Patent Application
20060234626
The present invention relates generally to communication satellites, and more particularly, to a system and method of increasing power efficiency for such satellites.
Communication satellite payloads typically use high power amplifiers to increase power of received signals. The high power amplifiers are grouped together in redundancy rings, such that each high power amplifier within the ring has the same operating frequency range, bandwidth, and output power. For example, a commercial satellite may include 32 Ku-Band 120 W traveling wave tube amplifiers, 24 C-Band 40 W traveling wave tube amplifiers, and 38 90 W Ka-Band traveling wave tube amplifiers. Each high power amplifier is required to meet down link power or effective isotropic radiated power (EIRP) requirements equivalent to adjacent high power amplifiers contained within a redundancy ring. However, different paths through the redundancy ring have varying amounts of loss and not all of the high power amplifiers have identical EIRP requirements. Therefore, extra power is provided to high power amplifiers that have lower EIRP requirements. This is generally not power efficient.
Additionally, current high power amplifiers are limited in that they are designed and manufactured for one specific performance application. A performance application may be associated with providing service to a specific service area or serving or providing backup to a particular business service plan. Differing performance applications require different output RF power levels.
In conventional satellite payloads a high power amplifier typically includes a power supply that monitors cathode current of a high power amplifier and adjusts anode voltage to maintain a constant cathode current via an analog feedback loop. Over time, as a cathode of the high power amplifier degrades, the power supply compensates for this change by adjusting anode voltage to maintain a constant cathode current for a single designed performance application.
Current power systems for satellites determine minimum EIRP for high power amplifiers and adjust saturated power output of the high power amplifiers to the minimum EIRP, thereby reducing power consumption of the systems. Reducing the amount of power consumed by various components is desired because it not only conserves energy but also allows additional transponders to be placed upon a satellite to generate additional revenue.
Unfortunately, although the above stated systems provide high power amplifiers with variable output power that may be externally adjusted when in orbit; they do not provide a technique for maintaining constant overall power efficiency when operating with non constant amplitude (envelope) signals such as those gain of the amplifier, thus also limiting their use for multiple performance applications. For example, when RF power of a high power amplifier, such as a flexible traveling wave tube as used in the present invention, is increased by a factor of two (3 dB), gain of the amplifier can increase by a factor of 32 (15 dB). Magnitude difference of this gain change is too large to tolerate in an analog environment and as such is present when multiple carriers are being amplified by the same amplifier. This limits and potentially prevents the high power amplifier from being used in multiple performance applications.
It would therefore be desirable to provide a high power amplifier system that has reduced operating power consumption and that is capable of maintaining an approximately constant power efficiency regardless of the variability in the amplitude or envelope of the signals being amplified.
The present invention provides a power control system for satellites receiving radio-frequency signals and including traveling wave tube amplifier systems having corresponding traveling wave tubes. An anode voltage of the traveling wave tube amplifier system is used to modulate or saturate the current of the electron beam of the traveling wave tube so that the output power of the traveling wave tube amplifier system, from the saturated electron beam, is proportional to a waveform envelope (i.e. envelope-to-anode voltage) of the radio-frequency signals.
One advantage of the present invention is that the traveling wave tube amplifier system radio-frequency (RF) output power at this “effective saturation point” is as efficient (or nearly as efficient) at converting direct current (DC) power to RF power as the classical “saturation point” operation of the non-anode modulated traveling wave tube amplifier.
Another advantage of the present invention is that it provides a high power amplifier that is able to be externally controlled to maintain overall gain and efficiency, allowing the high power amplifier to be flexed over an RF output power dynamic range. In so doing, the present invention provides a high power amplifier that is capable of being used in multiple performance applications. By maintaining overall gain and efficiency, the present invention saves on power consumption.
Furthermore, the present invention reduces the amount of power consumed by a high power amplifier circuit by controlling the saturated output power level of a high power amplifier therein. In reducing the amount of power consumed in the high power amplifiers, the overall size of the spacecraft power system may be reduced or additional revenue generating transponders may be deployed on a satellite. Also, in controlling the saturation output power level, the present invention provides more DC power reduction than merely backing off drive power to the high power amplifiers.
The present invention itself, together with attendant advantages, will be best understood by reference to the following detailed description, taken in conjunction with the accompanying drawings.
In the following figures the same reference numerals will be used to identify the same components. While the present invention is described with respect to a system and method of controlling saturated output power of a power control system for a satellite, the present invention may be adapted for various high power amplifier applications known in the art. It should be understood that the present invention is applicable to various types of high power amplifiers as well as various numbers of high power amplifiers. Also, the present invention may be employed in groups or rings of high power amplifiers.
