WO2008142628A2

WO2008142628A2 - Receiver calibrating system and method

Info

Publication number: WO2008142628A2
Application number: PCT/IB2008/051947
Authority: WO
Inventors: Olivier Charlon
Original assignee: Nxp B.V.
Priority date: 2007-05-18
Filing date: 2008-05-17
Publication date: 2008-11-27
Also published as: WO2008142628A3

Abstract

In accordance with the invention a system and method for calibrating a receiver which provide calibration of Radio Frequency (RF) impairments namely quadrature imbalance, Direct Current (DC) offset, and channel filter corner frequency are described.

Description

RECEIVER CALIBRATING SYSTEM AND METHOD BACKGROUND

The exchange of information through wireless communication systems continues to grow with the increasing use of pagers, cellular telephones, Wireless Local Area Networks (WLANs) and other wireless communication devices and networks. Technological advancements have made it possible to integrate more and more components into a single chip, called System-On-Chip (SoC). Embedded systems are the common cores in SoC design and are considered to be the vital microelectronic components of a digital logic system. As the number of electronic components on the chip increases, circuit impairments such as noise, device matching and absolute passives value worsen. As a result, calibration techniques are required to guarantee viable production yield. A typical wireless communication system includes a transmitter and a receiver using a superheterodyne architecture employing a low Intermediate Frequency (IF.) In a low IF superheterodyne receiver, a received Radio Frequency (RF) signal is downconverted to a low IF by two mixers driven by quadrature phases of a Local Oscillator (LO). Instead of using a low IF, some wireless communications systems may use a zero IF or direct conversion architecture which also employs quadrature mixing.

Nevertheless, both low IF and zero IF receivers employ quadrature mixing and should address a common problem of matching or balancing the amplitudes and phases of an in- phase and quadrature phase branch, or channel, of the received RF signal. A mismatch between the two channels may be the result of bipolar transistors, Metal Oxide Semiconductor (MOS) transistors and finite tolerances of capacitor and resistor values used to implement the analog components of the receiver LO generation circuit. While balanced channels in a quadrature mixing receiver correspond to a pure frequency translation, mismatches introduce a frequency translation that results in a mixture of an image and a desired signal when the in-phase and quadrature phase channels of the received RF signal are not exactly 90 degrees out of phase, and the image is seen as interference.

The quadrature imbalance refers to the process of quadrature phase splitting, which introduces some degree of cross talk between data streams (I and Q data streams), and wherein a level of I data will be superimposed on Q, and vice versa. Since the two channels are noise-like in nature and uncorrelated, the cross talk signal will appear as degradation in carrier-to-noise of the independent data streams. Quadrature cross talk is normally expressed in terms of I and Q gain and phase imbalance.

Several typical receiver calibration techniques for calibrating receiver parameters, such as quadrature imbalance, DC offset, and channel filter corner frequency tuning, are currently used in communication systems. Typical receiver calibration techniques are broadly classified into signal based and noise based.

Loop back is a conventional signal-based technique in which a reference signal is generated from a calibrated transmit path. The signal is looped back to receiver inputs. The loop back technique lacks receiver calibration accuracy limited by transmitter signal's calibration accuracy. Also, RF input balance, noise, and undesired coupling mechaninisms reduce performance of the receiver.

Tone generator is another typical signal based technique in which a dedicated circuit is used for generating reference signals. This technique requires a complex dedicated circuit which increases complexity and die area. The tone generator can disturb the performance of the receiver.

