Intermodulation Distortion Products Research Articles

Photonic Downconversion of RF Signals With Improved Conversion Efficiency and SFDR

A photonic microwave downconverter with improved conversion efficiency and spurious-free dynamic range (SFDR) is proposed and experimentally demonstrated based on a dual-parallel Mach–Zehnder modulator (DPMZM) and a digital signal post-processing algorithm. The radio frequency (RF) signal and the local oscillator (LO) signal are fed to the two sub-MZMs of the DPMZM, respectively, leading to an infinite isolation between the RF and LO ports. By biasing the two sub-MZMs and the parent MZM of the DPMZM at the minimum transmission point, the optical carrier can be greatly suppressed. As a result, the conversion efficiency is improved for the same power impinged on the photodetector. A preliminary experiment shows that a conversion efficiency of −12.7 dB can be achieved. On the other hand, without emulating the inverse link transfer function, where exact parameters of the photonic link should be known, a simple post-processing algorithm which just needs the modulation index of the LO signal is employed to suppress the intermodulation distortion products. The SFDR of the downconverter is improved from 101.5 dB $\cdot $ Hz $^{{2/3}}$ to 114.5 dB $\cdot $ Hz $^{{4/5}}$ by using the proposed digital linearization algorithm.

A role for acoustic distortion in novel rapid frequency modulation behaviour in free-flying male mosquitoes

We describe a new stereotypical acoustic behaviour by male mosquitoes in response to the fundamental frequency of female flight tones during mating sequences. This male-specific free-flight behaviour consists of phonotactic flight beginning with a steep increase in wing-beat frequency (WBF) followed by rapid frequency modulation (RFM) of WBF in the lead up to copula formation. Male RFM behaviour involves remarkably fast changes in WBF and can be elicited without acoustic feedback or physical presence of the female. RFM features are highly consistent, even in response to artificial tones that do not carry the multi-harmonic components of natural female flight tones. Comparison between audiograms of the robust RFM behaviour and the electrical responses of the auditory Johnston's organ (JO) reveals that the male JO is tuned not to the female WBF per se but, remarkably, to the difference between the male and female WBFs. This difference is generated in the JO responses as a result of intermodulation distortion products (DPs) caused by non-linear interaction between male-female flight tones in the vibrations of the antenna. We propose that male mosquitoes rely on their own flight tones in making use of DPs to acoustically detect, locate and orientate towards flying females. We argue that the previously documented flight-tone harmonic convergence of flying male and female mosquitoes could be a consequence of WBF adjustments so that DPs generated through flight-tone interaction fall within the optimal frequency ranges for JO detection.

Digital Mitigation of Transmitter-Induced Receiver Desensitization in Carrier Aggregation FDD Transceivers

Carrier aggregation transmissions in frequency division duplexing devices reduce the duplexing distance between the transmitter (TX) and receiver (RX) bands. As a consequence, the spurious intermodulation distortion products created by the nonlinear RF front-end of the TX may easily extend over to the RX band, potentially causing own RX desensitization. In this paper, we propose an efficient and computationally feasible adaptive digital identification and cancellation technique to mitigate the RX desensitization. We first show that the spurious leakage signal at own RX band depends on an equivalent leakage channel that models the overall signal leakage path including the TX nonlinearities, duplexer filter responses, and RX path. The parameters of the equivalent leakage channel can be efficiently estimated with the least squares or the recursive least squares algorithm, using the actual digital transmit data as a reference, and then used to regenerate and cancel the leakage interference from the received signal. The performance of the proposed technique is evaluated with extensive computer simulations, as well as with practical real-world RF measurements, demonstrating excellent calibration properties with up to 19-dB improvement in singal-to-interference plus noise ratio of the desired received signal.

Circuit model of uni-traveling carrier photodiode with high power and enhanced bandwidth technique

An electrical equivalent circuit model of InGaAs/InP uni-travelling carrier photodiode (UTC-PD) is presented. The model is suitable to be built on any electrical circuit simulator to optimize the design parameters of the device. Its performance in terms of bandwidth, linearity, third order inter-modulation distortion and output photo current are investigated. Simulation techniques to measure inter-modulation distortion products and linearity are given in detail. The result obtained by the modeling technique is validated through a comparison with the reported experimental results. It is shown that 3 dB cutoff frequency and output photocurrent can be substantially improved by inserting a small inductance in series with the load together with choosing an optimum absorption layer width in UTC-PD. This technique does not sacrifice the device linearity to a large extent.

Concurrent Multi-Band Envelope Modulated Power Amplifier Linearized Using Extended Phase-Aligned DPD

This paper reports an envelope tracking (ET) transmitter architecture for multi-band/widely-spaced carrier signals. Due to the bandwidth limitations of the envelope modulator, the dynamic supply voltage is controlled by combining the low frequency envelopes of the multi-inputs rather than the true composite envelope of the input signals. The ET transmitter is tested for dual-band and tri-band power amplifiers (PAs) operating in concurrent mode using various bandwidth carrier aggregated long term evolution (LTE) signals. The transmitter is linearized using a phase-aligned Volterra digital predistortion that considers the inter-modulation distortion (IMD) products around the carrier frequencies as well as the third-order IMD products in the tri-band case. Experimental results are shown to validate the proposed techniques for linearization and efficiency enhancement of the multi-band ET transmitter and compared to the case of fixed supply biasing of the PA.

