WO2009089405A1

WO2009089405A1 - Bandwidth extension of an iq mixer by way of baseband correction

Info

Publication number: WO2009089405A1
Application number: PCT/US2009/030523
Authority: WO
Inventors: Joel B. Simoneau; L. Wilson Pearson
Original assignee: Clemson University
Priority date: 2008-01-11
Filing date: 2009-01-09
Publication date: 2009-07-16

Abstract

Disclosed is a methodology for extending the bandwidth of a transmit IQ mixer by compensating for the magnitude and phase mismatch in the mixer with the use of baseband compensation constants. In phase and quadrature signals may be split, adjusted by compensation constants, and re-combined at baseband frequencies before being applied to mixers for further combination to produce a compensated radio frequency (RF) output. The correction system is amenable to real-time implementation, thereby providing substantial resiliency to changes in the operating environment.

Description

BANDWIDTH EXTENSION OF AN IQ MIXER BY WAY OF BASEBAND

CORRECTION

FIELD QF THE INVENTION

[0001] The present subject matter relates to transmitter mixers. More particularly, the present subject matter relates to software defined radio transmitters and to improvements in IQ mixer bandwidth associated with such transmitters.

BACKGROUND OF THE INVENTION

[0002] In software defined radio (SDR) transmitters, it is attractive to apply complex signal processing in software at baseband or a low intermediate frequency (IF). By employing complex signal processing, the data conversion rate and therefore power consumption and cost may be kept as low as possible. The use of certain radio frequency components such as a phase shifter or a high-Q radio frequency (RF) filter may also be avoided employing such methodology.

[0003] The application of complex signal processing to a transmitter is dependent on the phase and amplitude match between channels in an IQ mixer that serves as an upconverter. In the context of dynamic radio, in which the transmitter is frequency-agile, this complex signal processing is even more attractive because it can potentially obviate the use of a frequency-agile post-selection RF filter, and may also be employed to give a broadband phase shift for beam-steering purposes. Some commercial off-the-shelf IQ mixer units exhibit a 100% local oscillator bandwidth (e.g. 1.5 to 4.5 GHz). A useful target for civilian dynamic radio is to achieve a bandwidth of 300 MHz to 6 GHz — i.e., 181% bandwidth. Most previous design concepts would allow some switching over this band. [0004] Various designs have been proposed for extending the bandwidth of an IQ mixer. In E. Tiiliharju's PIiD. Dissertation for the Dept. Elect. Eng., Helsinki Univ. of Tech., Helsinki, Finland, 2006 entitled, "Integration of Broadband Direct-Conversion Quadrature Modulators," the author presents switched lumped circuit hybrids to produce a resulting total bandwidth of .56-4.76 GHz (157% bandwidth) with a phase variation of less than 2 degrees and a magnitude variation of less than .3 dB. Recently, Mo, et. al. have created a 180-degree hybrid coupler with 120% bandwidth and 25 degrees phase variation over the band as described in their publication: "A Broadband Compact Microstrip Rat- Race Hybrid Using a Novel CPW Inverter," IEEE Trans. Microwave Theory and Tech., Vol. 55, no. 1, January, 2007, pp. 161-167.

[0005] Other recent work by Gruszczynski et al. entitled, "Design of Compensated Coupled-Stripline 3-dB Directional Couplers, Phase Shifters, and Magic-T's — Part I: Single-Section Coupled-Line Circuits," published in IEEE Trans. Microwave Theory and Tech., Vol. 54, no. 11, November, 2006, pp. 3986-3994 and by Gruszczynski et al. entitled "Design of Compensated Coupled-Stripline 3-dB Directional Couplers, Phase Shifters, and Magic-T's — Part II: Broadband Coupled-Line Circuits," published in IEEE Trans. Microwave Theory and Tech., Vol. 54, no. 11, November, 2006, pp. 3501-3507 present an analog compensation approach that achieves decade bandwidth hybrids, but in three- section devices. The multiple sections create a large footprint for the component. [0006] In the Zheng et al. article entitled "Self tuned fully integrated high image rejection low IF receivers: architecture and performance." published in Proc. Int. Symp. on Circ. and Sys., 2003. Vol. 2, pgs 165-168 May 25-28, 2003, an amplifier configuration is used to correct for inaccuracies in an IQ mixer in receive mode, with analog variable-gain amplifiers (VGA) implementing two compensation constants. [0007] While the prior art has offered multiple approaches to provide bandwidth extension, it would be beneficial to consider approaches that exploit more of the degrees of freedom available to a designer.

