Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
1999-07-30
2001-01-09
Pascal, Robert (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C375S296000, C455S126000
Reexamination Certificate
active
06172564
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the reduction of distortion in electrical circuitry. More particularly, the present invention relates to distortion reduction circuitry particularly suitable for amplifiers.
Radio frequency (RF) systems, such as cellular and microwave communication systems, typically require input signals to be fed into an RF power amplifier. Unfortunately, all amplifiers tend to distort the input signal at some power level and to generally cause the amplifier output to contain undesirable distortion products, spurious products, noise perturbations and intermodulation (IM) products. For purposes of simplicity, these will collectively be referred to as IM products. These IM products can cause undesirable interference over the amplifier frequency range. It is for these reasons that most amplifiers must incorporate some form of correction mechanism into the signal flow path through the amplifier.
One conventional method for reducing IM products uses a spectral analysis approach. This approach involves scanning the output of the amplifier using a receiver tuned to the frequency of candidate IM products. The IM level is measured and a linearizer is manually adjusted to minimize the IM product. This procedure is repeated until each of the IM products have a magnitude below a predetermined acceptable level. In a similar approach, as described in U.S. Pat. No. 4,580,105 issued to Myer, a portion of the output signal is taken and combined with an input signal which has been adjusted in phase and amplitude. The signal combination isolates the distortion component which is then adjusted in phase and gain. This isolation distortion component is then added back to the output signal by means of a coupler to eliminate the distortion component. Unfortunately, the amount of distortion reduction available using this feed forward technique is limited by the accuracy of the gain and phase adjustments.
In addition, there appears to be a problem in the number of samples realized in several basic feed forward techniques. Samples in these systems are available only at the time that the phase detector circuit detects a correlated signal. This leads to poor error signal resolution and it is this error signal that is used to cancel corresponding spurs.
Moreover, it is believed that many cellular communication base stations do not function at full capacity because of serious linearity problems in the base station's components. In addition, the need for increased capacity accompanying the expansion of cellular communications is forcing requirements for transmission standards of RF equipment to become too stringent to be met by construction of most present day amplifier circuits. Moreover, both time division multiple access (TDMA) and code division multiple access (CDMA) modulation require greater linearity and an absence of IM products that can not be routinely obtained by presently available high efficiency RF power amplifiers. Accordingly, it is clear that present correction techniques for eliminating IM products are not acceptable for evolving communications systems.
Thus, there is a need for a cancellation circuit which reduces IM products in electronic circuitry such as power amplifiers.
It would also be highly advantageous if the IM cancellation circuit were inexpensive to manufacture and highly reliable.
Moreover, it would be highly advantageous if the IM cancellation could be provided in a compact and lightweight constriction.
SUMMARY OF THE INVENTION
Briefly, in accordance with the invention, I provide an improved apparatus and method for automatically reducing IM products in electrical circuits. It is believed that this IM cancellation circuit is particularly suitable for application with amplifiers which amplify an input carrier signal.
In its most basic form, the IM product cancellation circuit includes a first coupler for sampling an input carrier signal. The coupler isolates a portion of the signal, preferably 6 dB below the input carrier signal so as to not substantially affect the energy level of the input carrier signal. This sample signal is then routed to a stepping spectrum analyzer which detects and measures the frequency and amplitude of the signal components of the carrier signal including its intended components and any IM products in the carrier signal. The stepping spectrum analyzer transmits the information relating to the frequency and amplitude of the different signal components in the carrier signal to a processor. Meanwhile, the processor stores information relating to the identity of the amplitude and/or frequency or frequency bands of intended signal components in the carrier signal. This information is typically in the form of look-up tables which identifies the amplitude or frequency, or in the case of TDMA or CDMA systems, the frequency band where the intended signal components are expected to reside. In this manner, the processor, by referencing this look-up table, can determine whether the signal components detected and measured by the stepping spectrum analyzer are intended signal components or unintended IM products.
The IM product canceling circuit further includes a variable voltage controlled oscillator, variable amplifier and variable phase shifter. The voltage controlled oscillator, amplifier and phase shifter are connected in series for creating an oscillator signal which can be controlled in amplitude and phase. The operation of the voltage controlled oscillator, amplifier and phase shifter are all controlled by the processor. Upon recognition and measurement of an IM product by the stepping spectrum analyzer and processor, the processor controls the voltage controlled oscillator, amplifier and phase shifter to produce an oscillating signal which is substantially identical in amplitude and frequency to a detected IM product but shifted in phase by 180°. This oscillating signal acts as an IM cancellation signal as the signal is fed back into the original carrier signal by means of a coupler or the like, such as a combiner. The original carrier signal is then output from the coupler retaining all of the original characteristics of the intended signal components except that the unintended IM product is canceled by the cancellation signal.
In a preferred embodiment, the IM cancellation circuit includes a feedback circuit. After the IM product is canceled, the output carrier signal is sampled by splitting the output signal with a coupler or the like to produce a sample of the output carrier signal. This sample of the output carrier signal is routed back to, in turn, the stepping spectrum analyzer and processor. The stepping spectrum analyzer and processor are thus able to confirm that the IM product has been canceled. In the alternative, where it has been determined that the cancellation signal is being transmitted at an incorrect frequency, amplitude or phase, the voltage controlled oscillator, amplifier and phase shifter are automatically adjusted by the processor until the cancellation signal correctly cancels the unintended IM product.
As would be understood by those skilled in the art, the aforementioned described cancellation circuit would only cancel a single unintended IM product. Where the carrier signal is likely to have more than one unintended IM product, the cancellation circuit is provided with a plurality of variable voltage controlled oscillators, variable amplifiers and variable phase shifters. Each set of these voltage controlled oscillators, amplifiers and phase shifters are connected in series to the processor for creating a plurality of cancellation signals where necessary.
In an additional preferred embodiment, the IM product cancellation signal is adapted for use with an amplifier. A portion of the output from the amplifier is forwarded to the stepping spectrum analyzer for recognition and measurement of not only the intended signal components but also any IM products. This information is sent to the processor which determines whether the signals received by the stepping spectrum analyzer are
Choe Henry
Drummond & Duckworth
Pascal Robert
LandOfFree
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