# Patent

## Title
I/Q calibration techniques

## Abstract

A receiver includes a static I/Q calibration block and a correlation/integration block. The static I/Q calibration block is configured to substantially eliminate mismatches between in-phase and quadrature components of a portion of the spectrum having associated I/Q mismatches that are relatively frequency-independent. The correlation/integration block is configured to substantially eliminate mismatches between the in-phase and quadrature components of portions of the spectrum having associated I/Q mismatches that are relatively frequency-dependent in accordance with a pair of signals generated by the static I/C calibration block.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application is a continuation application of U.S. application Ser. No. 12/240,734, filed Sep. 29, 2008, which claims benefit under 35 USC 119(e) of U.S. provisional application No. 60/976,695, filed Oct. 1, 2007, and U.S. provisional application No. 60/977,020, filed Oct. 2, 2007, both entitled “I/Q Calibration Techniques”, the contents of which are incorporated herein by reference in their entirety.


BACKGROUND OF THE INVENTION
In systems with analog or RF complex signal paths, such as direct conversion and low-IF receiver systems, it is necessary to calibrate the system's balance of in-phase (I) and quadrature (Q) amplitude and phase signals. This is important because imbalances generate interference within the received signal by folding the negative frequencies in the complex path onto the desired signal. In a direct conversion receiver, this process causes the signal to fold onto itself, and is a well-known problem. The extent to which the interference from folding is rejected is referred to as sideband rejection.
A number of techniques have been developed to perform static I/Q calibration and to estimate the phase and amplitude imbalance. I/Q imbalance, or mismatch, is conventionally modeled as constant across the communication channel. This approximation is acceptable in narrow-band systems where the mismatch is dominated by RF contributions associated with mismatches occurring in components such as local oscillators (LO) and mixers. Such RF-based mismatches are referred to herein as static I/Q mismatches or static I/Q imbalances because they are treated as constant across the channel frequency.
However, in wideband communications systems there are mismatches that occur in the baseband circuits. Such mismatches make it difficult to achieve sideband rejection of greater than 45 dB across the band. For example, frequency-dependent mismatches in the baseband analog signal path can result in substantial degradation of sideband rejection to the 40 dB level, when 60 dB or better is required for demanding applications such as broadcast analog television applications.
FIG. 1 illustrates a simplified direct conversion receiver, as known in the prior art. I/Q mismatches introduced by LO 18's phase and amplitude differences as well as differences between I-path and Q-path mixers 22 and 12 generally result in static I/Q mismatches. Analog filter 24 disposed in the I-path, and an analog filter 14 disposed in the Q-path are adapted to reduce levels of signal spectrum that are close to the filter passband edges. Such signals often have relatively sharp transition bands, resulting in the presence of poles with high Q-factors. These poles are particularly sensitive to analog component mismatches. The resulting transfer function difference between the analog filters in the I and Q paths creates frequency-dependent mismatches. Such mismatches degrade the sideband rejection performance and continue to increase towards the filter band edge.
BRIEF SUMMARY OF THE INVENTION
A receiver, in accordance with one embodiment of the present invention, includes, in part, a local oscillator; first and second frequency conversion modules, first, second, third and fourth filter, a calibration block, and a correlation/integration block. The first frequency conversion module is responsive to the local oscillator signal and to a received signal to generate an in-phase signal. The second frequency conversion module is responsive to a phase-shifted local oscillator signal and to the received signal to generate a quadrature signal. The first filter is responsive to the in-phase signal, and the second filter is responsive to the quadrature. The third filter is responsive to the first filter, and the fourth filter is responsive to the second analog filter. The calibration block is responsive to the outputs of the third and fourth filters. The correlator/integrator is responsive to the outputs of the calibration block and is adapted to generate a first feedback signal applied to the third filter and a second feedback signal applied to the fourth filter. The first and second feedback signals are operative to vary frequency characteristics of the third and fourth filters so as to compensate for frequency dependent mismatches in the first and second filters. In one embodiment, the third and fourth filters are digital filters.
In one embodiment, the calibration block includes, in part, a first low-pass filter responsive to the third filter, a second low-pass filter responsive to the fourth filter; a phase detection block adapted to detect a difference between phases of the signals generated by the first and second low-pass filters, a first multiplier adapted to multiply the detected phase difference by an output of the third filter, a first signal combiner adapted to subtract an output of the first multiplier from the output of the third filter to generate a first output signal, an amplitude detection block adapted to detect a difference between amplitudes of the signals generated by the first and second low-pass filters, and a second multiplier adapted to multiply the detected amplitude difference by an output of the third filter to generate a second output signal.
