an octave or higher bandwidth, the worst-case harmonic content is typically about 8 dB below the fundamental at saturated output power. Under the same operating conditions, vacuum tube harmonic content is only 2 dB below the fundamental. These higher harmonics require the transmitter to meet more stringent filtering requirements, resulting in larger and more expensive components for the entire ECM system.
GaN for Strength
While GaN devices offer significant improvements in power density, power, and bandwidth compared to other heterojunction semiconductor technologies, a single device or MMIC still cannot deliver enough power for most ECM system transmitters. Power requirements are typically 100 W or more in the 2 to 7.5 GHz range. Figure 1 shows the output of a single Qorvo GaN power MMIC. This packaged MMIC is rated for 10 W in the 1 to 8 GHz range, but output power drops to a minimum of 8 W at an 85°C backside temperature. Providing 100 W over the frequency band and temperature range required by ECM systems would require more than 10 of these MMICs.
There are many ways to power the combining devices to achieve SSPA. For ECM system transmitters, the method used must have low loss and wide bandwidth. Many combining techniques use two-port binary combiners, such as Wilkinson or magic tees. Combining two MMICs requires one two-port combiner, combining four MMICs requires three combiners, and combining 16 MMICs requires 15 combining elements. Magic tees have relatively low losses, but they generally operate at a bandwidth of only about 10%, and a double-ridged magic tee has only about one octave of bandwidth, which is not enough for 2 to 7.5 GHz ECM requirements. In two-way combining, four stages of combining are required to achieve the required power. At these frequencies, the loss of a typical double-ridged magic tee is 0.3 dB, so the total loss through the combiner is 1.2 dB. Combining the 30% efficient GaN PA MMICs shown in Figure 1 through a 16-way magic tee gives a combined output efficiency of about 23%, delivering about 95 W at 85°C and 6 GHz. However, a typical double-ridge magic tee network is only effective over one multiple of the bandwidth (e.g., 2 to 4 GHz).
Space combination
Spatial combining techniques can have lower losses than circuit-based techniques. Spatium is Qorvo’s patented approach to coaxial spatial power combining (see Figure 2 ). It uses a broadband polar finline antenna to transmit to/from a coaxial pattern, split into multiple microstrip circuits, and then uses power MMICs to amplify and combine the power from these circuits. It uses free space as the combining medium, providing an efficient, compact, broadband way to combine multiple power MMICs in one stage. A typical Spatium design combines 16 devices in one stage with only 0.5 dB combining loss.
Combining the 16 MMICs in Figure 1 gives an SSPA efficiency of 27%, while each MMIC has an efficiency of 30%. Using magic tee combining gives an efficiency of 23%, which is a significant difference. The increased combining efficiency allows for higher output power from a given base power and reduces heat dissipation.
The actual Spatium amplifier design combines 16 radial blades, each with a Qorvo GaN MMIC PA. Figure 3 shows the measured output power and clamp surface temperature; the temperature of the baseplate under the MMIC is about 12°C higher than the clamp temperature, so the maximum baseplate temperature is 85°C. The device can achieve more than 100 W of power between 2 and 7.5 GHz with an average efficiency of 25%.
Thermal Design
Thermal management is one of the design challenges when using solid-state amplifiers in ECM transmitters. In a typical application, the outer surface of the clamp around the Spatium SSPA is conduction cooled from one or more sides (see Figure 4 ). For some systems, a liquid coolant may be used, and for others, a heat sink with a fan. The clamp is designed so that it contacts all of the blades in the Spatium and provides a conduction path to a cold plate or heat sink. Spatium blades and clamps can be made from different metals, including aluminum and copper. The trade-offs between size, weight, and power will determine the appropriate material for a given application.
Figure 4: Spatium amplifier conducting heat from the MMIC PA through the clamp.
Figure 5: Thermal simulation of the Spatium SSPA, showing a cross section of the architecture.
The thermal impedance from the back of the MMIC to the mounting board can be calculated and used to obtain the back MMIC temperature. From the thermal resistance of the MMIC and the package, the junction temperature of the MMIC can be calculated, and then the reliability of the SSPA can be estimated using this junction temperature. Figure 5 shows a thermal simulation of the SSPA shown in Figure 4, where the MMICs are operated at saturated output power and the efficiency is the lowest efficiency in the band (e.g., each MMIC dissipates 25 W). The thermal model shows that the temperature rise from the coldest point outside the clamp to the back of the packaged MMIC is about 12°C, and the temperature rise from the back of the package to the output transistor connection is an additional 164°C, assuming a thermal resistance of 6.56°C/W. The junction temperature of the MMIC is estimated to be 247°C, and the temperature of the clamp surface is maintained at 71°C. At a junction temperature of 247°C, the MTBF of the MMIC is about 1.2 million hours.
The MTBF of the entire Spatium module is the MTBF of a single MMIC divided by the number of MMICs: 75,000 hours. This calculation treats the failure of a single MMIC as a failure of the entire amplifier assembly, which is a worst-case assumption because the performance of a Spatium amplifier degrades with a single MMIC failure (e.g., output power decreases by approximately 0.7 dB for each MMIC failure).
For TWT, MIL-HDBK-217F Notice 2 provides the following formula for calculating MTBF in a fixed ground environment:
Where P is the rated power in Watts, ranging from 1 mW to 40 kW, and F is the operating frequency in GHz, ranging from 100 MHz to 18 GHz. Using this formula, the MTBF of a TWT with an output power of 150 W at 7.5 GHz is 29,609 hours. This is approximately 2.5 times lower than a comparable solid-state Spatium power amplifier module under similar environmental conditions.
Table 1
Summarize
For the first time, GaN MMICs and broadband spatial combining technologies such as Spatium allow ECM system designers to use reliable solid-state amplifiers in place of TWTAs. The ability to transmit hundreds of watts of power over a wide frequency band while staying within the fundamental power range provided by the platform and dissipating the heat for reliable operation opens new opportunities for solid-state ECM transmitters to be used in systems. Table 1 shows the frequency, power, and efficiency that can be achieved using three recent Spatium amplifiers. These SSPAs are much smaller in size and weight than the boxes previously occupied by TWTAs. ■
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