1. Field of the Invention
The present invention relates to the measurement of communications signals.
2. State of the Art
Predistortion is commonly used in communications signal transmitters to compensate for signal distortion introduced by transmitter components. In wireless communications, for example, the final stage of a power amplifier is often the source of significant distortion, in particular AM/AM distortion and AM/PM distortion. Device characterization attempts to identify the distortion characteristics of a device to enable predistortion to be performed, the predistortion and the distortion ideally cancelling each other, leaving a distortion-free signal.
Device characterization requires signal measurement. Measurement circuitry, however, may also introduce significant distortion. In the case of a wireless communications system, for example, a typical characterization setup for characterizing a power amplifier is as shown in FIG. 1. A suitably-chosen RF stimulus (or xe2x80x9cprobe signalxe2x80x9d) 101 is applied to a power amplifier 103, which produces a communications signal 105. The communications signal is applied to an I/Q demodulator and data converter 107, which produces digital I and Q signals 109. An I/Q compensator 111 alters the digital I and Q signals to compensate to the extent possible for non-idealities of the I/Q demodulator. Compensated digital I and Q 113 are input to a data analysis unit 115 (e.g., a stored-program computer), which produces estimates of an AM/AM distortion curve of the power amplifier and a AM/PM distortion curve of the power amplifier.
The amplifier of FIG. 1 may be a conventional linear amplifier, as shown in FIG. 2, or may be a non-linear amplifier of a type shown in FIG. 3.
A more detailed diagram of a main portion of the conventional measurement setup of FIG. 1 is shown in FIG. 4. An output signal 405 of a power amplifier 403 is applied to an RF port of a mixer 407. Applied to a local oscillator port of the mixer is a local oscillator signal 409 having a frequency equal to the sum of a carrier frequency fc of the RF signal and a desired intermediate frequency fIF. The carrier frequency may be in the range of many hundreds of megahertz, for example, while the intermediate frequency may be in the range of many tens of megahertz. The mixer 407 produces an output signal 411 at the intermediate frequency. The intermediate frequency signal is split into two signals, one of which is applied to an I (in-phase processing) chain 420I and one of which is applied to a Q (quadrature processing) chain 420Q. The I and Q chains are largely identical, each having a mixer 421, followed by a low-pass filter 423, a video amplifier 425, a further low-pass filter 427, and an A/D converter 429. A local oscillator (not shown) produces a reference signal 431 that is applied directly to the mixer of the I chain. A 90 degree phase shifter 433 produces a quadrature version of the reference signal, which is applied to the mixer of the Q chain. Digital outputs of the I and Q chains are combined to form a single complex number stream z(t), the combination being effected by a j operator 435 and a summation block 437.
The foregoing setup is acceptable where accuracies of 10%, or perhaps even some few percent, are acceptable. In high-data-rate wireless communications systems, however, accuracies of less than 1% may be required. The conventional setup of FIG. 1 does not allow for such accuracy to be obtained.
More particularly, the I/Q video amplifiers 423 are a source of various types of errors, including I/Q gain mismatch, different DC offsets, and non-linearity. The standard approach to signal measurement is particularly susceptible to the latter two sources of error. Furthermore, the power amplifier 403 itself exhibits what may also be termed DC offset, as a result of AM/AM distortion.
Assume that the RF stimulus 401 exhibits a sawtooth envelope. Whereas ideally the output signals of the I/Q video amplifiers would have a sawtooth envelope varying between a minimum amplitude a0 and a maximum amplitude a1, as illustrated in FIG. 5, instead, these amplitude levels are shifted by a combination of DC offsets from the power amplifier (the offset of interest, caused by AM/AM distortion of the power amplifier, denoted herein by DCPA(AM/AM)) and from the I/Q video amplifiers (xe2x80x9cnuisancexe2x80x9d offsets, denoted herein by DCVA). Identifying the different offset components is analogous to being asked to guess two numbers given their sumxe2x80x94the offsets are, in practical terms, inseparable.
An additional source of significant error is the phase shifter 433, which typically will have a tolerance of a few degrees of phase. If an accuracy of a fraction of a degree is desired, clearly such an error is intolerable.
The present invention, generally speaking, provides a method and apparatus for accurately measuring a communications signal. In accordance with one aspect of the invention, DC offset effects and nonlinearities attributable to a communications amplifier are made spectrally separable from DC offset effects and nonlinearities attributable to the measurement apparatus. Spectral separability may be accomplished, for example, by adding an offset frequency to a local oscillator used by a downconverter of the measurement apparatus. As a result, the signal of interest is moved away from baseband (zero frequency) to the offset frequency. Similarly, other nonlinearities in the video amplifier (such as third order distortion) manifest theselves mostly at harmonics of the offset frequency. Hence spectral separability between the power amplifier characteristics and these nonlinear impairments of the measurement system is achieved. At the offset frequency, the signal of interest may be represented by a single signal component (e.g., the in-phase component I) instead of requiring an I/Q representation. The measurement apparatus may therefore be dramatically simplified, and errors stemming from undesired interaction of I and Q signal processing chains may be avoided. In accordance with another aspect of the invention, the effects of noise are mitigated through different averaging methods.