In one embodiment, the correlator/integrator block further includes, in part, a first mixer responsive to the first output signal and to a first oscillating signal having an oscillation frequency falling within a frequency band filtered out by the first filter thereby to generate a third signal, a second mixer responsive to the first output signal and to a second oscillating signal having a ninety degrees phase shift with respect to the first oscillating signal thereby to generating a fourth signal, a third mixer responsive to the second output signal and to the first oscillating signal thereby to generate a fifth signal, and a fourth mixer responsive to the oscillating signal thereby to generate a sixth signal. The third and fifth signals define a first complex signal about an offset frequency. The fourth and sixth signal define a second complex signal about the offset frequency. The correlation/integration block is adapted to integrate the first and second complex signals to generate the first and second feedback signals.
A method for performing calibration in a receiver, in accordance with one embodiment of the present invention, includes, in part, frequency converting a received RF signal to a first in-phase signal and a first quadrature signal; filtering the first in-phase signal in an in-phase signal path to generate a second in-phase signal; filtering the first quadrature signal in a quadrature signal path to generate a second quadrature phase signal; performing a first digital filtering operation in response to the second in-phase signal and further in response to a first feedback signal to generate a third in-phase signal; and performing a second digital filtering operation in response to the second quadrature signal and a second feedback signal to generate a third quadrature signal. The first and second feedback signals are adapted to compensate for frequency dependent mismatches in the in-phase and quadrature signal paths.
In one embodiment, the method further includes, in part, detecting a difference between the phases of the second in-phase and quadrature signals; multiplying the detected phase difference by the second in-phase signal to generate a first multiplied signal; subtracting the first multiplied signal from the second quadrature signal to generate a first output signal; detecting a difference between amplitudes of the second in-phase and quadrature signals; and multiplying the detected amplitude difference by the second in-phase signal to generate a second output signal.
In one embodiment, the method further includes, in part, frequency converting the first output signal to a third in-phase signal in response to a first oscillating signal having an oscillation frequency falling within a frequency band used to filter the in-phase signal; frequency converting the first output signal to a fourth in-phase signal in response to a second oscillating signal having ninety degrees phase shift with respect to the first oscillation frequency; frequency converting the second output signal to a third quadrature signal in response to the first oscillating signal; and frequency converting the second output signal to a fourth quadrature signal in response to the second oscillating signal. The third in-phase and third quadrature signals define a first complex signal about an offset frequency. The fourth in-phase and fourth quadrature signals define a second complex signal about the offset frequency. The method further includes, in part, integrating the first and second complex signals to generate the first and second feedback signals.
A method for processing a received signal over wireless communication receiver, in accordance with another embodiment of the present invention, includes, in part, removing mismatches between in-phase and quadrature components of a first portion of a frequency spectrum of the receive signal; and removing mismatches between the in-phase and quadrature components of a second portion of the frequency spectrum of the receive signal. The method further includes, removing the mismatches between the in-phase and quadrature components of the second portion of the frequency spectrum of the receive signal in accordance with a pair of feedback signals generated as a result of removing the mismatches between in-phase and quadrature components of the first portion of a frequency spectrum of the receive signal.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified block diagram of a direct conversion receiver, as known in the prior art.
FIG. 2 is a simplified block diagram of a direct conversion receiver, in accordance with one exemplary embodiment of the present invention.
FIG. 3 is a block diagram of the static I/Q calibration block of the direct conversion receiver of FIG. 2, in accordance with one exemplary embodiment of the present invention.
FIG. 4 is a spectrum of a signal received by the static I/Q calibration block of FIG. 3.
FIG. 5 is a block diagram of the correlation/integration block of the direct conversion receiver of FIG. 2, in accordance with one exemplary embodiment of the present invention.



