Cartesian Feedback Research Articles

On the Performance of Cartesian Feedback and Feedforward Linearization Structures Operating at 28 GHz

Some performance results of the Cartesian feedback and feedforward linearization techniques applied to a class-A power amplifier operating at 28 GHz are presented. The performance of the combination of HMMC-5040 (driver) and HMMC-5033 (power amplifier) is used as benchmark for simulation analysis. This analysis is addressed to show the key aspects on spectral regrowth due to phase margin and loop gain for Cartesian feedback, and time delay, phase and gain mismatch for feedforward. A 16 QAM digital signal at 10 Mbits/s filtered with a squared raised cosine filter with /spl alpha/=0, 25 is used as test signal.

A direct-conversion CMOS transceiver for the 802.11a/b/g WLAN standard utilizing a Cartesian feedback transmitter

A fully integrated dual-band transceiver is implemented in 0.18-/spl mu/m CMOS and is compliant with the IEEE 802.11a/b/g standards. The direct-conversion transceiver occupies 12 mm/sup 2/ in a QFN-40 package. A fractional-N synthesizer operates at twice the channel frequency, covering continuously bands from 4.9 to 5.9 GHz, as well as the 2.4-GHz band. The 5- and 2.4-GHz receivers achieve a sensitivity level below -73 dBm in the 54-Mb/s mode and below -93 dBm in the 6-Mb/s mode, while consuming 230 mW. A fast RSSI-channel power-detection system allows to power-down signal processing in the listen mode. The 5- and 2.4-GHz transmitters implement a wideband Cartesian feedback loop for enhanced EVM performances and improved spectrum masks compliance. The transmitters deliver -2-dBm average power with an EVM of 3% in the 54-Mb/s mode while consuming 300 mW.

The NMR multi-transmit phased array: a Cartesian feedback approach

The use of Cartesian feedback is proposed to solve the problem of using an array of coils for the purposes of transmission in magnetic resonance imaging. The difficulties caused by direct and sample-mediated coil interactions are briefly examined, and the known solutions of using power-mismatched pre-amplifiers and transmitters noted. It is then shown that, without loss of transmitter efficiency, a high effective impedance may be created in series with each coil in the array by the use of Cartesian negative feedback. A bench experiment is described that confirms the theory. The solution is also viable for signal reception and is more efficacious than pre-amplifier damping, albeit over a smaller bandwidth.

Automatic phase alignment for a fully integrated cartesian feedback power amplifier system

A phase-alignment system is used in the first reported IC to fully integrate a power amplifier, Cartesian feedback linearization circuitry, and a phase-alignment system. The phase-alignment system consumes 8.8 mW from a 2.5-V supply and employs a new technique for offset-free analog multiplication that enables it to function without manual trimming. Phase alignment of better than 9/spl deg/ is maintained for drifts as large as /spl plusmn/90/spl deg/. We demonstrate how the phase-alignment system improves the stability margins of the fully integrated Cartesian feedback system. The power amplifier itself, integrated on the same die, operates at 2 GHz and delivers a maximum of 14.2 dBm of output power into a 50-/spl Omega/ load. The IC was fabricated in a 0.25-/spl mu/m CMOS process.

An automatic phase adjustment scheme for RF and Cartesian feedback linearizers

Amplifier linearization schemes based on modulation feedback (e.g., Cartesian feedback) require a phase correction circuit to stop instability and limit the level of the out-of-band noise floor. It is shown that the optimum phase corrector setting corresponds to the minimum output power condition of the amplifier. A new and very simple method by which the phase correction circuit is adjusted is described. The scheme uses the supply current to the final stage amplifier as a measure of the output power and minimizes this by using a simple direct search algorithm. Phase correction to within 5/spl deg/ of optimum can be obtained after about 50 iterations. The performance is limited by hysteresis in the supply current feedback signal.

Amplifier linearization using RF feedback and feedforward techniques

The performance of feedback as a distortion reduction technique is highly dependent on the integrity of the feedback path. Any error or noise generated in this path is directly reflected into the output of the amplifier. Linearized RF power amplifiers (PAs) using Cartesian feedback require a demodulator in the feedback loop, and this is a potential source of linear errors, nonlinear errors, and noise. RF feedback with Cartesian compensation is proposed as a technique for overcoming some of these problems. The scheme is most suited to systems requiring an RF input. In addition, the RF nature of the input, feedback, and error signals enables the addition of a feedforward loop to further improve the linearization capability while still maintaining good efficiency. Design equations and simulation results are given for such a system. Disadvantages include the limited bandwidth (estimated at 1 MHz) and the need for additional circuits to generate the RF input signal when included in an integrated transmitter.

