Power modulation systems and methods that separately amplify...

Details Power modulation systems and methods that separately amplify... Power modulation systems and methods that separately amplify...

1999-05-17

2003-07-08

Liu, Shuwang (Department: 2634)

Pulse or digital communications

Transmitters

Antinoise or distortion

C375S295000, C330S149000, C332S145000

Reexamination Certificate

active

06590940

ABSTRACT:

FILED OF THE INVENTION
This invention relates to modulation systems and methods and more particularly to systems and methods that can efficiently modulate a signal onto a carrier.
BACKGROUND OF THE INVENTION
Modulation systems and methods are widely used in transmitters to modulate information including voice and/or data onto a carrier. The carrier may be a final carrier or an intermediate carrier. The carrier frequency can be in UHF, VHF, RF, microwave or any other frequency band. Modulators are also referred to as “mixers” or “multipliers”. For example, in a mobile radiotelephone, a modulator is used in the radiotelephone transmitter.
In modern radiotelephone communications, mobile radiotelephones continue to decrease in size, cost and power consumption. In order to satisfy these objectives, it is generally desirable to provide modulation systems and methods that can provide high power modulation while reducing the amount of battery power that is consumed. Unfortunately, the power amplifier of a modulator may consume excessive power due to efficiency limitations therein. More specifically, it is known to provide linear Class-A or Class-AB power amplifiers that may have efficiencies as low as 30 percent or less. Thus, large amounts of battery power may be wasted as heat.
A major breakthrough in high efficiency modulation systems and methods is described in application Ser. No. 09/195,384 to the present co-inventor Camp, Jr., et al., entitled Circuit and Method for Linearizing Amplitude Modulation in a Power Amplifier (now U.S. Pat. No. 6,191,653); application Ser. No. 09/195,129 to the present co-inventor Camp, Jr., et al., entitled Circuit and Method for I/Q Modulation with Independent, High Efficiency Amplitude Modulation; application Ser. No. 09/207,167 to the present co-inventor Camp., Jr., entitled Amplitude Modulation to Phase Modulation Cancellation Method in an RF Amplifier (now U.S. Pat. No. 6,295,442) and application Ser. No. 09/226,478 to the present co-inventor Camp, Jr., et al., entitled Power I/Q Modulation Systems and Methods (now U.S. Pat. No. 6,181,199). Each of these copending applications is assigned to the assignee of the present application and the disclosures of all these applications are hereby incorporated herein by reference. These applications describe systems and methods for modulating phase and amplitude separately using high efficiency amplifiers.
FIG. 1
is a block diagram of power modulation systems and methods that separately amplify amplitude and phase according to the above-cited copending applications. As shown in
FIG. 1
, these power modulation systems and methods
100
include a waveform generator
102
that generates an amplitude waveform G(A(t))
104
and a phase waveform F(&phgr;(t))
106
from a plurality of data symbols
108
. A phase modulator
110
, such as a voltage controlled oscillator (VCO) and/or a phase locked loop (PLL), phase modulates the phase waveform
106
to produce a phase modulated waveform
112
. An RF driver amplifier
114
may be included in order to overdrive a power amplifier
116
. The power amplifier
116
is preferably a Class-C power amplifier that includes a power supply input
116
a
, a signal input
116
b
, and a power output
116
c
. The phase modulated waveform
112
is applied to the signal input
116
b
of the Class-C power amplifier
116
, either directly from the phase modulator
110
or via the RF driver amplifier
114
.
Continuing with the description of
FIG. 1
, a Class-D amplifier
120
is responsive to the amplitude waveform
104
to supply a variable supply voltage to the power supply input
116
a
of the power amplifier
116
. An analog-to-digital converter, for example a delta sigma modulator
124
that operates from a clock frequency f
cl
, converts the amplitude waveform
104
to a digital signal that is applied to the Class-D amplifier
120
. The output of the Class-D amplifier
120
is then lowpass filtered, for example at a frequency f
c
, by a lowpass filter
126
. The amplified amplitude waveform
122
is applied to the power supply input
116
a
of the power amplifier
116
, and the phase modulated waveform
112
is applied to the signal input
116
b
of the power amplifier
116
, to produce a power modulated waveform
130
of the data symbols
108
at the power output
116
c
. Thus, power modulation systems and methods of
FIG. 1
control the supply voltage to the Class-C power amplifier
116
using the Class-D power amplifier
120
in order to maintain a high overall efficiency.
As power modulators are used with increasingly higher frequencies, it may be desirable to further extend the frequency response thereof. For example, satellite radiotelephone communication systems may operate at relatively high frequencies compared to terrestrial radiotelephone communication systems. Accordingly, it may be desirable to extend the frequency response of power modulation systems and methods that separately modulate phase and amplitude.
The frequency response of the power modulation systems and methods of
FIG. 1
may be extended by increasing the cutoff frequency f
c
of the lowpass filter
126
. Unfortunately, if the cutoff frequency of the lowpass filter is increased, noise from the delta sigma modulator
124
may be imparted on the power modulated waveform. Alternatively, a higher clock frequency f
cl
may be used for the delta sigma modulator. Unfortunately, this may increase the cost and/or decrease the efficiency of the delta sigma modulator
124
and/or the Class-D amplifier
120
. Accordingly, there continues to be a need for power modulation systems and methods that can separately modulate phase and amplitude at high frequencies.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide improved power modulation systems and methods.
It is another object of the invention to provide improved power modulation systems and methods that can separately modulate amplitude and phase.
It is yet another object of the present invention to provide systems and methods that can extend the frequency response of power modulation systems and methods that separately modulate amplitude and phase.
These and other objects are provided, according to the present invention by separately amplifying a low frequency portion of an amplitude waveform and a high frequency portion of the amplitude waveform. The low frequency portion, generally containing most of the modulation energy, is preferably amplified at high efficiency to produce an amplified low frequency portion, while the high frequency portion may be amplified at lower efficiency to produce an amplified high frequency portion. The amplified low frequency portion and the amplified high frequency portion are combined to produce a combined amplified amplitude waveform that is applied to the power supply input of a high efficiency power amplifier.
The invention stems from the realization that the energy versus frequency in a conventional amplitude modulation spectrum generally diminishes rapidly with increasing frequency. Accordingly, efficient modulation is used for the low frequency portion of the amplitude waveform. Amplitude modulation frequency components above that frequency may be supplied using more conventional linear methods of modulation and amplification. Thus, the bulk of the amplitude modulation power may be supplied via efficient amplification, while the remaining frequency components, representing very little power, may be supplied via a less efficient but more convenient conventional linear control circuit. Overall high efficiency therefore may be maintained while allowing an extended bandwidth signal to be modulated.
More specifically, power modulation systems and methods according to the invention generate an amplitude waveform and phase waveform from a plurality of data symbols. The amplitude waveform includes a low frequency portion and a high frequency portion. The phase waveform is phase modulated to produce a phase modulated waveform. A power amplifier, preferably a Class-C power amplifier, includes a pow

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