DETAILED DESCRIPTION OF THE INVENTION
In accordance with one embodiment of the present invention, I/Q mismatches (skews) in a direct conversion receiver is substantially minimized FIG. 2 is a block diagram of a direct conversion receiver 500 shown as including, in part, an analog front end 100, a digital baseband 200, a static I/Q calibration block 300, and a correlator/integrator 400. The baseband analog I/Q signal path mismatches, such as those from the analog filters 14 and 24, are modeled as a systematic frequency response scaling in the analog signal path transfer function. In other words, mismatches in the analog signal path are treated as being frequency dependent.
FIG. 4 is a spectrum 370 of a signal received by receiver 500. Spectrum 370 is shown as having a first portion 355 associated with I/Q mismatches that are relatively frequency-independent, and second portions 350, 352 associated with I/Q mismatches that are frequency-dependent. Portions 350 and 352 are closer to edges of the bandpass filter and are alternatively referred to hereinbelow as offset portions. As described above, the static I/Q mismatch, corresponding to the mismatch in the spectrum portion 355 of the received signal, e.g., at the outputs of filters 14 and 24, is relatively frequency independent and may be eliminated using any one of a number of conventional techniques. Mismatches in the offset portions 350, and 352 (also shown in FIG. 4 as dF) of the received spectrum are treated as being frequency dependent. As described further below, mismatches in the portion 355 of the spectrum between the I/Q channels is extracted using static I/Q calibration block 300. Mismatches in the offset portions 350, 352 of the spectrum are extracted using correlator/integrator block 400.
To eliminate the I/Q mismatches, the frequency response of digital filter 32 disposed in the I-path is varied to mimic the frequency response of the analog filter 24 disposed in the Q-path, and the frequency response of digital filter 42 disposed in the Q-path is varied to mimic the frequency response of the analog filter 14 disposed in the I-path. The complex spectrum characterizing the opposing portions of the offset, namely spectrum portions 350 and 352, provide an estimate of the I/Q skew introduced by the analog signal path mismatches. The transfer functions of the digital filters 32 and 42 are scaled in a direction opposite to the estimated I/Q skew present in the baseband analog filters in order to yield nearly identical overall transfer functions in I and Q paths, as described further below.
Digital baseband block 200 includes circuitry that calibrates mismatches in wideband applications. As described above, static I/Q calibration block 300 detects mismatches between I-channel and Q channel of the spectrum portion 355. Correlator/integrator 400 detects mismatches in the offset portions 350 and 352 of the received spectrum 370 and, in response, generates signals X1 and X2 (−X1). Signals X1 and X2 respectively adjust the frequency characteristics of digital filters 32 and 42, thereby to cancel the frequency dependent mismatches of analog filters 14 and 24. The result is that mismatches between analog filters 24 and 14 are canceled by nearly equal but opposite mismatches between digital filters 42 and 32, in turn, allowing the cascaded I and Q transfer functions to match across a range of frequencies.
RF amplifier 10 is configured to receive and amplify input signal VRF. RF amplifier 10 may be configured to receive a signal, for example, from an antenna or wired connection, such as a single ended wireline, a differential wireline, a twisted pair, a coaxial cable, a transmission line, a waveguide, an optical receiver configured to receive an optical signal over an optical fiber, and the like. In one embodiment, RF amplifier 10 may be a Low Noise Amplifier (LNA). In another embodiment, RF amplifier 10 may be a variable gain amplifier. RF amplifier 10 may be configured as a single-stage or multi-stage amplifier.
The output signal of RF amplifier 10 is shown as being coupled to inputs of first and second frequency conversion modules, 12 and 22. Frequency conversion modules 12 and 22 are shown as being mixers in exemplary embodiment of FIG. 2. Mixers 12 and 22 are configured to generate in-phase (I) and quadrature (Q) frequency down-converted signal components. Mixer 12 is shown as being disposed in the in-phase signal path and mixer 22 is shown as being disposed in the quadrature signal path.
