Method and apparatus providing calibration technique for RF performance tuning
A method is disclosed for operating a RF receiver of a communications equipment, as is circuitry for implementing the method. The method includes, while operating under the control of a data processor of the communications equipment, generating a calibration signal; injecting the calibration signal into a low noise amplifier (LNA) of the RF receiver; measuring a downconverted response of the receiver at a plurality of different frequencies of the calibration signal, or measuring the downconverted response of the receiver at a plurality of different LNA tuning combinations using a fixed calibration frequency, and at least one of tuning a resonance frequency of at least one LNA resonator based on the measured downconverted response so as to compensate at least for variations in component values that comprise the at least one resonator, or adjusting the linearity of the receiver.
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CROSS-REFERENCE TO RELATED APPLICATIONS:
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.
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.
The following abbreviations are herewith defined.
- ADC analog-to-digital converter
- AM amplitude modulation
- ASIC application specific integrated circuit
- BB baseband
- CDMA code division multiple access
- CMRR common-mode-rejection ratio
- CPU central processing unit
- DS-CDMA direct sequence CDMA
- DSP digital signal processing
- FDD frequency division duplexing
- FM frequency modulation
- FPGA field programmable gate array
- IC integrated circuit
- ICP input compression point
- IF intermediate frequency
- IIP2 second-order input intercept point
- IIP3 third-order input intercept point
- IMD2 second-order intermodulation product
- IMD3 third-order intermodulation product
- LNA low noise amplifier
- LO local oscillator
- PM phase modulation
- PD phase detector
- PDF phase-frequency detector
- RX receiver
- RF radio frequency
- RSSI received signal strength indicator
- TX transmitter
- VCO voltage controlled oscillator
- WCDMA wide-band CDMA
- 3G third-generation (cellular system)
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.
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.
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.
However, a wide-band LNA can be implemented according to
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
The foregoing and other problems are overcome, and other advantages are realized, in accordance with the presently preferred embodiments of these teachings.
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.
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.
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.
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.
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.
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.
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.
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.
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.
One important benefit that is derived from the use of the teachings of this invention is that it becomes possible to use a relatively narrow-band LNA in the radio receiver, and to then perform the calibration internally to the RF IC. The narrow-band LNA attenuates more effectively the out-of-band interferers, which enables the linearity requirements of the receiver blocks following the LNA, as well as the LNA itself, to be relaxed. This has particular significance in the full duplex case where reception occurs simultaneously with transmission, and where the transmitter power leaks into the receiver input. In that linearity performance requirements typically dominate the receiver power consumption, by relaxing the linearity performance requirements, the average power consumption can be reduced significantly.
Another important benefit that is derived from the use of the teachings of this invention is that due to the large leaked transmitter power an external filter is typically needed between the output of the LNA and input to the mixer, even in the direct conversion receiver, otherwise the mixer linearity requirements become unreasonable. However, by enabling the use of a narrow band LNA, in combination with the calibration procedure therefor, the large and expensive external filter can be eliminated, and the linearity requirements of the mixer can be relaxed as well. This results in both cost and power consumption savings.
The use of this invention also provides the possibility to employ the same oscillator used in the resonance calibration for tuning the IIP2 of the receiver by a simple amplitude modulation method in the oscillator. In this case there is potentially no need to calibrate the IIP2 during fabrication. The IIP3 can also be maximized by correctly biasing or switching certain structures. This invention provides an opportunity to tune or calibrate IIP3 performance internally in order to optimize the circuit performance. In general, the internal optimization of IIP3 can lead to reduced power consumption. A two-tone test signal for the IIP3 optimization can be generated either internally or externally.
This invention provides a RF receiver of a communications equipment, such as a mobile station or a base station, that has calibration circuitry that operates under the control of a data processor of the communications equipment for use in calibrating RF circuitry of the communications equipment in the field, as opposed to only calibrating the circuitry in a factory in a production and/or testing environment. Furthermore, all of the disclosed calibrations can also be performed during production. In this case no external test signals are required to be connected to the device under test, leading to simplifications and increased efficiencies during the fabrication test cycle.
A method is disclosed for operating a RF receiver of a communications equipment, as is circuitry for implementing the method. The method includes, while operating under the control of a data processor of the communications equipment, generating a calibration signal; injecting the calibration signal into a low noise amplifier (LNA) of the RF receiver; measuring a downconverted response of the receiver at a plurality of different internal states of the receiver using the calibration signal at more than one frequency, or measuring the downconverted response of the receiver at a plurality of different LNA tuning combinations using a fixed calibration frequency, and at least one of tuning a resonance frequency of at least one LNA resonator based on the measured downconverted response so as to compensate at least for variations in component values that comprise the at least one resonator, or adjusting the linearity of the receiver.
