Historically, the term microwave power module (MPM) has been associated with a small, fully integrated, self-contained radio frequency (RF) amplifier that combines both solid-state and microwave vacuum electronics technologies. Typically, the output power of these MPMs is on the order of about 100 Watts CW over an octave bandwidth. The MPMs require both a solid-state amplifier at the front end and a microwave vacuum electronics amplifier at the back end. However, such MPMs cannot be utilized for communications because the MPMs are not optimized for linearity or efficiency. Also, the MPMs can be very expensive to manufacture, particularly when modules are produced in very small quantities for space applications. Also, a kilovolt (kV) class power supply is required to power the traveling-wave tube amplifier, which is a part of the microwave vacuum electronics.
The innovation presented here is a wideband, high-power, high-efficiency, solid-state microwave power module (SSMPM) or amplifier for a multifunction spacecraft payload that operates, depending on the need, as a radar system, communication system, or a navigation system. The construction of the module is based on a wideband multistage amplifier design. The low-power stage is a high-efficiency GaAs pHEMT-based MMIC distributed amplifier. The medium-power stage is either a high-efficiency GaAs pHEMT or GaN HEMT-based MMIC distributed amplifier. The high-power stage is a high-efficiency GaN HEMT-based MMIC distributed amplifier.
The gate and drain voltages and currents for the amplifier stages are provided by an electronic power conditioner (EPC). The EPC is a DC-to-DC power converter that transforms the spacecraft bus voltage into appropriate voltages required by the amplifier stages. In addition, a DC power management circuit is included to manage the correct power-up and power-down sequence to ensure that the negative gate voltage is applied before the amplifier is turned on. A DC blanking control is also provided to quickly turn the amplifier off if a fault condition arises. Moreover, an RF output monitor such as a temperature sensor or a detector/reference diode pair is located near or on the highpower GaN die, in the output stage, to monitor an over-temperature condition. The detector/reference diode pair also monitors the RF output power level. The packaged unit is conduction cooled. The mode of operation involves amplifying the incoming signal in the pre-amplifier stage, and boosting the power to a level sufficient to drive the medium-power amplifier across its dynamic range. Likewise, the output of the medium-power amplifier then drives the high-power amplifier across its dynamic range. The dynamic range is the difference between 1dB compression point (P=1dB) and the minimum detectable output power (Pout min).
The module can also be constructed with three or more narrow-band GaAs pHEMT low-power MMIC amplifiers that cover different frequency bands designated for radar, communications, and navigation at the input. A wideband SP3T PIN diode then switches each amplifier in and out of the circuit as required. The advantage of this circuit is that the noise figure of the low-power stage can be optimized for a particular function. As an alternate, the switch can be replaced by a bandpass filter. The bandpass filters prevent any cross talk among the three signal paths. The advantage of this approach is that it eliminates the PIN diodes and the switch control circuitry.
Typically, the navigation and radar functions are performed at S-band and X-band frequencies, respectively. However, for communications, a K-band or a Ka-band narrow-band, high-linearity, GaAs pHEMT, medium-power MMIC amplifier is employed, which directly drives the high-power GaN MMIC distributed amplifier. The two bandpass filter circuits at the input to the high-power amplifier ensure that the cross talk between the two signal paths is minimal.