Local oscillator (LO) 18 is configured to generate a local oscillating signal that is applied to mixer 12. Phase shifter 35 shifts the phase of the LO signal by ninety degrees to generate a quadrature LO signal that is applied to mixer 22. The output of mixer 12 is an in-phase signal that is supplied to filter 14. The output signal of filter 12 is amplified by amplifier 16 and subsequently digitized by analog-to-digital converter (ADC) 30. The output of mixer 22 is a quadrature signal that is supplied to filter 24.
The output signal of filter 14 is amplified by amplifier 24 and subsequently digitized by ADC 40. ADCs 30 and 40 respectively apply their output signals to digital filters 32 and 42. The output of digital filter 32 is amplified by variable gain block (stage) 34, and the output of digital filter 42 is amplified by variable gain block 44, as shown.
FIG. 3 is a block diagram of static I/Q calibration block 300 of the direct conversion receiver 200, in accordance with one exemplary embodiment of the present invention. As described above, I/Q calibration block 300 is adapted to detect mismatches in the spectrum portion 355 between the In-phase (I) and quadrature (Q) paths (channels) of receiver 500. Referring to FIGS. 3 and 4 concurrently, low-pass filters 302 and 304 filter out portions 350 and 352 of the total received spectrum 370 associated with the I-channel and Q-channel, that are received respectively from variable gain stages 34 and 44.
Phase error estimation block 306 is adapted to detect a difference between the phases of the I/Q signals received from low-pass filters 302, and 304. Amplitude error estimation block 308 is adapted to detect a difference between the amplitudes of the I/Q signals received from low-pass filters 302, and 304. The phase and amplitude error estimation is performed on the spectrum portion 355 having an I/Q mismatch that is frequency independent. Multiplier 314 multiplies the detected phase difference by the signal present on the I-channel to generate a correction signal A. Correction signal A is subtracted from the signal present on the Q-channel to generate a first output signal Q-out. Multiplier 312 multiplies the detected amplitude difference by the signal present on the I-channel to generate signal I-out. Signals I-out and Q-out represent spectrum portion 370 of the received signal after removal of mismatches therefrom, and are applied to correlator/integrator 400, as shown in FIG. 2.
FIG. 5 is a block diagram of correlator/integrator 400 (hereinafter alternatively referred to as integrator), in accordance with one exemplary embodiment of the present invention. Integrator 400 is adapted to remove mismatches between the in-phase and quadrature components of the offset portions 350, 352 of the frequency spectrum of the receive signal. Integrator 400 is shown as including, in part, a local oscillator (LO) 410 that is tuned to frequency F_offset falling within the offset portions 350 and 352 of the received spectrum.
Signal I_OUT is applied to multipliers (or mixers) 402 and 406. Signal Q_OUT is applied to mixers 404 and 408. Phase shifter 415 generates an oscillating signal that has a 90 degrees phase shift with respect to the output signal of LO 410. The spectrum of the output signals generated by mixers 402, 404, 406, and 408 is centered at DC in one embodiment. Low-pass filters 412, 414, 416 and 418 are adapted to maintain the signal spectrum around ±F_offset by filtering out the known spectrum 355 from the spectrums present at the output of the mixers 402, 404, 406, and 408. Signals E and G represent I-channel and Q-channel signals around frequency (+F_offset), and are collectively referred to as spectrum b+. Signals F and H represent I-channel and Q-channel signals around frequency (−F_offset), and are collectively referred to spectrum b−. Integration/correlation block 420 correlates spectrums b+ and b− to generate signals X1 and X2 (−X1), as described further below. Signals X1 and X2 are applied to digital filters 32 and 34, respectively, as shown in FIG. 5.
Referring to FIG. 2, the baseband analog signal path transfer functions associated with analog filters 14 and 24 may be represented as shown below:

