Amplifiers – With amplifier condition indicating or testing means
Patent
1996-12-02
1998-05-05
Mottola, Steven
Amplifiers
With amplifier condition indicating or testing means
455115, 330149, H03F 126
Patent
active
057480370
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a method of calibration of a power amplifier for a radio transmitter, and is particularly relevant to the field of training of linear power amplifiers.
BACKGROUND OF THE INVENTION
A typical linear transmitter uses a class AB power amplifier (PA) with a negative feedback technique, such as Cartesian feedback, to achieve high linearity and prevent adjacent channel interference. The Cartesian feedback loop is a closed loop that includes a non linear RF (radio frequency) PA. The PA's linearity performance improves in proportion to the loop gain when it is closed in the loop.
U.S. Pat. No. 5,066,923 of Motorola describes a method of training a power amplifier for, among other advantages, improved efficiency.
There are a number of disadvantages with known linear transmitters. In particular, training of the amplifier allows the PA to operate in its non-linear region for a short time when "clipping" of the PA occurs. During this time there is an increase in interference in the adjacent channel. Moreover, this occurs at the amplifier's maximum power. Allowing the amplifier to output more power than necessary causes damage and decrease the mean time between failures.
SUMMARY OF THE INVENTION
According to the present invention, a method of calibrating a power amplifier for a radio transmitter is provided, comprising the steps of: entering a calibration mode of operation; providing an increasing input signal to the amplifier until the amplifier commences clipping; reducing the maximum input level of said input signal and identifying a value for said input which does not cause clipping; entering an operational mode; periodically providing an increasing input signal to the amplifier up to said value and determining whether clipping occurs.
In this manner, the maximum limit of training is set in the calibration mode. The maximum limit is set at a level known to be below the clip point. Thereafter, training signals are not allowed to exceed that level.
In a preferred embodiment, the maximum level is set at 0.4 dB below the clip point. This represents negligible reduction in efficiency. The margin at which the maximum level is set below the clip point can have other values. It is preferred that the margin is in the range of about 1.5 dB to about 0.4 dB. Any greater margin would sacrifice efficiency and any lower margin would increase the risk of clipping and interference.
Various steps can be taken as described below and as claimed, to (a) check that clipping is not occurring or about to occur and (b) to avoid further clipping in the event that clipping occurs.
According to another aspect of the invention, a radio transmitter comprising a power amplifier having an input and an output, controlled input signal generating means for supplying a signal to the input, output monitoring means for determining when the amplifier commences clipping, memory means for storing a maximum input signal value and control means arranged to provide an input signal to the amplifier, wherein the control means are arranged to store in said memory means a maximum input signal value and to control the input signal generating means to generate periodically an increasing input signal up to a value not exceeding said stored value. The stored value represents an input signal value below a value at which clipping will commence.
Preferred embodiments of the invention are now described, by way of example only, with reference to the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a radio transmitter circuit with a power amplifier.
FIG. 2 shows diagrams of various signals in the circuit of FIG. 1 on a time axis during training of the amplifier of FIG. 1.
FIGS. 3 and 4 show attenuator level and output power level respectively for the circuit of FIG. 1, with dotted lines representing modified operation in accordance with the preferred embodiments of the invention.
FIG. 5 is a diagram of input voltage on a time axis during training in accordance with the preferred embodiments of the invention
REFERENCES:
patent: 5041793 (1991-08-01), Gailus
patent: 5066923 (1991-11-01), Gailus et al.
patent: 5239693 (1993-08-01), Gailus et al.
patent: 5564087 (1996-10-01), Cygan et al.
patent: 5574992 (1996-11-01), Cygan et al.
Ben-Ayun Moshe
Horowitz Ilan Yehezkel
Rozental Mark
Motorola Inc.
Mottola Steven
Scutch, III Frank M.
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