The VSX3622, a 1.5 kw X-Band GaN Power Amplifier for Radar Application George Solomon, Dave Riffelmacher, Matt Boucher, Mike Tracy, Brian Carlson, Todd Treado Communications & Power Industries LLC, Beverly Microwave Division Abstract Solid State Power Amplifiers (SSPAs) incorporating GaN transistors provide compact and efficient sources of microwave power. CPI has developed a 1.5 kw X-band SSPA, CPI model VSX3622, for radar applications. The VSX3622 SSPA combines the power from two VSX3614 SSPAs, which each operate at nominally 1 kw, saturated. This paper will present details of the design and performance data for both amplifiers. Background The Beverly Microwave Division of Communications & Power Industries LLC (CPI BMD) has been manufacturing microwave and radar components for more than 6 years. Communications & Power Industries LLC (CPI) is a global company with a world-class global network of service centers. CPI BMD develops, manufactures, and repairs radar components and systems having full compliance to military standards. Our design and manufacturing processes are geared for military as well as high-reliability commercial workmanship. CPI BMD is an ISO 91/AS91 certified manufacturer. Figure 1 VSX3614 Solid State Power Amplifier The VSX3614 design has been optimized for duty cycles to 1% duty and to allow multiple amplifiers to be combined efficiently using waveguide combiners to produce a multi kilowatt transmitter. The VSX363 design has been optimized for duty cycles up to 2%. VSX3614 SSPA Design For a given output power, size, weight, and efficiency are critically important for mobile applications. Solid state power amplifiers incorporating GaN devices have quickly found a home in mobile applications due to the high power density and efficiency of the GaN transistors. CPI has capitalized on these attributes for the development of the VSX3614, the VSX363, and the VSX3622 X-Band SSPAs for radar applications. Figure 2 VSX363 Solid State Power Amplifier VSX3614 Electrical Design The VSX3614 design is based on a 1 watt power device. The system block diagram is shown in Figure 3.
- m ) B ( d 7 63 56 49 42 35 28 21 14 7-1 8 1 22 31 9 8 2 54 55 34 74 14 RFAMP_DRIVE_1 RFAmp_Dri ve_2 Isol ator_14 Isol ator_7 2 Coupler 1_2 Radi al_2 3 Isol ator_16-18 Split2_1 15 4 Node Split3 77-18 WG_COM B_1 2 Coupler 1_1 Isolator_1 RFAmp_2 Isolator_2 Radial_1 RFAmp_3 Isolator_3 Isolator_13 Split6_1 RFAmp_4 Isolator_6 RFAmp_5 Coupler1_2 Coupler1_1 Isolator_5 WG_COMB_ RFAmp_6 Port_2 ZO=5 CW Isolator_4 CWSource_1 Isolator_15 RFAMP_DRIVE_ Isolator_16 RFAmp_Drive_2 Split2_1 REV_PO_DET ZO=5 RFAmp_7 Isolator_12 FWD_POW_DET ZO=5 RFAmp_8 Isolator_11 Radial_2 RFAmp_9 Isolator_1 Isolator_14 Split3 Figure 3 Schematic design of VSX3614 and VSX363 SSPAs Isolator_9 Isolator_8 1 Isolator_7 2 The design consists of a two stage pre- driver with power split two ways and then amplified. Each secondary driver feeds a 6-way radial power divider and the 12 power stages. The signals are then recombined in two 6-way isolated, radial combiners. The isolated radial in-phase combining structure is used to sum the powers from individual transistors while maintaining isolation between adjacent devices. The combiners provide greater than 2-dB return loss for the transistors. The 12 power devices are mounted directly to the heat exchanger to provide the best thermal interface with the lowest thermal resistance and optimum heat spreading. Figure 4 shows a computational simulation of the temperature profile of the VSX3614 at the transistor to baseplate interface. The maximum temperature rise is less than 3 C. The SSPA s cascaded gain and output power at Psat