[0008] Further, it would be beneficial to provide an amplifier configuration that has improved suitability for any intermediate frequency (IF) from zero to the maximum output bandwidth of the data converter.

[0009] While various implementations of transmitter mixers have been developed, and while various software defined radio transmitters have been developed, no design has emerged that generally encompasses all of the desired characteristics as hereafter presented in accordance with the subject technology.

SUMMARY OF THE INVENTION

[0OiO] In view of the recognized features encountered in the prior art and addressed by the present subject matter, improved apparatus and methodology for providing IQ mixer bandwidth extension have been provided.

[0011] In accordance with the present technology, a method for extending the bandwidth of a conventional transmit IQ mixer employing compensation constants that are a function of the local oscillator frequency in order to attain magnitude and phase match for the IQ mixer has been provided.

[0012] In one of its forms, signal processing is employed in the baseband portion of the transmitter in order to reduce the number of radio frequency components and also reduce the required data conversion rate.

[0013] Another positive aspect of this type of device is that the present methodology may be implemented in either hardware or software.

[0014] In accordance with certain aspects of other embodiments of the present subject matter, methodologies have been developed to develop compensation coefficients either during manufacture or on the fly using real time feedback.

[0015] In accordance with yet additional aspects of further embodiments of the present subject matter, apparatus and accompanying methodologies have been developed to provide phase shift and dynamic range compensation for an IQ mixer.

[0016] Additional objects and advantages of the present subject matter are set forth in, or will be apparent to, those of ordinary skill in the art from the detailed description herein.

Also, it should be further appreciated that modifications and variations to the specifically illustrated, referred and discussed features and elements hereof may be practiced in various embodiments and uses of the invention without departing from the spirit and scope of the subject matter. Variations may include, but are not limited to, substitution of equivalent means, features, or steps for those illustrated, referenced, or discussed, and the functional, operational, or positional reversal of various parts, features, steps, or the like.

[0017] Still further, it is to be understood that different embodiments, as well as different presently preferred embodiments, of the present subject matter may include various combinations or configurations of presently disclosed features, steps, or elements, or their equivalents including combinations of features, parts, or steps or configurations thereof not expressly shown in the figures or stated in the detailed description of such figures. Those of ordinary skill in the art will better appreciate the features and aspects of such embodiments, and others, upon review of the remainder of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which: [0019] Figure 1 illustrates a block diagram of IQ mixer compensation scheme in accordance with the present subject matter;

[0020] Figure 2a is a phasor diagram of an IQ mixer output with magnitude and phase mismatch;

[0021] Figure 2b is a phasor diagram of an IQ mixer output with compensation constants employed per present disclosure where resulting vectors are illustrated in solid line and summands are illustrated in dashed lines;

[0022] Figure 3 is a plot of Conversion Gain vs. Angle mismatch in an exemplary IQ

Mixer constructed in accordance with the present technology;

[0023] Figure 4 is a plot of Image rejection vs. Frequency for the compensated system per present disclosure; and

[0024] Figure 5 is an exemplary plot of Dynamic Range Gain vs. Frequency in an uncompensated IQ mixer.

[0025] Repeat use of reference characters throughout the present specification and appended drawings is intended to represent same or analogous features or elements of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] As discussed in the Summary of the Invention section, the present subject matter is particularly concerned with improved apparatus and methodology for providing IQ mixer bandwidth extension.

[0027] Selected combinations of aspects of the disclosed technology correspond to a plurality of different embodiments of the present invention. It should be noted that each of the exemplary embodiments presented and discussed herein should not insinuate limitations of the present subject matter. Features or steps illustrated or described as part of one embodiment may be used in combination with aspects of another embodiment to yield yet further embodiments. Additionally, certain features may be interchanged with similar devices or features not expressly mentioned which perform the same or similar function. [0028] Reference will now be made in detail to the presently preferred embodiments of the subject IQ mixer bandwidth extension apparatus and methodology. Referring now to the drawings, Figure 1 illustrates a block diagram for a hybrid mixer 100 and an exemplary compensation circuit 110. The dividers 112, 114 and combiners 122, 124 as illustrated are "ideal" in the sense that that an input voltage on a divider is present on both outputs and the output voltage of a combiner is the sum of the two input voltages.

