Telecommunications – Transmitter – Plural modulation
Reexamination Certificate
1998-12-07
2001-09-25
Hunter, Daniel (Department: 2684)
Telecommunications
Transmitter
Plural modulation
C455S108000, C455S205000
Reexamination Certificate
active
06295442
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to RF amplifiers and, more particularly, to a circuit and method for cancelling the effects of unintended phase modulation that can occur when a power amplifier is amplitude modulated.
BACKGROUND OF THE INVENTION
Radio frequency (RF) transmitters, such as used in cellular telephones, develop an RF signal to be transmitted through the air. Information is carried in the signal via some form of modulation such as frequency modulation, phase modulation, amplitude modulation, or a combination of these.
It may be desirable to create a modulated signal with both amplitude and phase modulation. With the necessity of developing small and lightweight devices, particularly cellular telephones, it is important that such amplifier circuits use a minimum of components. One possibility is to use a combination of phase modulation of a master oscillator at a carrier frequency and amplitude modulation of a power amplifier. However, unintended phase modulation can occur when the power amplifier is amplitude modulated. This problem has been addressed by including the amplitude and modulated power amplifier within the total phase locked loop. This can cause the out of band signal that occurs if as a loop is locking to be broadcast unless a switch with very large isolation is included. The feedback point from the voltage-controlled oscillator can be shifted to the output of the power amplifier only after the loop has locked and is on frequency. This shift can cause disruptions of the loop and cause the loop to lose lock.
The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.
SUMMARY OF THE INVENTION
In accordance with the invention there is provided a circuit and method for correcting a phase modulation command based on an amplitude modulation command.
Broadly, there is disclosed herein an RF amplifier including command means developing a phase modulation command representing a desired phase modulation of an RF signal, and an amplitude modulation command representing a desired amplitude modulation of the RF signal. An oscillator develops an RF input signal phase modulated based on the phase modulation command. A power amplifier receives the RF input signal and amplifies the RF input signal based on the amplitude modulation command to develop an RF output signal. A modulation control is operatively associated with the oscillator. The modulation control includes phase memory means for storing phase correction information correlating the amplitude modulation commands to a phase modulation error and phase control means for varying the phase modulation command based on the phase modulation error to correct for unintended phase errors created by amplitude modulation of the power amplifier.
It is a feature of the invention that the phase control means comprises a summer that sums the phase modulation command and the phase modulation error and the sum is an input to the oscillator. It is another feature of the invention that the modulation control comprises a programmed processor.
It is a further feature of the invention that the phase memory means stores a mathematical function correlating the amplitude modulation command to the phase modulation error.
It is still a further feature of the invention that the phase memory means stores a list of amplitude modulation command values to phase modulation error values.
It is still another feature of the invention to provide means for determining actual phase modulation of the RF output signal and wherein the modulation control periodically updates the phase correction information using the monitored RF output signal phase modulation and the desired phase modulation.
It is still another feature of the invention to provide means for determining actual phase modulation of the RF output signal and wherein the modulation control compares the actual phase modulation to the phase modulation error to vary the phase modulation command. The control means comprises a summer that sums the phase modulation command with an output of the comparison of the actual phase modulation to the phase modulation error.
It is still a further feature of the invention to provide amplitude memory means for storing amplitude correction information correlating actual amplitude of the RF output signal relative to the desired amplitude modulation and amplitude control means for varying power amplifier supply voltage based on the amplitude modulation command modified responsive to the amplitude correction information. The modulation control determines phase modulation error based on the amplitude modulation command modified responsive to the correction information. Means are also provided for monitoring amplitude of the RF output signal. The amplitude control means periodically updates the amplitude correction information using the monitor RF output signal and the desired amplitude modulation. The amplitude control means varies power amplifier supply voltage based on a difference between monitored amplitude and the amplitude modulation command. The oscillator comprises a phase modulated voltage-controlled oscillator.
In accordance with another aspect of the invention, there is disclosed a method of developing an RF signal with both phase and amplitude modulation. The method comprises the steps of developing a phase modulation command representing a desired phase modulation of an RF signal, developing an amplitude modulation command representing a desired modulation of the RF signal, shifting the phase of the RF signal to provide an RF input signal phase modulated based on the phase modulation command, amplifying the RF input signal based on the amplitude modulation to develop an RF output signal, storing phase correction information correlating the amplitude modulation command to a phase modulation error, and varying the phase modulation command based on the phase modulation error to correct for unintended phase errors created by amplitude modulation of the RF input signal.
Further features and advantages of the invention will be readily apparent from the specification and from the drawings.
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Camp, Jr. William O.
Gore Charles
Mannerstrale Jacob
Schlang Jeffrey A.
Coats & Bennett
Ericsson Inc.
Hunter Daniel
Nguyen Thuan T.
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