HaI(ω)=Ha(αIω)  (1)

HaQ(ω)=Ha(αQω)  (2)

where αI and αQ are ideally unity, but in practice are not equal to one another due to various mismatches between the components disposed in RF/analog front end 100. Ha represents the ideal analog signal path transfer function. In some embodiments, automatic calibration restricts the range of deviation of αI and αQ so that on average they deviate by, for example, less than 1% from unity.

Portions 350 and 352 of spectrum 370, shown in FIG. 4, are correlated and integrated by correlator/integrator 400, to generate a value X1, as shown below:

X1=∫B+(t)·b−*(t)dt  (3)

where b+(t) and b−(t) represent time domain transformation of signal spectrums b+ and b− described above, and notation * represents the convolution operation. Correlator/integrator 400 is well known. The width dF of the spectral slices 350 and 352 on either end of the spectrum can be adjusted to optimize performance of a particular system. In the digital domain, the baseband analog signal path transfer functions are replicated using digital filters 32 and 42, which have the following transfer functions:

HdI(z−1,X1)=Hd((1+β·X1)·z−1)  (4)

HdQ(z−1,X1)=Hd((1−β·X1)·z−1)  (5)

The frequency responses of digital filters 32 and 42 are scaled with input signals X1 and coefficient β. In some embodiments, this frequency scaling can be implemented in other ways, e.g., using programmable filter taps in the filter.
The feedback circuit disposed between filters 32, 42, static I/Q calibration block 300 and integrator/correlator 400 operates as follows. As X1 increases, the frequency response of filter 32 moves to reduce the correlation while the frequency response of filter 42 moves in the opposite direction, so that

2β·X1≅αQ−αI  (6)

Because deviation of αI and αQ from unity is small, the phase and amplitude differences between the I and Q baseband analog signal paths are approximately dependent only on the difference between αI and αQ and only weakly dependent on their absolute values.
The above embodiments of the present invention are illustrative and not limiting. Various alternatives and equivalents are possible. The invention is not limited by the type of integrated circuit in which the present disclosure may be disposed. Nor is the disclosure limited to any specific type of process technology, e.g., CMOS, Bipolar, or BICMOS that may be used to manufacture the present disclosure. Other additions, subtractions or modifications are obvious in view of the present disclosure and are intended to fall within the scope of the appended claims.

## Claims

1. A receiver comprising:
an analog front end configured to generate a baseband signal from a received radio frequency signal;
an analog-to-digital converter configured to convert the baseband signal to a digital baseband signal;
a calibration block configured to compensate for I/Q mismatches in a first portion of the digital baseband signal that is substantially frequency independent, the calibration block comprising a phase error estimator block configured to detect a phase difference between I/Q signals to correct the Q signal and an amplitude error estimator block configured to detect an amplitude difference between the I/Q signals to correct the I signal; and
a correlator/integrator responsive to a feedback signal generated by the calibration block, said correlator/integrator configured to compensate for I/Q mismatches in a second portion of the digital baseband signal that is frequency dependent, the feedback signal being representative of the first portion of the digital baseband whose substantially frequency independent I/Q mismatches are compensated,
wherein the calibration block further comprises a low-pass filter coupled to the phase error estimator block and the amplitude error estimator block and configured to pass the first portion and stop the second portion of the digital baseband signal.

2. The receiver of claim 1 further comprising:
an analog filter disposed in the analog front end and configured to filter the baseband signal; and
a digital filter coupled to the analog-to-digital converter and configured to mimic a frequency response of the analog filter.

3. The receiver of claim 2 wherein the digital filter is a finite impulse response filter having a plurality of coefficients.

4. The receiver of claim 3 wherein the coefficients are controlled by the correlator/integrator.

5. A method for removing an I/Q mismatch in a receiver, the method comprising:
frequency converting a received RF signal to a baseband signal;
converting the baseband signal to a digital baseband signal;
passing a first portion and stopping a second portion of the digital baseband signal via a low-pass filter;
compensating, via a first block, for I/Q mismatches in the first portion of the digital baseband signal that is substantially frequency independent; and
compensating, via a second block responsive to a feedback signal from the first block, for I/Q mismatches in the second portion of the digital baseband signal that is frequency dependent, the feedback signal being representative of the first portion of the digital baseband signal whose substantially frequency independent I/Q mismatches are compensated,
wherein the first block comprises a phase error estimator block configured to detect a phase difference between I/Q signals to correct the Q signal and an amplitude error estimator block configured to detect an amplitude difference between the I/Q signals to correct the I signal.

6. The method of claim 5 further comprising:
filtering the baseband analog signal by an analog filter;
mimicking a frequency response of the analog filter by a digital filter.

7. The method of claim 6 wherein the digital filter is a finite impulse response having a plurality of coefficients.

8. The method of claim 7 wherein the plurality of coefficients is controlled by the second block.

9. The receiver of claim 1 wherein the feedback signal comprises a phase corrected Q signal and an amplitude corrected I signal.