In the following description, various operating parameters and components are described for one constructed embodiment. These specific parameters and components are included as examples and are not meant to be limiting.
The present invention is a method for dynamically changing the maximum output power of a traveling wave tube and the corresponding “saturation point” of the traveling wave tube amplifier system 19 such that the traveling wave tube amplifier system 19 can operate at maximum efficiency most of the time. This can increase the power efficiency of the traveling wave tube amplifier system 19 in proportion to approximately the average amount of time that a received radio-frequency (RF) signal is not at its peak amplitude.
Referring now to
Referring now to
The communication system 14 also includes a command antenna 36 receiving command signals 38 from the ground station 16. The command antenna 36 is coupled to a command receiver 40, and ultimately to a spacecraft control processor or controller 42. The controller 42 is also part of the power control system 12. The spacecraft control processor 42 may be coupled to various components including the amplifier circuits 28.
The controller 42 is preferably microprocessor based such as a computer having a central processing unit, memory (RAM and/or ROM), and associated input and output buses. The controller 42 may be a portion of a central main control unit, or may be a stand-alone controller as shown.
Referring now to
The power amplifier 46 has an anode voltage operating range, and amplifies the received signals 21. In one embodiment of the present invention, the power amplifier 46 includes an output power range approximately equal to 3 dB. The power amplifier 46 may be one of various types of high power amplifiers and include amplifiers, such as a traveling wave tube amplifier system 19, as shown, or a solid-state power amplifier (not shown). Each of the high power amplifiers 46 has a minimum isotropic radiated power output. The minimum effective isotropic radiated power varies within rings or groupings of the high power amplifiers 46. The saturated power output of the high power amplifiers 46 within groups of high power amplifiers may be reduced on an individual basis to reduce the amount of power consumption for the satellite 10.
The power amplifier 46 includes an electronic power conditioner 56 that is coupled within the power amplifier 46 and adjusts anode voltage of the power amplifier 46 in response to a command signal or a saturated power output adjustment signal 58. The power conditioner 56, although shown as being an integral part of the power amplifier 46 may be separate from the power amplifier 46.
The power conditioner 56 includes a current sense circuit 60 that adjusts anode voltage of the high power amplifier 46 to maintain a constant cathode current at a predetermined level. The power conditioner 56 may also maintain a constant cathode current through the high power amplifier 46 via a servo loop (not shown), as known in the art. The power conditioner 56 has a telemetry and command interface 62 that may be part of the controller 42. Telemetry and command interface 62 may also act as a controller in and of itself and be part of the controller 42, or part of the power conditioner 56. Power conditioner 56 includes a DC power input 64 that is used to supply power to the power amplifier 46. The power conditioner 56 has a command input 66 and a telemetry output 68.
As mentioned, the power conditioner 56 includes a programmable circuit 76. Although, the programmable circuit 76 is shown as being an integral part of the power conditioner 56. Also, any number of programmable circuits may be utilized within the amplifier circuit 28, even though only one is shown in
In one embodiment of the present invention, the programmable circuit 76 includes a look-up table 72, containing various parameters, such as gain, offset, set-point, anode voltage, cathode current, and phase and their respective relationship to each other for a plurality of complementary states of associated output saturated power. The table 72 may be established during and from ground testing, and any number of look-up tables may be utilized.
Referring to
The digital signal processor 80 includes two signal paths, one for processing the RF signals (RF signal path 89) and the other for separating out and processing the envelope of the RF signal (envelope signal path 91). Simultaneous to the anode modulation of the electron beam current in the envelope signal path 91, the RF applied to the RF input of the traveling wave tube amplifier system 19 is modulated through the RF signal path 89. Prior to application to the traveling wave tube amplifier system RF input, the RF signal is modified in such a manner that when it is applied to the RF input of the traveling wave tube 90 along with the envelope modulated electron beam, the traveling wave tube generates a distortion free “saturated” output power at each moment in time (“distortion free” generally refers to a signal free of Amplitude-to-Amplitude Conversion—“AM/AM”, Amplitude-to-Amplitude Conversion “AM/PM”, and so called “memory effects”).
The traditional term for separating the envelope (envelope signal path 91) from the signal (RF signal path 89) and resizing an amplifier to improve power added efficiency is known as “Kahn Amplification”. The traditional implementation of “Kahn Amplification” was applied to solid state power amplifiers by modulating the “DC Bias” line of the transistor collector. The modulation was achieved at a rate proportional to the signal bandwidth and at a power level proportional to the device power level. Generally the product of the bandwidth and the power level determines the difficulty or feasibility of constructing a modulator. In solid state power amplifiers having low power levels (0.25-10 Watts) and their small bandwidths for traditional cell phone or broadcast radio applications (5 Khz-200 Khz), the modulator is relatively simple and inexpensive to implement.