[0029] The results derived from the circuit of Figure 1 can be corrected for "real" divider performance with scale factors appropriate to the dividers provided by selected devices providing correction factors Cn, C₂₁, C_1Q, and C_2Q. Such selected device, as well as the dividers and combiners, will depend on whether the present technology is implemented in software or hardware but generally may correspond to adjustable gain amplifiers whose gain may be controlled by D/A converters or by equivalently functioning hardware or software.

[0030] An in-phase signal /„, is applied to an input terminal of divider 112 while quadrature signal Q_1n is applied to an input terminal of divider 114. The signals delivered to the input ports 132, 134 of the output RF combiner 130 are denoted Si and S₂. The final

RF output from combiner 130 is denoted as S.

[0031] Figure 1 illustrates a partitioning of the components into baseband (or IF) operation and RF operation, which are joined at the mixers 142, 144. Mixer module 100 containing a local oscillator with an angular frequency ωw that is configured to produce outputs cos(<ø£o0 and Asin(ωιot+φ) as represented by the illustrated oscillators 152, 154, respectively.

[0032] One motivation for seeking exceptional bandwidth from IQ mixers is their application in frequency- agile radio. The mixer "error," that is, the extent of its departure from ideal, is a function of the local oscillator frequency CO_LO- Consequently, the compensation elements in the baseband block 110 will need to take on values based on (Di_O- If the compensation is implemented in software, as is also contemplated by the present disclosure, prior to D/A conversion, this frequency dependence compensation is easily implemented. On the other hand, when the present scheme is implemented with analog compensation, the frequency programming presents some practical issues. [0033] A matrix description of the signals in the network can be constructed as

S. - [M][C] 4 (3.1)

Q₁ m where M is a 2 x 2 matrix characterizing the mixer, and C is a 2 x 2 matrix comprising the four correction factors C_n, C₂₁, C,_Q, and C_2Q shown in Figure 1. The mixer matrix relates baseband quantities to RF quantities. If the mixer was "ideal" and no compensation was needed, equation 3.1 would reduce through Cs becoming an identity matrix and M's taking the form associated with a perfect quadrature mixer as follows:

1 0

(3.2)

& o JK

[0034] If the mixer departs from the ideal, with no correction, equation 3.1 becomes

Here, A is a non-unity amplitude factor of the quadrature channel and φ is the phase error between the two channels, with the in-phase channel taken as the reference. The corrected signal matrix then is:

Expanding the matrix product to obtain Sj and S₂ and summing these two signals gives

S - (c +C Ae'^)l +(c +C Ae^ji?^/rΦΑθ (3.5)

By setting

[Vi-* ,-^J'A C, 10 + C₂₀Ae "A-* (3.6)

an RF output is created with signals in mutual quadrature and proportional, respectively, to /,„ and Q_1n.

[0035] Equation 3.6 is complex valued and can be separated into two real-valued equations. The C_υ parameters provide four degrees of freedom, while the present objective is to correct for A and φ. If the parameters are normalized by setting Ci_/ =1 and require that

this results in four equations and four unknowns. The constraint on the magnitude of the cross terms serves to balance signals into the D/A converters in the top and bottom halves of the system. The 4 x 4 system can be solved to obtain:

C_u = l , (3.7a) -tan φ -sin φ

^C\O = T lT±sec ³Tø = cos^ JT±Tl > ^md (³ -^7b)

Since the choice of plus or minus is arbitrary, we select it to be the sign of cosφ so that C_\Q and C_2/ remain finite for all values of φ .