is shown in Figure 5. G A I N C P, C X-Band SSPA Cascaded Power and Gain Isol ator_15 CP CGAIN SSPA OUTPUT POWER Node 2, 6.475 dbm SSPA Node 2, 45.21 db Figure 5 Cascaded gain and output power for VSX3614 SSPA The CPI-proprietary combiners enable an overall amplifier efficiency of greater than 15% in the VSX3614 SSPA, where overall amplifier efficiency is defined as the ratio of RF output power to DC input power. The RF bandwidth is greater than 2% The VSX3614 SSPA is O-ring sealed and internally temperature compensated. Packaged GaN FETS and MMICS ensure high reliability under extreme environmental conditions. Table 1 summarizes the data for the VSX3614 SSPA. Figure 4 Thermal profile of VSX3614 SSPA
Frequency Range 7.6 to 9.6 GHz Peak RF Power 1 kw, saturated Pulse Width.2 to 1 microsecond Small Signal Gain 5 db small signal,45 db at nominal output power Duty Cycle 1% Pulse Droop.5 db Output Power Flatness 1 db, over selected bandwidths Harmonic Output -4 dbc maximum Inter Pulse Noise Power Density -165 dbc/hz maximum Prime Power 42 VDC at 13 Amps Weight 11 pounds, including air heat exchanger Table 1 Key Parameters for VSX3614 SSPA Output power as a function of frequency and temperature is plotted in Figure 6 for the VSX3614 SSPA. This data was taken at 1 µs pulse widths at 1% duty. Output Power dbm 65 64 63 62 61 6 59 58 57 56 VSX 3614 Output Power 55 7.6 8.1 8.6 9.1 9.6 Frequency GHz VSX3614 Saturated Power Figure 6 VSX 3614 output power at room temperature VSX3614 SSPA Life Test CPI tests the GaN transistors and the SSPA design under pulsed RF conditions to validate the robustness of the devices and amplifier design. A 1+ hour life test was conducted on the VSX3614 amplifier while operating at 1 µs pulse width and 5% duty factor. The VSX3614 SSPA was operated, at ambient temperature, in a rack assembly that mimicked the cooling air flow of a system configuration. The RF output was monitored for peak power using a USB pulsed-power sensor and the phase stability was monitored using a quadrature-detector-type phase bridge. The power and phase data was automatically logged every 5 minutes for the duration of the life test. Power supply voltages and currents and the ambient temperatures were recorded periodically. CPI s 1.5 kw Power Amplifier, the VSX3622 Figure 7 VSX 3622 is two VSX3614 SSPAs combined with fan box and slots for power supply The VSX3622 amplifier was designed for mobile, air-cooled applications. As such, the overall size and weight and efficiency are driven by the choice of the power combiners. The generation of the 1.5 kw of output power from the coherent addition of two lower power SSPA bricks. The output combiner used for this amplifier is a half-height WR9 magic T with a load port for combining isolation. The system will also include a waveguide isolator, forward and reverse power samplers in the system packaging. Frequency Range 7.6 to 9.6 GHz Peak RF Power 1.5 kw, saturated Pulse Width.2 to 1 microsecond Small Signal Gain 5 db small signal, 45 db nominal output power Duty Cycle 1% Pulse Droop.5 db Output Power Flatness 1 db, over selected bandwidths Harmonic Output -4 dbc maximum Inter Pulse Noise -165 dbc/hz maximum Power Density Prime Power 42 VDC at 25 Amps @ 1% Duty Cycle Table 2 Key Parameters for VSX3622 SSPA CPI conducted a temperature test on the VSX3622 SSPA. The pair of amplifiers comprising the VSX3622 SSPA were mounted on a chassis and cooled with a fan mounted in the box at the end of the enclosure. As shown in Figure 7, the two adjacent slots will house the system power supply cooled by the same fan. The thermal profile shown in Figure 8 depicts the chamber ambient air in blue and the output from four