Measuring the quadrature cross-correlated noise is a noise based typical receiver calibration technique which lacks in accuracy. Filter tuning in this method is limited due to dynamic range limitations. SUMMARY

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 illustrates a receiver calibrating system in an embodiment in accordance with the invention; and

FIG. 2 is a flow diagram illustrating the sequence of steps in a method for calibrating a receiver according to an embodiment in accordance with the invention. DETAILED DESCRIPTION

Embodiments in accordance with the invention offer a system and method for calibrating a receiver which provides calibration of Radio Frequency (RF) impairments namely quadrature imbalance, Direct Current (DC) offset, and channel filter corner frequency. In deep-submicron Complementary Metal Oxide Silicon (CMOS) processes (<120nm gate length), it is possible to generate extremely fast digital signal edges with high frequency content, up to radio frequencies, typically greater than IGHz. The high frequency part of the digital signal can be used as a "test signal" to calibrate the RF receiver for its traditional impairments such as quadrature imbalance, DC offset and channel filter corner frequency calibrations. Embodiments in accordance with the invention may be implemented in software or firmware, as well as in programmable gate array devices, Application Specific Integrated Circuit (ASIC) and other hardware.

FIG. 1 illustrates at 100 the receiver calibrating system embodiment in accordance with the invention. The receiver calibrating system 100 includes a harmonic generator 105, a low noise amplifier 110, a down mixer 115, a set of channel filters 120, a set of Analog- to- Digital Converters (ADC) 125, and a digital signal processor 130.

A digital signal (clock) is presented to the harmonic generator 105. The harmonic generator 105 converts the digital signal to a differential signal with sharp edges in order to create a large harmonic content. The harmonic generator 105 output drives the Low Noise Amplifier (LNA) 110 inputs, and harmonics of the clock signal are present at RF 135, close to the LO (Local Oscillator) frequency. The harmonics are amplified by the LNA 110. The LNA outputs are then down converted from RF to quadrature baseband (BB) 140 using the down mixer 115.

The low frequency BB signals (I and Q) are filtered using the set of channel filters 120 and fed into the inputs of ADC 125. The harmonics quadrature imbalance is measured using a Digital Signal Processor (DSP) 130. Digital post distortion coefficients are calculated and applied to incoming received signals. Likewise, the DC offset and filter corner frequency is measured using DSP 130 and the calibration is partially done in the analog and digital domains. In accordance with the invention, digital signals can be injected after the LNA 110. Digital signals can also be injected after the down mixer 115 and digital signals can be injected after the channel filter 120. The common mode signals can also be injected to the LNA 110 input to calibrate second order effect. It should be appreciated that the calibration technique in accordance with invention is not limited to direct conversion receivers and can be also applied to near zero IF receivers.

FIG. 2 is a flow diagram illustrating the sequence of steps in a method for calibrating a receiver according to an embodiment in accordance with the invention. Step 205 includes the step of converting an input signal into a differential signal thereby generating large harmonic content of the input signal (using a harmonic generator 105). Step 210 includes the step of amplifying the harmonic content (using an LNA 110). Step 215 includes the step of down converting the harmonic content present in radio frequency to a quadrature baseband signal (using a down mixer 115). Step 220 includes the step of filtering the baseband signals (using a set of channel filters 120 operative Iy coupled the down mixer 115). Step 225 includes the step of converting the baseband signals into a digital signal (using an ADC 125). And step 230 includes the step of processing the digital signal by measuring impairments thereby calibrating the receiver (using a DSP 130).

Embodiments in accordance with the invention provide the following advantages: The receiver calibrating system and method according to the embodiments in accordance with the invention do not need a complex dedicated tone generator, and only requires a simple Complimentary Metal Oxide Semiconductor (CMOS) inverter circuit. Embodiments of the invention provide limited disturbance to receiver inputs, since CMOS inverter loading is negligible and also provide a large dynamic range. Quadrature imbalance can be measured across all base band / channel filter gain modes. The inverter dynamic range can be adjusted via the digital processor.

Embodiments in accordance with the invention can be applied to RF communications systems that use quadrature receivers as a means to convert quadrature analog signals or multiple signal paths to the digital domain. In particular, embodiments in accordance with the invention can be used for, but not restricted to, Wireless Local Area Network (WLAN), Television on Mobile (TVoM), Worldwide Interoperability for Microwave Access (WiMax), and Multiple Input Multiple Output (MIMO) and Cellular systems.

