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Method and apparatus providing calibration technique for rf performance tuningRelated Patent Categories: Telecommunications, Receiver Or Analog Modulated Signal Frequency Converter, Local Control Of Receiver Operation, Gain ControlMethod and apparatus providing calibration technique for rf performance tuning description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060160510, Method and apparatus providing calibration technique for rf performance tuning. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS: [0001] This application is a divisional application of U.S. patent application Ser. No. 10/099,626, filed on Mar. 15, 2005, by the same inventors herein and-both present and parent applications are assigned to Nokia Corporation. TECHNICAL FIELD [0002] These teachings relate generally to radio frequency (RF) receivers and, more specifically, relate to methods and apparatus for optimizing the performance of receivers such as those found in cellular telephones and other types of mobile communication devices and terminals. BACKGROUND [0003] The following abbreviations are herewith defined. [0004] ADC analog-to-digital converter [0005] AM amplitude modulation [0006] ASIC application specific integrated circuit [0007] BB baseband [0008] CDMA code division multiple access [0009] CMRR common-mode-rejection ratio [0010] CPU central processing unit [0011] DS-CDMA direct sequence CDMA [0012] DSP digital signal processing [0013] FDD frequency division duplexing [0014] FM frequency modulation [0015] FPGA field programmable gate array [0016] IC integrated circuit [0017] ICP input compression point [0018] IF intermediate frequency [0019] IIP2 second-order input intercept point [0020] IIP3 third-order input intercept point [0021] IMD2 second-order intermodulation product [0022] IMD3 third-order intermodulation product [0023] LNA low noise amplifier [0024] LO local oscillator [0025] PM phase modulation [0026] PD phase detector [0027] PDF phase-frequency detector [0028] RX receiver [0029] RF radio frequency [0030] RSSI received signal strength indicator [0031] TX transmitter [0032] VCO voltage controlled oscillator [0033] WCDMA wide-band CDMA [0034] 3G third-generation (cellular system) [0035] As is well known, passive components that are used in RFICs typically have relatively large process variations. This leads to a direct trade-off between the accuracy of the resonant or resonance frequency and the bandwidth of the circuit. As a result it is common practice to use relatively low-Q resonators in the RF signal path in order to ensure a sufficiently wide bandwidth and, thus, sufficient performance without requiring calibration. Additionally, calibrations performed during fabrication are preferably avoided in order to reduce cost. The use of a narrow bandwidth (narrow band) LNA in the RF receiver enables the elimination of a bandpass filter after the LNA, and thus reduces cost. Since the passive component process variations can be large, however, some calibration is normally needed, and the cost savings may not be as great as one would at first expect. [0036] As such, what is required is a simple implementation of a calibration technique that can be used to tune resonators in analog circuits, as well as a technique to utilize (relatively) narrow-band resonators in a radio system. [0037] In most applications a relatively wide-band LNA is used, which is insensitive to process variations, and if necessary an external filter is placed between the LNA and a downstream mixer in order to reduce transmitter leakage (an undesired signal coupled into the receiver from the transmitter). In addition, some structures that use additional resonators in the LNA-mixer interface, or in the LNA topology itself have been presented. Reference in this regard can be made to J. A. Macedo, M. A. Copeland, "A 1.9-GHz. Silicon Receiver with Monolithic Image Filtering", IEEE J. Solid-State Circuits, vol. 33, pp. 378-386, March 1998, as well as to H. Samavati, H. R. Rategh, T. H. Lee, "A 5-GHz CMOS Wireless LAN Receiver Front End", IEEE J. Solid-State Circuits, vol. 35, pp. 765-772, March 2000. While primarily intended for image rejection purposes, the problems associated with filtering out-of-band signal components are basically the same as when filtering transmitter leakage. However, although the LNA structure with two resonators has been described in the prior art, an adequate solution to the calibration and optimal scaling with current of the two resonator LNA has not previously been proposed. [0038] FIG. 1 shows a conventional direct conversion receiver 1. After the antenna 2 the desired radio band (e.g., WCDMA/GSM/or other) is selected using a bandpass filter 3 in front of the first (variable) amplifying stage 4. The signal is then downconverted with mixers 5 to a zero IF (i.e. direct conversion) using quadrature local oscillator (LO) signals 6 (in 90 degree phase shift) that are tuned