[0036] The compensation scheme according to the present technology may also be expressed in the phasor domain as a scalar multiplication and phasor addition. Figures 2a and 2b illustrate this point. In Figure 2a, the uncorrected output is shown with magnitude and phase errors on the quadrature component. In Figure 2b, the compensation scheme is illustrated, showing the 90 degree phase shift between the two resulting vectors. As can be seen from the constants in equation 3.7, the dynamic range at the output of the D/ A converter that compensates for the amplitude will be decreased by |201og _^4| dB.

[0037] The trade-off encountered in the phase compensation of this scheme is the ratio of radio frequency amplitude to baseband input.

This term may be interpreted as a conversion loss, since it is always less than or equal to one, and it multiplies the magnitude of the upconverted signal. It may be seen from this expression how the method fails at φ = {in + 1) π/1 . Since the magnitude of the RF amplitudes is zero, the signal is in fact not upconverted at all for this value of phase error. A plot of the conversion gain introduced versus angle is given in Figure 3. [0038] For a given mixer, the parameters A and φ are not known and generally are not directly observable. However, Weaver's method for generation of a single sideband signal as described in his article entitled "A third method of generation and detection of single sideband signals," published in Proc. IRE, vol. 44, Dec, 1956, pp. 1703-5, provides a means for indirect observation of IQ quality.

[0039] The parameter A may be measured by taking the ratio of two successive output measurements: one with a signal applied to the quadrature input of the mixer and a second with the same signal applied to the in-phase input. The Weaver method is sensitive to phase mismatch in an IQ mixer. In the method, a cosine at an intermediate frequency (IF) is used as the in-phase input and a sin at that IF is fed to the quadrature input. With a perfectly matched IQ mixer, the signal at the sum frequency of the LO and IF will cancel, and the signal at the difference frequency will sum together, providing a single-sideband output. The cancellation at the sum frequency depends on accurate magnitude and phase match in the IQ mixer. Therefore, observation of the sum-frequency signal provides a measure amplitude/phase tracking in the mixer channels.

[0040] Simple circuitry that allows feedback of the output spectrum in a software- defined radio (SDR) transmitter into the digital processing can be employed with the present subject matter to allow the processor to monitor the sum-frequency sideband as defined by Weaver as previously noted. By monitoring the output spectrum one can then view the adjustment of the constants in Figure 1 as an optimization problem with the sum- signal amplitude to be minimized as a function of the compensation constants. This optimization can be executed in a processor, essentially amounting to a tuning of the compensation to optimize the IQ tracking in the hybrid. This procedure can be repeated across the LO frequency range and tabulated. The procedure may be automated to correct the tracking tuning "on the fly" as it were, allowing correction as mixer parameter values change in a system's operating environment.

[0041] In order to demonstrate the effectiveness of the present technology, an Eclipse Microwave IQ8012 IQ mixer with a specified bandwidth of 8-12 GHz (40% Bandwidth) was employed to obtain experimental results. This IQ mixer has a nominal image rejection of 25 dB over the 8-12 GHz band, which corresponds to a phase match within +7 degrees and a magnitude accuracy within + IdB. The compensation scheme described here above with respect to Figure 1 was employed. At each frequency, the value of A and φ that gave the greatest image rejection was used.

[0042] The results of the compensation scheme for image rejection versus frequency are shown in Figure 4. In the frequency range of 0.75 to 20 GHz (185% bandwidth), it is shown that the amplitude and phase mismatch are +0.3 dB and +1.8 degrees respectively. This rejection is achieved employing stored data that compensates for the device characteristics derived in a single set of measurements at room temperature. Such data may be stored in a memory associated with a processor configured to control the correction factors Cn, C₂₁, C₁Q, and C₂Q shown in Figure 1. Experience with automatic sidelobe canceling as described in the Simoneau article previously mentioned suggests that the unwanted harmonic can be suppressed at levels below -60 dB.

[0043] Two types of conversion loss are incurred through the use of this scheme. One is a result of the compensation scheme, as stated in equation 3.8. The other is a result of the mixer circuit operating outside its designed bandwidth. For the low frequency ranges, a conversion loss in excess of 12 dB above the in-band value was incurred by this second type. These results are, in part, a consequence of employing an off-the-shelf device in the present demonstration. Generally the use of a hybrid and diode switch combination will obviate most of the excess conversion loss.