For satellite applications with traveling wave tube amplifiers, following the traditional approach proposed by Kahn, the collector voltage of the traveling wave tube would have to be modulated. With typical satellite bandwidths of 25-500 MHz and power levels on the order of 50-500 Watts, modulating the cathode is orders of magnitude more difficult than it is for SSPAs. This would be extremely difficult with prior technology, and hence prior to the present invention there were no commercial Kahn traveling wave tube amplifiers.
By considering the problem of modulating the electron beam rather than the cathode itself, several other possible implementations of Kahn are generated. The anode voltage, for example, is an excellent point to modulate the beam because although it is a high voltage, the anode draws nearly zero current and hence nearly zero power. Thus, the modulator itself is not required to provide any power, only a high voltage varying at a high frequency.
The digital signal processor 80 receives, for example, high power amplifier radiated power outputs and processes these signals, as discussed below. The digital signal processor 80 includes an oversample module 92, a digital limiter 94, a complex look-up table 96, a digital delay 98, an envelope module 100, and a look-up table with envelope-to-anode voltage conversion 102.
In one embodiment of the present invention, prior to up-conversion, the digital signal processor 80 or a digital signal processing circuit, which may include an Application Specific Integrated Circuit (ASIC) or Field Programmable Gate Array (FPGA), is used to adjust the envelope and RF modulation signals in order to compensate for the non-linearities of the modulator circuits, the traveling wave tube amplifier 90, and the relative delay of the envelope and the RF signals such that the signal coming out of the RF output port is a linearly amplified reproduction of the RF signal.
Input RF signals are received in the digital signal processor oversample module 92, which adds in-phase (I) and quadrature (Q) signals, i.e. I+Q of the signal. In one embodiment, the waveform is “over-sampled” by three-to-five times the bandwidth of the signal to allow for correct accounting of the non-linear quality of the typical values in the look-up tables 96, 102.
Signals from the oversample module 92 are received in the digital limiter 94 and limited thereby. In other words, the digital limiter 94 is a nonlinear electronic circuit having output limited in amplitude; used to limit the instantaneous amplitude of the oversample signal waveform (i.e. to clip off the peaks of the waveform). This limited signal is then linearized in the complex look-up table 96, which cycles AM/AM and AM/PM conversions.
The look-up table signal is received in the digital delay 98, which delays the RF signal of the RF signal path 89 for synchronization with the envelope signal of the envelope signal path 91. The delay signal is then converted to an analog signal in the first digital-to-analog converter 84. If, as in one embodiment, the RF Signal is in digital form, it is “up-converted” in the up-converter 88 to an appropriate frequency prior to being applied to the traveling wave tube RF Input 103 of the traveling wave tube 90.
The envelope module 100, on the envelope signal path 91, separates the RF envelope from the RF signal. The envelope of the RF signal is used to modulate at least one of the voltage or the current of the electron beam in the traveling wave tube. Signals from the envelope module 100 are received in the look-up table 102, which converts the envelope signal to an anode voltage signal. The look-up table signals are received in the second digital-to-analog converter 84 and converted to analog signals, which are amplified in the high voltage amplifier and impedance transformer 86 prior to reception in the anode control 104 of the traveling wave tube 90.
The RF signals amplified by the traveling wave tube amplifier system 19, with the use of non-constant envelope waveforms such as bandwidth limited quadrature phase-shift keying (QPSK), bandwidth efficient modulation (BEM) or quadrature amplitude modulation (QAM), orthogonal frequency-division-multiplexing (OFDM) and multi-carrier, include both phase and amplitude modulation. The present invention operates the traveling wave tube amplifier system 19 in a “backed off” state so that the signal peaks are not distorted. With this method; however, when the RF signal has amplitude that is below the peak, the traveling wave tube amplifier system 19 may be operating in a power inefficient manner. The typical non-constant envelope waveform RF signal is on average at its peak amplitude for only a fraction of the time. The traveling wave tube amplifier system 19 is exceptionally efficient when it is operated at its “saturation point” where the input amplitude of the signal is equal to the input power level that achieves the maximum output power of the traveling wave tube.
The envelope of the RF signal (i.e. envelope signal in the envelope signal path 91) is used to modulate at least one of the voltage and current of the electron beam of the traveling wave tube 90.
In one embodiment of the present invention, the anode voltage of the traveling wave tube amplifier system 19 is used to modulate the current of the electron beam in such a manner that the output power of the traveling wave tube amplifier system 19 from the saturated electron beam is proportional to the waveform envelope (i.e. envelope-to-anode voltage). The traveling wave tube amplifier system 19 radio-frequency (RF) output power at this “effective saturation point” is as efficient (or nearly as efficient) at converting direct current (DC) power to RF power as the classical “saturation point” operation of the non-anode modulated traveling wave tube amplifier.
