Microwave Power Combiner
communications
Microwave Power Combiner (LEW-TOPS-72)
Enables MMIC amplifiers to be combined more efficiently for use in near-Earth and deep-space communications
Overview
To achieve power levels necessary for communications in space, several monolithic microwave integrated circuit (MMIC) amplifiers must be combined. One problem with this process is that conventional power combiners require MMICs with identical amplitude and phase characteristics, but even minor variations in manufacture cause MMIC power outputs to be unequal. To solve this problem, scientists at NASA's Glenn Research Center have developed a novel waveguide hybrid power combiner for solid-state power amplifiers based on MMICs. Glenn initially demonstrated the proof-of-concept at Ka-band frequencies, from 31.8 to 32.3 GHz, which is NASA's deep-space frequency band. However, the design can be scaled to other frequencies of interest.
The Technology
MMICs are a type of integrated circuit that operates at microwave frequencies to amplify electronic signals. The system has at least two power amplifiers; input ports to receive power from the amplifiers; at least one power combiner, which receives power from each input port and combines them to produce maximized power; an output port that sends this maximized power to its destination; and an isolated port, either grounded or match-terminated, that receives no or negligible power from the combiner. The output port can be connected to a load, and can employ more than one combiner, so that the power from another combiner and an input port can be combined, for example, in a 3-way unequal power combiner.
Glenn's Ka-band demonstration power combiner has an output return loss better than 20 dB, and a high degree of isolation between the output port and the isolated port, as well as between the two input ports. When the ratio of output power for two MMICs is two-to-one, the combined efficiency is better than 90%. However, the design is not limited to a two-to-one ratio; it can be customized to any arbitrary power output ratio. This means that a low-power gallium arsenide MMIC can be combined with a high-power gallium nitride MMIC, giving designers much more flexibility. The output impedance of the MMIC power amplifier is matched directly to the waveguide impedance, without first transitioning into a transmission line. This technique eliminates the losses associated with a transition and enhances the overall efficiency. Furthermore, the MMIC power combiner is dual purpose- run in reverse it serves as a power divider. To reduce the cost and weight the combiner can be manufactured using 3-D printing and metal-plated plastic. By combining MMIC amplifiers more efficiently, Glenn's technology greatly enhances communications from near-Earth and deep space-to-Earth.
Benefits
- Flexible: Enables the efficient combination of multiple MMICs, even when each possesses different output power
- Economical: 3-D printing permits use of metal-plated plastic, reducing material, cost, and weight
- Scalable: Features a design that can be scaled to any arbitrary power-combining ratio and port impedance and to a wide range of communication frequencies
- Versatile: Can be run in reverse to create a power divider
Applications
- Near-Earth and deep-space communication
- Microwave/Millimeter-wave frequency solid-state power amplifiers
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Similar Results
Solid-State Microwave Power Module
Typically, microwave power modules (MPMs) are useful only for radar and navigation purposes because they lack the linearity and efficiency required for communications. In standard configurations, conventional MPMs require both a solid-state amplifier at the front end and a microwave vacuum electronics amplifier at the back end. By contrast, Glenn's design features a wideband multi-stage distributed amplifier system. The low-power stage is a high-efficiency gallium arsenide (GaAs) pseudomorphic high-electron-mobility transistor (pHEMT)-based monolithic microwave integrated circuit (MMIC) distributed amplifier. The medium-power stage is configured to pick up and amplify the low-power signal. This stage can be either another high-efficiency GaAs pHEMT or a gallium nitride (GaN) HEMT-based MMIC distributed amplifier, depending on the need. The high-power stage, configured to pick up the signal from the second amplifier, is a high-efficiency GaN HEMT-based MMIC distributed amplifier, which supplants the traveling-wave tube amplifier found in most microwave power modules.
In Glenn's novel MPM, the radar functions as a scatterometer, radiometer, and synthetic aperture imager. The high-speed communications system down-links science data acquired by Earth-observing instruments. The navigation system functions like a transponder for autonomous rendezvous and docking, and estimates the range information. Glenn's MPM gives systems the versatility to use a single power module to drive not only radar and navigation but also communications systems.