[0044] For the implementation with this specific mixer, the conversion loss is less than 3 dB over the entire range. A plot of the dynamic range lost versus frequency as a result of the parameter A in the uncompensated IQ mixer is given in Figure 5. It shows that up to 15 dB of dynamic range is lost because of the magnitude of the tracking imbalance (2.5 bits of dynamic range in a D/ A converter) between the in-phase and quadrature branches. [0045] The IQ mixer compensation scheme shown in Figure 1 was employed to extend the bandwidth of an IQ mixer from 8 to 12 GHz (40%) with magnitude and phase mismatch of + IdB and +7 degrees, respectively, to an IQ mixer with a bandwidth of .75- 20 GHz with a magnitude and phase mismatch of + .3 dB and +1.8 degrees, respectively. A maximum dynamic range loss of 15 dB was incurred. The conversion loss incurred due to phase match was less than 3 dB over the entire range; however, a conversion loss of greater than 12 dB was incurred due to the output of the mixer operating outside its designed frequency.

[0046] While the present subject matter has been described in detail with respect to specific embodiments thereof, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing may readily produce alterations to, variations of, and equivalents to such embodiments. Accordingly, the scope of the present disclosure is by way of example rather than by way of limitation, and the subject disclosure does not preclude inclusion of such modifications, variations and/or additions to the present subject matter as would be readily apparent to one of ordinary skill in the art.

Claims

What is claimed is:

1. An IQ mixer, comprising: a first mixer having first and second inputs and an output; a second mixer having first and second inputs and an output; a combiner having first and second inputs and an output, the output of said first mixer coupled to the first input of the combiner and the output of said second mixer coupled to the second input of the combiner; a first oscillator having an output coupled to one of the first and second inputs of the first mixer; and a second oscillator having an output coupled to one of the first and second inputs of the second mixer.

2. The mixer of claim 1, wherein the output frequency of the first oscillators is equal to the output frequency of the second oscillator.

3. The mixer of claim 2, wherein the output of the second oscillator is shifted in phase from the output of the first oscillator.

4. The mixer of claim 3, wherein the output of the second oscillator is shifted in phase by 90^° plus an offset amount from the output of the first oscillator.

5. The mixer of claim 1, further comprising: a first divider having a first input and first and second outputs; a second divider having a first input and first and second outputs; a second combiner having first and second inputs and an output; and a third combiner having first and second inputs and an output, wherein the first output of the first divider is coupled to the first input of the second combiner, the second output of the first divider is coupled to the first input of the third combiner, the first output of the second divider is coupled to the second input of the third combiner, the second output of the second divider is coupled to the second input of the second combiner, the output of the first combiner is coupled to one of the first and second inputs of the first mixer, and the output of the second combiner is coupled to one of the inputs of the second mixer.

6. The mixer of claim 5, wherein the first output of the first divider is coupled to the first input of the second combiner by way of a first amplifier, the second output of the first divider is coupled to the first input of the third combiner by way of a second amplifier, the first output of the second divider is coupled to the second input of the third combiner by way of a third amplifier, the second output of the second divider is coupled to the second input of the second combiner by way of a fourth amplifier.

7. The mixer of claim 6, wherein the first, second, third, and fourth amplifiers are adjustable gain amplifiers.

8. A method for providing IQ mixer bandwidth extension, comprising providing an IQ mixer including an in phase mixer and a quadrature mixer, each mixer having two inputs and an output; applying a phase shifted signal to one input of each of the phase and quadrature mixers; combining the output of each mixer to provide a combined output signal; applying an in phase signal to a second input of the in phase mixer; applying a quadrature signal to a second input of the quadrature mixer; monitoring the combined output signal; and adjusting the in phase and quadrature signals based on variations in the combined output signal.

9. The method of claim 8, further comprising; providing one or more adjustable gain amplifiers coupled to the second inputs of the in phase mixer and the quadrature mixer, wherein adjusting comprises adjusting the gain of the one or more amplifiers.

10. The method of claim 8, further comprising: applying a portion of the in phase signal to each of the in phase and quadrature mixers; and applying a portion of the quadrature signal to each of the in phase and quadrature mixers, wherein adjusting comprises varying the portions of the signals applied to each of the in phase and quadrature mixers.