with a synthesizer 7 at the carrier frequency of the received channel. After downconversion the signal is applied to baseband amplifiers 8, 10 and filters 9, and in a digital communications systems the information is converted into digital form with an analog-to-digital converter (A/D) 11 and further then digitally filtered 12. Channel decoding 13 and other necessary digital functions to recover the transmitted information are be performed after the A/D 11. Gain control is an important function to extend the input signal range in all receiver architectures, and is used as well in the instant invention to adjust signal levels during calibration. An RSSI block 14 provides a signal to a gain control logic block 15 that functions to adjust the gain of the amplifiers 4, 8 and 10 to maintain the received signal at a desired level. [0039] FIG. 2A shows the construction of a typical low noise amplifier (LNA) that is used as the first amplifier 4 in the receiver chain, while FIG. 2B shows a typical voltage controlled oscillator (VCO) 7A used in the synthesizer 7. The VCO 7A generates the high-frequency signal from which the quadrature LO signals are generated. Both of these devices use a resonator that can be implemented on the RF IC or with an external tank circuit. Although the resonators in FIGS. 2A and 2B appear slightly different, they perform electrically exactly the same resonance function. Generally, for the purposes of this invention all described resonators can be considered to be an LC tank circuit containing an inductor, a capacitor and a resistor. The resistor is not necessarily shown in all drawings, and in most cases the resistor is actually the parasitic resistance, which degrades the quality of the tank circuit in all physical realizations, and hence must be taken into account. [0040] However, a wide-band LNA can be implemented according to FIG. 2A using a high-quality LC tank and a parallel separate resistor R to enhance the bandwidth by lowering the quality factor (Q value) of the tank circuit. This technique is generally used in many LNA implementations, as the current IC technologies provide inductors and capacitors that produce too high a Q value for the tank circuit, if process variations are taken into account. This problem is illustrated in FIG. 3A. Due to process variations the center frequency can vary by too large an amount between samples in order to cover the entire band of interest (i.e., bandwidth of the system) without requiring tuning (calibration) Therefore, tuning is required in the case of a narrow-band LNA. In the case of a wide-band structure (shown by the dashed lines), the process variations have a much smaller effect on the amplification in the band of interest. The difference between the two approaches in attenuating the out-of-band interferer, such as the transmitter leakage, is shown in FIG. 3B. The benefit of using a narrow-band LNA is obvious, and even relatively small improvements in the attenuation can relax the receiver specifications significantly. [0041] In making system calculations it can be shown that 6 dB attenuation in the maximum transmitter (TX) power leakage at some distance (in MHz) from the desired signal can relax the mixer 5 specification sufficiently so as to remove a filter from the LNA-mixer interface. The intermodulation of the TX leakage with an unwanted spurious signal is considered in this estimation. While a resonance circuit with sufficient performance can be implemented using current IC technologies, the accuracy of the resonant frequency is not acceptable without tuning. However, and as was described above, the requirement to provide tuning increases the cost, and is thus not desirable. SUMMARY OF THE PREFERRED EMBODIMENTS [0042] The foregoing and other problems are overcome, and other advantages are realized, in accordance with the presently preferred embodiments of these teachings. [0043] This invention describes an apparatus, a method and an algorithm for providing a calibration technique that allows for the use of narrow bandwidth resonators in the RF signal path and thus reduces the linearity requirements of those circuit blocks that benefit from additional attenuation of the out-of-band interferers. The presently preferred calibration technique also allows the enhancement of linearity performance with certain tuning techniques, without increasing the power consumption. Because in most cases the linearity requirement dominates the power consumption, the calibration technique preserves the same linearity performance with a reduced current consumption, and thus contributes to reducing the overall power consumption in the system. Another important benefit realized by the presently preferred calibration technique is that when certain performance parameters are relaxed after frequency band limitation, it becomes possible to eliminate external filtering while achieving sufficient performance with on-chip devices. One important application