thermocouples; two mounted on the body of the units and the other two mounted in the output air plenum. The graph shows that the unit operating at 1 µs pulse width and at 7% duty has a temperature rise of 15-18 C above ambient air temperature. 1..8 Pulse Droop vs Temperature @ Frequency 76.MHz 82.MHz 89.MHz 96.MHz 8 Temperature vs Time [db].6 6.4 4.2 [C] 2 2. 4 2 2 4 6 Temperature [C] Figure 1 Pulse amplitude droop vs. temperature for VSX3622 4 1 2 3 4 5 6 7 Time [Hours] Output power versus input power and frequency was measured at the temperature extremes and 25 C. Figure 8 Temperature profile for VSX 3622 2,5 VSX 3622 X Band SSPA at 25 C Data was measured and recorded every minute in the profile while frequency swept data was measured periodically. The RF output power was plotted across frequency and temperature. [dbm] 64.5 64. 63.5 63. 62.5 62. RF Power Out vs Frequency @ Temperature -4.C -3.C -2.C -1.C.C 1.C 2.C 3.C 4.C 5.C 55.C 2, 1,5 1, 5 7.6 7.7 7.8 7.9 8. 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 9. 9.1 9.2 9.3 9.4 9.5 9.6 Figure 11 Output power vs. frequency and input power at 25 C for VSX3622 dbm 61.5 61. 2,5 VSX 3622 X Band SSPA at -4 o C 6.5 6. 76 78 8 82 84 86 88 9 92 94 96 Frequency [MHz] Figure 9 Power vs. frequency and temperature for VSX3622 Power data from the pulsed power meter was measured at the 5% and 95% portion of the 1 µs pulse to record pulse amplitude droop. The RF output power droop is plotted as a function of frequency and temperature in Figure 1. 2, 1,5 1, 5 7.6 7.7 7.8 7.9 8. 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 9. 9.1 9.2 9.3 9.4 9.5 9.6 dbm Figure 12 Output power vs. frequency and input power at -4 C for VSX3622
2,5 VSX 3622 X Band SSPA at +55 o C In Figure 16 the pulse phase droop data is plotted. Figure 17 shows the AM and PM noise data. 2, 1,5 1, 5 dbm Phase [deg] I & Q [V].35.3.25.2.15.1.5..5 2 4 6 8 1 2.5 2. 1.5 1..5. 5 to 99% Pulse Top Data 7.6 7.7 7.8 7.9 8. 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 9. 9.1 9.2 9.3 9.4 9.5 9.6 Figure 13 Output power vs. frequency and input power at 55 C for VSX3622.5 1. 2 4 6 8 1 Time [us] Figure 16 Pulse droop and phase data as a function of time across a 1 µs pulse for VSX3622 The data measured at 55 C indicates the design could use more drive power at the higher end of the band to fully saturate the output devices. 9 1 11 AM and PM Noise AM PM Combined Pulsed waveforms are shown in Figures 14 and 15 at an operating frequency of 9. GHz. Figure 14 shows the output power of a 1 µs pulse at a pulse repetition frequency of 1 khz. Figure 15 shows the output power of a 1 µs pulse at a pulse repetition frequency of 1 khz. Power is measured with a pulsed power meter. These waveforms were taken at ambient temperature. [dbc/hz] 12 13 14 15 16 Frequency [Hz] Figure 17 AM and PM noise for VSX3622 Figure 14 Power at 1 µs pulse width and 1 khz pulse repetition frequency. Figure 15 Power at 1 µs pulse width and 1 khz pulse repetition frequency Figure 18 Four-amplifier module providing 2.5 kw at X-band Summary CPI has developed and extensively tested the VSX3614, the VSX363 and the VSX3622 SSPAs. CPI has demonstrated efficient and compact combining of multiple amplifiers at X-band. These amplifiers extend CPI s proud heritage of high-power, high-reliability RF transmitters into a new technology regime. CPI s GaN SSPAs can be readily combined into amplifiers with other form factors for power levels from 1 kw to 2 kw in a cost-effective manner at frequency ranges from L-band to X-band. Figure 18 shows one such form factor with four SSPAs power combined to generate 2.5 kw at X-band.