Noise performance of a Cartesian loop transmitter

The Cartesian loop transmitter is now a well-known linear transmitter architecture and is finding application in a number of mobile radio systems employing linear modulation technologies. In particular, systems utilizing /spl pi//4 DQPSK require a linear transmitter, and many emerging standards [e.g., trans-European trunked radio (TETRA)] provide applications for the Cartesian feedback linearization (CFL) technique. One problem with the CFL transmitter is that its output-noise performance is no longer dominated by that of the RF power module employed within it (as is the case in more conventional transmitter architectures). The use of significant degrees of attenuation, followed by a high level of gain, within the loop, means that the noise performance of the loop is significantly poorer than that of a conventional transmitter. There are a number of tradeoffs that are available to the designer of a CFL transmitter to aid in the optimization of the output-noise performance. This paper presents a derivation of the noise performance of a Cartesian loop transmitter and highlights the design methods that may be employed in order to optimize its noise performance. It also provides a comparison of the theoretically derived behavior with that of a practical transmitter operating in the TETRA (380-400 MHz) band.

Automatic removal of Cartesian feedback transmitter imperfections

The effects of feedback DC offset and gain/phase imbalance on the performance of the Cartesian feedback linear transmitter are described. A method for the automatic removal of these effects is presented and its validity confirmed by simulation results. The relevant theoretical background is also discussed.

Theory and Implementation of a Repetitive Robot Controller With Cartesian Trajectory Description

This paper presents a new repetitive learning controller for motion control of mechanical manipulators undergoing periodic tasks defined in Cartesian space. The controller does not require knowledge of the manipulator dynamic parameters beyond a simple geometric description. The desired task will be defined in Cartesian coordinates, and no inverse kinematics or inverse Jacobian will be calculated. The asymptotic stability of this algorithm is proven using the Lyapunov approach, and the nonlinear characteristics of the manipulator are explicitly taken into account. The results of implementation of this new repetitive learning controller on an IBM 7545 robotic manipulator are presented. Cartesian feedback was obtained from optical joint position encoders using forward kinematics, and velocity was estimated by simple numerical differentiation of the Cartesian position signal in software. The performance of the algorithm was compared to that of a simple PD feedback system, and a modified “Computed Torque” controller using inverse kinematics on the Cartesian path. The learning algorithm outperformed both of these controllers by a significant margin, exhibited convergence within approximately three cycles, and did not require inverse kinematics to execute the Cartesian path.

Stability analysis of Cartesian feedback linearisation for amplifiers with weak nonlinearities

RF amplifier linearisation by Cartesian feedback is analysed using a multiple input multiple output (MIMO) model. The effects of RF amplifier distortion, namely AM/AM and AM/PM distortion, are shown to degrade the stability of the Cartesian feedback loop. A versatile technique is presented that allows the amount of stability degradation to be determined for RF amplifiers provided distortion is not too large (i.e class A to class AB amplifiers). Loops with low stability margins are shown to exhibit out-of-band noise peaking. It is concluded from the noise analysis undertaken, that the highest proportion of gain possible should be placed ahead of the loop compensation to minimise out-of-band noise.

Twin-loop Cartesian transmitter (TLCT)

A novel technique which enhances the linearisation performance of the Cartesian feedback transmitter is introduced. The effectiveness of the technique has been illustrated by means of analysis and the results obtained from a hardware prototype.

The frequency-hopped Cartesian feedback linear transmitter

A frequency-hopped version of the Cartesian feedback linear transmitter for use in future mobile communication systems is introduced. The essential requirements for the successful operation of such a transmitter are discussed. Using the results obtained from a hardware prototype, the problems generated by a hopping carrier are outlined and solutions introduced.

Linearisation of RF multicarrier amplifiers usingCartesian feedback

A linearised multicarrier RF power amplifier using multiple Cartesian feedback loops is described. Measurements on a four channel system with a class AB power amplifier at 880 MHz showed that the intermodulation products were suppressed by 30 dB.

An assessment of the efficiency performance of linearization schemes in the Australian mobilesat system

AbstractThe efficiency of class C type amplifiers with digital predistortion linearizers is discussed. It is shown, by simulation with the AUSSAT MOBILESAT modulation schemes as examples, that the maximum efficiency from such a linearizer amplifier combination is significantly reduced for non‐constant envelope modulation schemes on that achievable with constant envelope schemes. This aspect is often neglected when linearizer performance with such amplifers is discussed. The results are generally applicable to Cartesian feedback linearization also.