1. Field of the Invention
The present invention relates to high efficiency microwave power amplifiers (HEMPA) used in communication devices. More particularly, the present invention relates to high efficiency microwave power amplifiers operating in the S-Band region, which provide communication for spacecrafts and satellites using systems such as the Tracking and Data Relay Satellite System (TDRSS) implemented by NASA.
2. Description of the Related Art
In communication systems, there is a strong need for power amplifiers with high efficiency to maximize the amount of talk time obtained from a power source. This need is especially important for communications involving spacecraft, where available power is very limited, and the amounts and types of payloads which are provided for scientific experiments are adversely affected by lower communication efficiencies.
The space shuttle orbiter communication system is used for transferring telemetry information regarding orbiter operating conditions and configurations, systems and payloads either directly with the ground, or through TDRSS. The commands sent to the orbiter system allow for functional or configurational changes.
In addition, documentation sent from ground control, which is printed on the orbiter""s teleprinter or text and graphics system, as well as voice communications among the flight crew members and/or between ground control, are within the control of this system.
In addition, certain communications in the TRDSS system are within the S-Band of the RF spectrum (microwaves ranging approximately from 1,700 to 2,300 MHz). In particular, S-band FM and S-Band PM systems can be used to transfer information between the space shuttle orbiter communication system and the ground on RF signals in the S-Band frequency range.
There are several known classes (e.g. A, B and C) of amplifiers, each class having various operational characteristics specific to the respective class.
For example, Class-D amplifiers (shown in FIG. 1), which have two transistors (Q1 and Q2) in a push-pull configuration, are suited for applications requiring reduced power. One advantage of using Class-D amplifiers is that the transistors function as switches instead of current sources, which is particularly advantageous for generating square waves.
In addition, Class-D amplifiers allow for high voltages to operate across a device and for large current to operate through the device, but not at the same time. This arrangement saves power, and is a primary reason why Class-D amplifier have very good efficiency.
Class-D amplifiers have been used extensively for frequencies ranging from the Low-Frequency (LF) to High Frequency (HF), with an efficiency of approximately 85-90 percent in the HF range.
At the lower frequency bands, Class-D amplifiers are assumed to have: (1) transistors operating with zero on-resistance; (2) infinite off-resistance; zero saturation voltage; (3) instantaneous switching and perfect timing; and (4) negligible capacitance and inductance.
However, at the microwave range of the RF spectrum, particularly in the S-band, the above assumptions regarding Class-D amplifiers are invalid. The operating efficiency of a Class-D Amplifier in the microwave range is well below the 85-90 percentage achieved at the HF range.
The efficiency losses of the Class-D Amplifier, particularly in the microwave range, result from switching, conduction and gate drive losses. As the operating frequency increases, so does the associated losses.
Accordingly, the Low Frequency and High Frequency assumptions regarding power generation for Class-D Amplifiers are not applicable in the S-Band region. There remains a need for new design techniques for a Class-D Amplifier which operates in the S-Band region with high efficiency.
Accordingly, it is an object of the present invention to develop a high efficiency microwave power amplifier.
To this end, according to the present invention, there is provided a process for providing a High Efficiency Microwave Power Amplifier (HEMPA) which propagates a microwave frequency square wave, wherein said method utilizes a circuit simulation program for simulating Field Effect Transistors (FETs) at high DC-to-RF efficiencies, and comprises the steps of:
(a) analyzing linear elements of a plurality of selected FETs in a frequency domain;
(b) analyzing non-linear elements of the plurality of selected FETs in a time domain;
(c) converting the time domain values obtained in step (b) into the frequency domain by using a Discrete Fourier Transform (DFT) of the time domain values;
(d) performing a series of DC and S-parameter simulated measurements based on predefined data specific to each one of the selected FETs contained in the simulation program and from values obtained in steps (a) to (c);
(e) extracting and isolating individual device parameters of the selected FETs by converting the S-parameters to one of admittance and impedance parameters to derive FET models for each of the selected FETs;
(f) employing the FET models derived in step (e) in a simulated amplification circuit of the simulation program to provide a final output of a HEMPA circuit based on iterative simulations of the amplification circuit utilizing microwave topology at microwave frequencies, wherein the iterative simulations of the amplification circuit include analyzing output values of a plurality of cascaded stages of the selected FETs, and said cascaded stages including at least an input stage and an output stage of FETs, with each stage having FETs arranged in a push-pull configuration;
(g) receiving the final output of the HEMPA circuit from the simulation program after the iterative simulations provided in step (f) are a sufficient quantity to provide values of components for connection with the cascaded stages of FETs to provide said HEMPA circuit.
In an embodiment, the FET models employed in step (f) include a driver stage cascaded between said input stage and said output stage.
In an embodiment, the iterative simulations may be provided for microwave frequencies in the S-band region. In another aspect of the invention, the push-pull configuration of FETs employed in step (f) in each of the cascaded stages are Class D amplifiers.
In another aspect of the invention, the extracting and isolating of the FETs to derive the models provided in step (e) may comprise:
(1) previewing device operation by verifying a value of gate current (Ig) vs. a voltage from gate-to-source (Vgs);
(2) measuring a drain current (Id) while sweeping a gate voltage the FETs;
(3) varying the drain current (Id) with respect to drain voltages (Vd) at a plurality of values of gate voltage to obtain a Family of Curves;
(4) previewing source, drain, and gate resistances of the FETs by using the Yang-Long method;
(5) providing a final measurement of the resistances previewed in step (4) by the Yang-Long method;
(6) extracting intrinsic and extrinsic parasitics from S-parameter data;
(7) measuring ideality (I) of the FETs by measuring the gate voltage of the FETs while opening the drain of said FETs;
(8) measuring values of Vg with Vd s at a constant value of Vds0;
(9) measuring Id vs. Vd while varying Vg; and
(10) performing sweeping S-parameter measurements by sweeping a signal applied to the gate and measuring the S-parameters at the drain of the FETs.
In an embodiment, the present process may include employing FET models recited in step (f) includes for each respective stage:
(1) using ideal component values in regards to frequency response and power loss during initial iterative simulations in the simulation program; and
(2) replacing the ideal component values recited in step (f)(1) with realistic values based on manufacturer""s specification of each respective component during latter iterative simulations so as to provide a more realistic response of said HEMPA circuit.
In addition, a high efficiency microwave power amplifier product according to the above processes are clearly within the scope of the invention.