The receiver internal states are preferably those associated with an adaptive receiver wherein receiver internal blocks including, but not limited to, the bias current(s), gain(s) and linearity can be tuned or adjusted, resulting in some state or performance of the receiver.
In a further aspect this invention operates a mobile station data processor, during a time that a receiver is not required, for at least partially disabling a receiver low noise amplifier, for generating a calibration signal within the mobile station and coupling the calibration signal into the receiver, measuring a downconverted response of the receiver to the calibration signal, and at least one of tuning a resonance frequency of at least one LNA resonator based on the measured downconverted response, or adjusting the linearity of the receiver chain.
In another aspect this invention operates a mobile station data processor, during a time that a receiver is required, for at least partially enabling the receiver low noise amplifier, for generating the calibration signal within the mobile station and coupling the calibration signal into the receiver, measuring a downconverted response of the receiver to the calibration signal, and at least one of tuning a resonance frequency of at least one LNA resonator based on the measured downconverted response, or adjusting the linearity of the receiver chain.
The downconverted calibration signal is preferably located outside of a receiver passband transfer function so that the calibration signal is not totally rejected, and preferably the downconverted calibration signal is separated from the received signal spectrum by bandpass filtering in the digital domain.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other aspects of these teachings are made more evident in the following Detailed Description of the Preferred Embodiments, when read in conjunction with the attached Drawing Figures, wherein:
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In order to place this invention into a suitable technological context, reference is first made to
The air interface standard can conform to any suitable standard or protocol, and may enable both voice and data traffic, such as data traffic enabling Internet 70 access and web page downloads. One suitable type of air interface is based on TDMA and may support a GSM or an advanced GSM protocol, although these teachings are not intended to be limited to TDMA or to GSM or GSM-related wireless systems. In fact, another wireless system and air interface, such as a WCDMA system, may serve at least a part of the geographical area served by the wireless communication system shown in
The network operator may also include a suitable type of Message Center (MC) 60 that receives and forwards messages for the mobile stations 100. Other types of messaging service may include Supplementary Data Services and one under currently development and known as Multimedia Messaging Service (MMS), wherein image messages, video messages, audio messages, text messages, executables and the like, and combinations thereof, can be transferred between the network and the mobile station 100.
The mobile station 100 typically includes a microcontrol unit (MCU) 120 having an output coupled to an input of a display 140 and an input coupled to an output of a keyboard or keypad 160. The mobile station 100 may be a handheld radiotelephone, such as a cellular telephone or a personal communicator. The mobile station 100 could also be contained within a card or module that is connected during use to another device. For example, the mobile station 10 could be contained within a PCMCIA or similar type of card or module that is installed during use within a portable data processor, such as a laptop or notebook computer, or even a computer that is wearable by the user.
The MCU 120 is assumed to include or be coupled to some type of a memory 130, including a read-only memory (ROM) for storing an operating program, as well as a random access memory (RAM) for temporarily storing required data, scratchpad memory, received packet data, packet data to be transmitted, and the like. A separate, removable SIM (not shown) can be provided as well, the SIM storing, for example, a preferred Public Land Mobile Network (PLMN) list and other subscriber-related information. The ROM is assumed, for the purposes of this invention, to store a program enabling the MCU 120 to execute the software routines, layers and protocols required to operate in the wireless communications system, as well as to provide a suitable user interface (UI), via display 140 and keypad 160, with a user. The stored program also is operable for executing, the methods and algorithms in accordance with these teachings, such as and including the methods illustrated in
The mobile station 100 also contains a wireless section that includes a digital signal processor (DSP) 180, or equivalent high speed processor or logic or control unit, as well as a wireless transceiver that includes a transmitter (Tx) 200 and a receiver (Rx) 220, both of which are coupled to an antenna 240 for communication with the network operator via the BTS 50. At least one local oscillator (LO) 260 forms a part of a frequency synthesizer and is provided for tuning the transceiver. Data, such as digitized voice and packet data, is transmitted and received through the antenna 240.
Of most interest to the, teachings of this invention is the receiver 220. The receiver 220 is assumed to be a direct conversion receiver, although in other embodiments other types of receivers, such as superheterodyne receivers, could be employed, and to include calibration functions in accordance with this invention, as will now be described in further detail.
This invention allows the use of a narrow-band LNA 4 in the radio receiver 220 by providing a convenient mechanism and method to calibrate the LNA. The calibration method can be done using an internal calibration procedure. While some calibration typically is performed after RF IC fabrication, the use of this invention can eliminate the requirement to provide external test signals and/or equipment during mobile station production. The calibration can also be performed later if desired, during idle operation modes and possibly during reception as well. Additionally, the calibration does not require another switch to connect the calibration signal to the LNA 4.