Signal Combiner for Wideband Communication
Through low-loss signal combination, Glenn is leading the way to optimize radio transmission remotely during self-checking routines. Glenn's signal combiner offers a simple method to minimize signal loss significantly when combining two signals. Using conventional combiners in bit-error-rate testing results in a loss of 3 to 4 dB per band, and with a directional coupler the secondary signal experiences losses of 10 dB or more. Moreover, during signal measurements, the additional components must be placed and later removed to prevent any impact to the measurement, making for a cumbersome process. Glenn's solution is to combine the primary and secondary signals in the frequency domain through the use of a frequency division diplexer/multiplexer in combination with a wideband ADC. The multiplexer selects one or more bands in the frequency domain, and the ADC performs a non-linear conversion to digital domain by folding out-of-band signals in with the primary signal. NASA makes use of subsampling a given band within the ADC bandwidth to fold it into another band of interest, effectively frequency-shifting them to a common frequency bandwidth.
Glenn's breakthrough method has two significant advantages over the conventional use of a power combiner or directional coupler in bit-error-rate testing: 1) it combines signal and noise (secondary signal) with very low loss, and 2) it enables the selection of the desired signal-to-noise ratio with no need for the later cumbersome removal of components. This streamlined process allows for invaluable in-situ or installed measurement. Glenn's novel technology has great potential for satellite, telecommunications, and wireless industries, especially with respect to equipment testing, measurement, calibration, and check-out.
Multimode Directional Coupler
Glenn's researchers originally created the MDC to improve the beacon sources for atmospheric propagation studies. These studies are typically conducted to test atmospheric conditions to determine the signal strength needed for satellite communications. A low-power transmitter (e.g., a beacon source) is attached to the satellite, and transmits a continuous waveform (CW) signal to a receiving station on Earth. However, when a separate frequency is desired, building a new beacon source for the transmitter on the satellite - especially one that will operate at higher frequencies - presents numerous challenges. For one, a single-frequency beacon source requires a temperature-stabilized oscillator for frequency generation separate from that provided by the spacecraft receiver.
To solve such problems, Glenn's innovators fabricated the MDC from two sections of waveguide: a primary waveguide for the fundamental frequency (Ku-band), and a secondary waveguide for the harmonics (Ka-band). These sections are joined together so that precision-machined slots in the second waveguide selectively couple the harmonics, for amplification and transmission. The harmonics can then be used as an additional beacon source with very small power losses to the fundamental signal. Once the separation takes place, the second or higher harmonic can be amplified and transmitted to a station on Earth. The efficiency and performance of the MDC can be optimized through appropriate computer modeling software and currently available high-precision fabrication techniques. Without the complexity and expense involved in building separate traveling wave tube amplifiers to generate additional frequencies, Glenn's MDC enables satellites to produce multiple signals that can be received by multiple stations - a significant leap forward in satellite productivity.
A two-way microwave power divider using microstrip transmission lines
The power divider use Klopfenstein tapered transmission lines on each output branch of the junction impedance that is matched the input port. Thus, the output lines are well matched to the input, and a reflected power of 1% can be easily achieved. Resistors are distributed along the transmission lines to provide isolation between the two output ports which prevents power of one output port from coupling to the other output port. A large amount of the power is dissipated in the resistors rather than exiting through any other ports in the system. Due to the symmetry of the design, very little power is dissipated during normal operation.
The resulting power divider is operable at high bandwidths as the tapered impedance match which have no upper frequency limitation. Additionally, the tapered lines eliminate many discontinuities in the layout which in turn reduce microwave junction effects. The power divider is capable of being manufactured using known methods and may be utilized in a compact microwave spectrometer.
Optical Tunable-Based Transmitter for Multiple High-Frequency Bands
NASA Glenn's researchers have developed a means of transporting multiple radio frequency carriers through a common optical beam. In contrast to RF infrastructure systems alone, this type of hybrid RF/optical system can provide a very high data-capacity signal communication and significantly reduce power, volume, and complexity. Based on an optical wavelength division multiplexing (WDM) technique, in which optical wavelengths are generated by a tunable diode laser (TDL), the system enables multiple microwave bands to be combined and transmitted all in one unit. The WDM technique uses a different optical wavelength to carry each separate and independent high-frequency microwave band (e.g., L, C, X, Ku, Ka, Q, or higher bands). Since each RF carrier operates at a different optical wavelength, the tunable diode laser can, with the use of an electronic tunable laser controller unit, adjust the spacing wavelength and thereby minimize any crosstalk effect.
Glenn's novel design features a tunable laser, configured to generate multiple optical wavelengths, along with an optical transmitter. The optical transmitter modulates each of the optical wavelengths with a corresponding RF band and then encodes each of the modulated optical wavelengths onto a single laser beam. In this way, the system can transmit multiple radio frequency bands using a single laser beam. Glenn's groundbreaking concept can greatly improve the system flexibility and scalability - not to mention the cost of - both ground and space communications.