The forgoing description sets forth numerous specific details to convey a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the invention may be practiced without these specific details. Well-known features are sometimes not described in detail in order to avoid obscuring the invention. Other variations and embodiments are possible in light of above teachings, and it is thus intended that the scope of invention not be limited by this Detailed Description, but only by the following Claims.

Claims

1. A receiver calibrating system comprising: a harmonic generator for generating a harmonic content of an input signal; an amplifier operatively coupled to said harmonic generator for amplifying said harmonic content; a down mixer operatively coupled to said amplifier for down converting the harmonic content to a quadrature baseband signal; at least one analog-to-digital converter for converting said baseband signal into a digital signal; and a digital signal processor operatively coupled to said analog-to-digital converter for processing said digital signal to measure impairments in the digital signal thereby calibrating said system.

2. The receiver calibrating system of claim 1 further comprising: at least one channel filter operatively coupled to said down mixer for filtering the baseband signals.

3. The receiver calibrating system of claim 1, wherein said impairments include quadrature imbalance, channel filter corner frequency and direct current offset.

4. The receiver calibrating system of claim 3, wherein a quadrature imbalance of the digital signal is measured using said digital signal processor, and digital post distortion coefficients of the digital signal are calculated and applied to the input signals.

5. The receiver calibrating system of claim 3, wherein said channel filter corner frequency and direct current offset are calculated using the digital signal processor, and calibration is performed in analog and digital domains.

6. The receiver calibrating system of claim 1 , wherein the amplifier comprises a low noise amplifier.

7. The receiver calibrating system of claim 1 , wherein said input signal comprises a digital signal and an impulse signal and combinations thereof.

8. The receiver calibrating system of claim 1, wherein the harmonic generator converts said input signal to a differential signal thereby generating a harmonic content of the input signal.

9. The receiver calibrating system of claim 1 , wherein the harmonic generator converts said input signal to a single ended signal thereby generating a harmonic content of the input signal.

10. A method for calibrating a receiver comprising: generating harmonic content of an input signal; amplifying said harmonic content; down converting the harmonic content to a quadrature baseband signal; converting said baseband signals into a digital signal; and processing said digital signal to measure impairments in the digital signal thereby calibrating said receiver.

11. The method of claim 10 further comprising: filtering the baseband signal using at least one channel filter.

12. The method of claim 10, wherein said impairments include quadrature imbalance, channel filter corner frequency and direct current offset.

13. The method of claim 12, wherein a quadrature imbalance of the digital signal is measured and digital post distortions coefficients of the digital signal are calculated and applied to the input signals.

14. The method of claim 12, wherein said channel filter corner frequency and direct current offset are calculated and calibration is performed in analog and digital domains.

15. The method of claim 10, wherein said input signal is converted to a differential signal thereby generating said harmonic content of the input signal.

16. The method of claim 10, wherein said input signal is converted to a single ended signal thereby generating said harmonic content of the input signal.

17. A computer program product used in a computer system to perform the method of calibrating a receiver comprising: generating harmonic content of an input signal; amplifying said harmonic content; down converting the harmonic content to a quadrature baseband signal; converting said baseband signals into a digital signal; and processing said digital signal to measure impairments in the digital signal thereby calibrating said receiver.

18. A computer program product as recited in claim 17 further comprising: filtering the baseband signal using at least one channel filter.

19. A computer program product as recited in claim 17, wherein said impairments include quadrature imbalance, channel filter corner frequency and direct current offset.

20. A computer program product as recited in claim 17, wherein a quadrature imbalance of the digital signal is measured and digital post distortions coefficients of the digital signal are calculated and applied to the input signals.

21. A computer program product as recited in claim 19, wherein said channel filter corner frequency and direct current offset are calculated and calibration is performed in analog and digital domains.

22. A computer program product as recited in claim 17, wherein said input signal in converted to a differential signal thereby generating said harmonic content of the input signal.

23. A computer program product as recited in claim 17, wherein said input signal is converted to a single ended signal thereby generating said harmonic content of the input signal.