For the purposes of this invention a wide bandwidth LNA 4 is one that is suitable for operation over a wide frequency bandwidth, whereas a narrow bandwidth resonator will typically have a higher Q value than the wide bandwidth resonator.
The same circuitry can be used also to tune the linearity of the receiver 220 by modulating the LO/auxiliary VCO signal. This permits as well the tuning of IIP2 without requiring external test signals, and also provides a technique to optimize IIP3.
While the calibration procedure consumes some amount of power during calibration, because the calibration needs to be performed either only once or at most infrequently, the increment in the average power consumption is negligible.
A receiver 220 embodiment, according to this invention, that uses an AM modulator 22 is shown in
In, the case of an unmodulated signal, or if the receiver 220 uses some intermediate frequency (IF), the calibration signal is not located at the same frequency as the LO signal.
Hence, an additional oscillator is preferred for use. An embodiment that combines the LNA 4 and the VCO for this purpose is described below. However, in the case of IIP2 (or IIP3) trimming the calibration signal (CS) is preferably located outside of the passband of the channel selection filter. That can be done, for example, by using a higher frequency in the modulated signal or preferably, in the case of IIP2, by using the TX local oscillator signal from the synthesizer 7 in
An embodiment of the calibration algorithm is shown in
Note that the method can include changing the resonance frequency after calibrating, and during normal operation, based on a current local oscillator frequency. That is, the center frequency of the LNA resonator is changed based on the operational channel. In this manner the rejection of the transmitted signal in frequency division duplex (FDD) systems, such as the WCDMA system, is maximized.
Further in accordance with this invention, for, the case where the LNA 4 has two resonators, a procedure can be performed to improve the out-of-band, attenuation performance and internal linearity. Such a LNA 4 structure is shown in
By way of contrast, the conventional single (load) resonator LNA 4 shown in
More particularly, in the receiver 220 the baseband signal processing blocks 8, 9 and 10 have an ideally infinite and a typically very high common-mode-rejection ratio (CMRR), and thus only the signal that is differential can pass through the chain. In principle, the IMD2 product is a common mode signal that can be rejected totally with an infinite CMRR. In practice, the process variations of the mixer 5 and baseband blocks following it will produce a small differential signal that originates from the common mode IMD2 product. This signal is undesirable and can be minimized by using the tuning procedure in accordance with this invention. By tuning the mixer 5 in such a way that the IMD2 products are again common mode, the CMRR of the analog baseband chain can suppress these signals automatically. One advantage of this tuning method is it's simplicity, as no additional circuit blocks at the analog baseband side are needed.
If several settings during the tuning cause the smallest response, then the measurement accuracy may not be sufficient. In that case it is possible to increase the measurement accuracy by increasing the signal level of the AM tone, or increasing the gain of the signal path. In the case of signal saturation of the receiver output, the opposite procedure can be performed during calibration. One possible IIP2 tuning technique is described in EP 0951138A1, “Method for reducing envelope distortion in radio receiver” by K. Kivekäs & A. Pärssinen, application date 18.04.2001, incorporated by reference herein in its entirety. The present invention extends the use of those techniques and presents a new calibration algorithm.
As was discussed earlier, instead of the synthesizer 7 an additional, or auxiliary oscillator, which uses the same resonator as the LNA 4, can be used to generate the calibration signal (CS).
The logic 36 may operate as follows: the logic 36 counts the up and down pulses of the PDF 34 separately and decides if the signal is lower or higher in frequency than the target frequency by comparing the output of the counters 38. If up counter has a larger count than the down counter, the decision is ‘go up’ and vice versa. The PLL 30 would thus oscillate between the two tuning words that are both acceptable. Other possibility is to begin from the lowest (or highest) frequency and count the tuning words up (or down) until the state of the phase detector 34 changes. In this case the logic 36 may count up and down pulses. If the system starts from the lowest frequency the result of the calibration is the state where the number of the counted down pulses exceeds the number of counted up pulses. The correct frequency is then one between the last tuning word that does not change the state of the phase detector 34 and the tuning word that does change the state of the phase detector 34. If the resonator includes an analog capacitance the tuning voltage is preferably set into the middle of the analog tuning range. Additionally, this method can be used to tune the local oscillator of the synthesizer 7.