for this technique is in the removal of external band selection filtering located between the LNA and the downconversion mixers, which are typically required due to the leakage of the transmitter power into the receiver input in full duplex systems having a simultaneous transmission and reception mode. The 3G CDMA systems are particular examples of such full duplex systems. [0044] In accordance with this invention logic is provided that changes the LNA resonator and detects the strongest response, as well, as an algorithm that selects the resonance frequency based on the strongest response and that can shift the resonance from the nominal according to the current LO signal. The invention also provides a method to shift the resonance according to a current scaling of the LNA. The invention also provides an additional oscillator mode in which the output signal is amplitude and/or frequency modulated. The invention also provides an algorithm that tunes the linearity performance of the receiver to a maximum by using the signals, produced by the additional oscillator or LO synthesizer and modulator, and measures the results at baseband. The invention beneficially provides a tuning method for a narrow-band LNA structure using two resonators, and an additional oscillator that uses the LNA resonator in a calibration mode, preferably in combination with a phase lock loop. [0045] In accordance with the teachings of this invention an additional high frequency signal with desired properties is connected to the LNA resonance circuit and the input stage of the LNA is switched off. By tuning the resonance frequency (and possibly the LO frequency) the maximum output signal level of the RF front-end of the receiver is detected. This maximum level indicates that the resonance of the LNA is approximately the same as with the known LO frequency. With this setting the resonance is brought closer to the band of interest and any process variations that affect the resonance, frequency are thereby compensated. By using this calibration procedure a relatively narrow bandwidth LNA can be employed in the receiver to filter out the out-of-band interferes, such as the own-transmitter leakage power when operating in the full duplex mode. [0046] The calibrated LNA resonance may also be tuned during reception based on the synthesizer frequency to which the receiver is to be tuned. In that the transmitted signal in the case of full duplex communication is located typically at a fixed separation in frequency as compared to the reception, the gain of the LNA with respect to the TX attenuation can be optimized for all frequency channels separately if the resonance of the LNA can be shifted according to the received radio channel. This property is beneficial in, for example, the full duplex 3G WCDMA system. [0047] The resonance frequency may also change slightly if the LNA biasing current varies. However, because the process variations of the resonator are calibrated for by the use of this invention, the known frequency shift as a function of the biasing current can be also taken into account when the bias of the LNA is changed. [0048] A narrow-band LNA can be established using a high-quality tank circuit in the resonator load. However, current IC technologies do not provide very high Q-values for the inductors using standard processing steps, and thus restrict the bandwidth limitation capabilities of the LNA. To circumvent this problem an LNA structure that uses two resonators is employed in order to reduce the LNA bandwidth, and in accordance with an aspect of this invention, a procedure is provided, for tuning the two resonators during calibration and during operation of the LNA/receiver. [0049] In order to maximize the linearity of the receiver, the calibration signal can be amplitude or frequency modulated. The amplitude modulated (AM) signal can be used for IIP2 tuning in the receiver while the frequency modulated (FM) signal can be used for IIP3 tuning. [0050] This invention thus provides circuitry and a method to generate suitable test signals for enabling internal receiver tuning, to detect the results and to perform an internal calibration cycle in the radio receiver to achieve an optimum level of performance. [0051] The calibration is preferably performed during idle time slots in a TDMA communications system, such as GSM, and/or during the start up of the mobile station, and/or during any other available idle time in any other radio system. Because the calibrations performed are related mainly to characterizing component process variations, they may be performed only once. However, the calibration can be performed as often as desired, so long as the LNA of the receiver can be switched off or at least partially disabled from operation. For example, the calibration procedure can be performed in a multi-mode or multi-band receiver when the particular receiver front-end is not in use. 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