Telecommunications – Transmitter – Amplitude modulation
Reexamination Certificate
1998-04-04
2001-07-03
Nguyen, Lee (Department: 2683)
Telecommunications
Transmitter
Amplitude modulation
C455S093000, C455S102000, C455S108000, C455S127500, C330S199000, C330S200000, C332S149000, C332S159000
Reexamination Certificate
active
06256482
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the field of radio transmitters and radio-frequency or microwave power amplifiers and more specifically to methods of improving efficiency in Kahn envelope-elimination-and-restoration (EER) circuits and in high-level amplitude modulation circuits and to modified Kahn envelope-elimination-and-restoration circuits.
BACKGROUND ART
AM radio signals are used in a variety of applications, including broadcast, non-directional navigation beacons, citizens-band radios, and aircraft communication. Various other radio signals with more complex modulations have time-varying amplitudes (envelopes) and can be regarded as having simultaneous amplitude and phase modulation. Examples of complex modulations include Single Sideband (SSB), Independent Sideband (ISB), Vestigial Sideband (VSB), multitone data, multiple carriers amplified simultaneously, and modem shaped-pulse digital-data modulation. Variable-amplitude radio signals are also required in applications such as magnetic-resonance imaging (MRI) and industrial-scientific-medical (ISM) devices.
AM transmitters can be implemented by a variety of techniques (see H. L. Krauss, C. W. Bostian, and F. H. Raab, “Solid State Radio Engineering” Chapter 15, New York, Wiley, 1980), but high-level amplitude modulation is widely regarded as preferable for both quality and efficiency. In high-level amplitude modulation, the main DC supply-voltage input to the final RF power amplifier is varied in proportion to the desired signal amplitude. The RF amplifier is operated in or close to saturation (i.e., at the top of or above its linear operating region). The amplitude (envelope) of the RF output is thereby caused to vary with the supply-voltage input. Throughout this specification and the appended claims, the terminology “high-level modulation,” “high-level modulator,” etc. refers to such modulation of the main DC supply-voltage input to the final RF power amplifier. It is worth noting that the terms “drain bias” or “collector bias” are sometimes used to refer to a supply-voltage input, especially in microwave engineering. In the present specification and appended claims, the term “supply-voltage input” is meant to include these connection points and any other kind of connection at which the supply voltage enters an amplifier.
High-level amplitude modulation can be used with more complex signals such as SSB through the Kahn Envelope-Elimination-and-Restoration (EER) technique (see L. R. Kahn, “Single Sideband Transmission by Envelope Elimination and Restoration,” Proc. IRE, vol. 40, no. 7, pp. 803-806, July 1952). In the classical form of a Kahn-technique transmitter, a limiter eliminates the envelope, producing a constant-amplitude, phase-modulated carrier which becomes the drive to the final amplifier. The detected envelope is amplified by an audio-frequency power amplifier. Amplitude modulation of the final RF power amplifier restores the envelope to the phase-modulated carrier, creating an amplified replica of the input signal. In a modern implementation, the envelope and phase-modulated carrier are produced by a combination of digital signal processing and synthesis.
High efficiency is needed for a variety of reasons. In high-power broadcast transmitters, efficiency determines the consumption of prime AC power and therefore the operating cost. In space-borne and portable transmitters, efficiency determines the size of the battery, power supply, and heat sink. Hence, highly efficient transmitters can be made much smaller and lighter than conventional transmitters. In all cases, improving efficiency reduces the heat dissipated in the RF-power devices, and the resultant lower temperatures increase reliability.
Efficiency can be improved by using a high-efficiency RF power amplifier, a high-efficiency modulator, and a technique such as Kahn EER. However, a limitation on efficiency for low signal levels remains. Often, transmitters must produce low-amplitude signals for a significant portion of the time; hence the efficiency in producing these signals dominates the overall average efficiency.
Drive power is a significant contributor to inefficiency when the transmitter is producing a low-level output. It is well known that the drive (for ideal power amplifiers) can be made to vary with the envelope of the output signal. However, in most real RF-power devices, the gain decreases at lower supply-voltage inputs, which causes them to cease amplification. Furthermore, efficient modulators such as class S modulators work best with fixed, known loads and behave erratically if their load (the RF power amplifier) ceases to draw current.
A detailed discussion of the impact of signal characteristics upon the average efficiency of power amplifiers is given by F. H. Raab, “Average Efficiency of Power Amplifiers,” Proc. RF TECHNOLOGY EXPO '86, Anaheim, Calif., pp. 474-486, Jan. 30-Feb. 1, 1986. The instantaneous efficiency (See
FIG. 1
) of an ideal class-A power amplifier increases with the square of its output voltage, reaching 50 percent at peak-envelope-power (PEP) output. The efficiency of an ideal class-B power amplifier increases linearly with the output voltage to 78.5 percent (=&pgr;/4) at PEP (see H. L. Krauss, C. W. Bostian, and F. H. Raab, “Solid State Radio Engineering,” Chapter 12, New York, Wiley, 1980). In practice, losses in MOSFETs due to resistance reduce the efficiency by 10 to 20 percent, resulting in maximum instantaneous efficiencies of about 40 and 60 percent for class-A and -B power amplifiers, respectively. The presence of load reactance degrades the efficiency even further.
The efficiency of switching-mode power amplifiers (classes D, E, and F) as well as class-C power amplifiers is generally higher than that of a linear power amplifier (class A or B). Because variation of the output amplitude is achieved through variation of the DC supply voltage, the instantaneous efficiency of these power amplifiers remains high for all signal levels. Given proper drive, the efficiency of a class-D power amplifier is subjected to only minor degradation by a reactive load.
Class-D power amplifiers typically achieve Peak Envelope Power (PEP) efficiencies from 75 to 90 percent. For power amplifiers that use Bipolar Junction Transistors (BJTs), the efficiency decreases at lower signal amplitudes because the BJT saturation voltage becomes a more significant fraction of the supply-voltage input. However, for power amplifiers that use MOSFETs, the instantaneous efficiency is largely independent of the output voltage. Saturated power amplifiers of any class generally maintain relatively constant efficiency near the value for peak output.
Continuous Wave (CW) and Frequency Modulation (FM) signals are characterized by constant envelopes and therefore are always at PEP. In contrast, SSB-voice, multitone-data, noise, and shaped-pulse data signals have time-varying envelopes with significant peak-to-average ratios &xgr; (typically 6-10 dB).
The probability-density functions (PDFs) of
FIG. 2
represent the relative amounts of time that the envelope spends in the vicinity of the corresponding output voltage. The Rayleigh PDF is produced by noise or a multitone signal, while the Laplacian PDF is produced by SSB speech. The PDF of square-root raised-cosine offset quadrature-amplitude modulation (SRRC DQAM in
FIG. 2
) is typical of that of most modern shaped-pulse digital modulations (See L. Sundstrom, “The Effect Of Quantization In A Digital Signal Component Separator For LINC Transmitters,” IEEE Trans. Veh. Technol., vol. 45, no. 2, pp. 346-352, May 1996).
Upon comparison of the instantaneous-efficiency and PDF curves in
FIG. 2
, it is immediately apparent that the instantaneous efficiencies differ greatly at the signal amplitudes that are most prevalent in real amplitude-modulated signals. To compare different amplifiers with different signals, it is useful to define the average efficiency as
η
AVG
=
P
oAVG
P
iAVG
where P
oAVG
and P
iAVG
are the average power output and input, respectively.
The aver
Nguyen Lee
Nguyen Simon
Touw Ted
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