A further, non-limiting embodiment for tuning the VCO is described in a commonly assigned patent application “Self-contained tuning Of the VCO center frequency”, by Pauli Seppinen and Kalle Asikainen, U.S. patent application Ser. No.: 10/024,084, filed 17 Dec. 2001, and incorporated by reference herein. In this approach there is a method for tuning an adjustable oscillator having at least one resonance circuit. The frequency of the oscillator is adjusted by changing the resonance frequency of the resonance circuit by means of a control signal for which a minimum value and a maximum value are selected. In the execution of the method at least one target value is selected for the control signal, the frequency of the adjustable oscillator is adjusted to substantially correspond to the target value, and the value of the control signal and the target value are compared. When the value of the control signal is substantially different from the target value, a tuning signal is produced to change the resonance frequency of the at least one resonance circuit.
The nodes to which the auxiliary oscillator is connected in
The teachings of this invention can be implemented with just a few additional active components including RF transistors, possibly a simple AM modulator (a simple switching device in its basic form, as shown in
The teachings of this invention can be applied to all digital radio systems. On the other hand, the benefits may vary between systems. For example, the use of the narrowband resonator is important in CDMA systems in order to establish the maximum attenuation for the TX leakage. On the other hand, in a GSM system it becomes possible to calibrate IIP2 internally. The feature may relax the calibration requirements during production, and/or it may add the possibility to use adaptivity in the mixers during operation. This is true because the IIP2 can be calibrated at a larger set of test points without adding production cost. This invention is suitable for use in both the mobile station 100 and in the base stations 50.
While described in the context of presently preferred embodiments of this invention, those having skill in the art should appreciate that variations in form and detail may be made, and that all such changes will still fall within the scope of the teachings of this invention. For example, this invention should not be viewed as being limited to the specific LNA circuit embodiments shown in the Figures and described above, nor for use with only specific frequencies, ranges of frequencies, types of receivers, or with only certain wireless communications standards and protocols, such as only with GSM or WCDMA systems.
Furthermore, the calibration can be made during a reception time, instead of during a receiver idle time. In this case, and during a time that the receiver 220 is required, the data processor 120, 180 of the mobile station 100 is operated to enable the LNA 4, as well as for generating the calibration signal within the mobile station 100, coupling the calibration signal into the receiver 220, measuring the downconverted response of the receiver 220 to the calibration signal, and then at least one of tuning the resonance frequency of at least one LNA resonator based on the measured downconverted response, or adjusting the linearity of the receiver chain. In this embodiment the downconverted calibration signal is preferably located outside of a receiver passband transfer function so that the calibration signal is not totally rejected. That is, the downconverted calibration signal is not totally rejected, but is instead rejected or attenuated 20 dB/decade, if the filter or filters (e.g., filters 9 and 12) are first order filters. In this case the calibration signal is attenuated by a known amount as it passes through the receiver chain, and can thus be measured and characterized as described above. In this embodiment the downconverted calibration signal may be separated from the received signal spectrum by bandpass filtering in the digital domain.
1. A low noise amplifier comprising at least one gain element and at least one resonant circuit coupled to said gain element, further comprising at least one further gain element coupled to said resonant circuit for operating as an oscillator for generating a calibration signal for calibrating at least said low noise amplifier.
2. A low noise amplifier as in claim 1, and further comprising a phase lock loop circuit coupled to said oscillator, where said oscillator output is set to a center frequency of said low noise amplifier.
3. A low noise amplifier as in claim 1, wherein said at least one resonant circuit comprises an LC tank circuit.
4. An integrated circuit comprising a low noise amplifier, the low noise amplifier comprising at least one gain element configured to be coupled to at least one resonant circuit, further comprising at least one further gain element configured to be coupled to said resonant circuit for operating as an oscillator for generating a calibration signal for calibrating at least said low noise amplifier.
5. An integrated circuit as in claim 4, wherein said low noise amplifier further comprises a phase lock loop circuit coupled to said oscillator, where said oscillator output is set to a center frequency of said low noise amplifier.
6. An integrated circuit as in claim 4, wherein said at least one resonant circuit comprises an LC tank circuit.
7. An integrated circuit as in claim 4, wherein said at least one resonant circuit is external to said integrated circuit.
8. An integrated circuit as in claim 4, wherein said at least one resonant circuit is internal to said integrated circuit.
9. A method for operating a mobile station comprising, during a time that a receiver is not required, operating a data processor of said mobile station for at least partially disabling a receiver low noise amplifier LNA, generating a calibration signal within said mobile station, coupling said calibration signal into said receiver, measuring a downconverted response of the receiver to said calibration signal, and at least one of tuning a resonance frequency of at least one LNA resonator based on the measured downconverted response, or adjusting the linearity of a receiver chain.
International Classification: H04B 1/06 (20060101); H04B 7/00 (20060101);