An alternate embodiment of the present invention includes any other method for modulating the electron beam of the traveling wave tube amplifier system 19 using the envelope of the RF signal. For example, three other implementations include: a) directly modulating the cathode for power added efficiency, b) splitting the helix from ground and modulating the helix and c) using the envelope to modulate a field emitter array (FEA) “cold cathode”.
One primary point of novelty of the present invention is the variation of “Kahn modulation” that results in greater modulation bandwidth and resulting output power. Specifically, the present invention boosts the maximum power output of traveling wave tube amplifiers by modulating the electron beam rather than the cathode.
The present invention is a break-through in two respects. While the general prior art implementations of “Kahn Modulation” can greatly improve the power added efficiency compared with non-beam modulated amplification techniques (on the order of a factor of two), these implementations are generally not applicable for broadband (>>1 MHz) or high power (>50 W) systems. The invention implements a novel variation of Kahn that is both broadband and high power.
From a physics point of view, the effect of modulating the electron beam can be accomplished via other processes, such as modulating the electron beam directly, i.e. via the cathode. Typically the speed or bandwidth of any analog part and the power it consumes is limited by the amount of power that flows though it.
Referring now to
In operation block 204, the controller 42 or the command interface generates a saturated power output adjustment signal for the high power amplifier 46 in response to the command signal.
The command signal includes a new power level that is contained in a look-up table, such as look-up table 72, along with associated adjustment values for a corresponding state.
The command signal may be in the form of a saturated power output adjustment signal and may be directly received by the power conditioner rather than from the controller 42. The saturated power output adjustment signal contains a desired output power level for the power amplifier 46.
In operation block 206 the envelope of the RF signal is used to modulate at least one of the voltage and current of the electron beam of the traveling wave tube 90. The anode voltage of the traveling wave tube 90 is adjusted through the envelop signal within an anode voltage range to provide the constant cathode current associated with that saturated output power state.
The power conditioner 56 then resets an operating set-point of the high power amplifier circuit 28 in response to the saturated power output adjustment signal through use of the look-up table 72. The power conditioner 56 determines an appropriate cathode current for a commanded output power in response to the saturated power output adjustment signal. The look-up table 72 includes a relationship between output power level and cathode current. The power conditioner 56 is capable of determining appropriate anode voltage for specified cathode current for a requested output power level of the traveling wave tube 19.
For the traveling wave tube amplifier system 19, the anode voltage is used by the power conditioner 56 for power control. For other amplifiers, such as a solid-state power amplifier (not shown) the drain voltage of a field effect transistor may be used to control the saturated output power.
In step 208, amplifier 46 gain offsets the received signals in response to the saturated power output adjustment signal through use of the look-up table 72. Overall gain offset for a particular power level may include adjustment of any of the attenuators 48. The look-up table 72 also contains a relationship between cathode current and nominal gain, such that as cathode current changes the amplifier 46 selects an appropriate gain for a current cathode current level. In gain offsetting the received signals the amplifier 46 compensates change in anode voltage to maintain an approximately constant overall gain of the power amplifier circuit 28. For example, when the output power is increased or the anode voltage is increased the gain is decreased and vice versa.
In step 210, the linearizer 50 compensates for the cathode current adjustment by adjusting nominal saturated phase and gain compression of the received signals, for the new saturated power level, to aid in providing an adjustment to minimize distortion in the output power of the power amplifier 46. Linearizer settings are adjusted through use of the look-up table 72
In step 212, the traveling wave tube 19 amplifies the received signals in response to change in the cathode current.
In step 214, the amplifier circuit 28 continues monitoring and maintaining operation at the new set-point.
The above-described steps are meant to be an illustrative example; the steps may be performed synchronously, sequentially, simultaneously, or in a different order depending upon the application.
The present invention provides a power control system for a satellite that is capable of operating in multiple performance applications due to its ability to conduct in-orbit adjustments of output power of high power amplifier circuits while maintaining a constant overall gain.
In operation, a power control method for a traveling wave tube amplifier system includes receiving an RF signal; generating a processed RF signal from the RF signal; generating an envelope of the RF signal from the RF signal; generating an electron beam; and modulating at least one of a voltage or a current of an electron beam generated in the traveling wave tube amplifier system with the envelope of the RF signal.
While the invention has been described in connection with one or more embodiments, it should be understood that the invention is not limited to those embodiments. On the contrary, the invention is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the appended claims.