Performance of 2–3.6 GHz Five-Port/Three-Phase Demodulators with Baseband AnalogI/QRegeneration Circuit in Direct-Conversion Receivers

A comparison between performance of a five-port demodulator (FPD) and a three-phase demodulator (TPD) with both architectures connected to a so-called basebandI/Qregeneration circuit is carried out. In order to compare these two receivers to a classical architecture, the performance of anI/Qdemodulator (IQD) is also presented. Measured results show the superiority of TPD and IQD over FPD in terms of residual DC offsets and 2nd order intermodulation distortion (IMD2) products, noise figure (NF), and sensitivity due to the use of active balanced mixers instead of diode power detectors. Lastly, demodulation of three noncontiguous RF carriers shows the stronger potential of the three-phase demodulator (TPD) for applications in future long term evolution-advanced (LTE-A) receivers through EVM and constellation diagram measurements.

A Generalized 2-D Linearity Enhancement Architecture for Concurrent Dual-Band Wireless Transmitters

A novel generalized two dimensional (2-D) digital predistortion (DPD) architecture for concurrent dual-band transmitters is presented. The proposed architecture is based on simultaneous injection of in-band intermodulation (IM) and cross-modulation (CM) distortion products. A general fundamental-frequency model for concurrent dual-band transmitter is developed theoretically and proofed experimentally. An individual impact of each IM and CM distortion component is theoretically derived and analyzed. It is clearly proven by theoretical analysis and experiments that the proposed predistorter improves the in-band and out-of-band performances of both signals. The simulation and experimental studies are performed using various signal sets. The proposed predistorter does not depend on frequency separation between bands and has low computational complexity in comparison with concurrent dual-band DPD's state-of-the-art. In addition, it is clearly shown in experiments that when using iterative simultaneous injections, a phenomenon known as distortion compensation limit can be reduced. A presence of memory effects and their mitigation in frequency domain for each band of concurrent dual-band transmitter are also demonstrated in experiments.

A 10W, 2-5 GHz Highly Linear Power Amplifier

In this paper, a 2 to 5 GHz (116% fractional bandwidth), highly linear power amplifier – based on GaN HEMT- with an output power excess of 10W is presented. The load pull technique is used to reach the optimum performance within the useful bandwidth. Electromagnetic simulations are carried out to model the matching networks on the substrate. The design is optimized based on the results of these electromagnetic simulations. Wideband matching, together with stability circuits are adopted to achieve a flat power gain of 7.7 ± 0.75 dB over the entire operating bandwidth, providing optimal fundamental and harmonic impedances within more than one octave of operation. The designed power amplifier exhibits a small signal gain more than 7.85 ± 0.75 dB, third-harmonic intermodulation distortion products (IMD3) far below -35 dBc at an input power of 29 dBm over the entire frequency band, and a power added efficiency (PAE) better than 15%. Using a two-tone test bench with a frequency spacing of 100KHz, a maximum value of the output OIP2 and OIP3 are found tobe greater than 55 dBm and 45 dBm, respectively.

An Accurate Predistorter Based on a Feedforward Hammerstein Structure

This paper presents a feedforward Hammerstein structure that is proposed for the modeling and digital predistortion of the dynamic nonlinear behavior of the wideband radio frequency power amplifiers and wireless transmitters. This model consists of two loops, one used as a signal cancellation loop and the second used as distortion injection loop. The main signal cancellation loop is responsible for the characterizing of the PA dynamics through a Hammerstein model. Indeed, the distortion injection loop was found to complement the main signal cancellation loop by adding an accurate means of modeling the nonlinear dynamics of the intermodulation distorted signal for the better mimicking of the intermodulation distortion products especially in the out-of-band regions. Its accuracy is assessed in behavioral modeling and digital predistortion, by comparing it to other state-of-the-art models. The measurement results show an adjacent channel power ratio of almost $-$ 50 dBc and a normalized mean square error of less than $-$ 42 dB.

Dynamic range enhancements in radio receivers by means of frequency retranslation

In this study, a receiver architecture is presented that may be used for linearisation or for large signal interference suppression. In linearisation mode, an error signal that consists of intermodulation distortion products is obtained and used for predistorting the receiver. Experimental results demonstrate >;16 dB improvement of third-order input intercept point when used in a superheterodyne topology. When used in a direct conversion receiver topology, 7-dB improvement of second-order input intercept point is obtained. In interference suppression mode, an error signal that consists of high-level interferers spectrally close to a small wanted signal is obtained and used for predistorting the receiver. Experimental results demonstrate 14-dB enhancement of spurious-free dynamic range of the receiver.

Mathematical Model of Laser Transmitter in Feed-Forward Mitigating Technique for Radio over Fiber System

This paper discusses a compensation system for nonlinear distortion of laser transmitter in Radio over Fiber application at frequency 5.2GHz, where feed-forward linearization technique is the applied technique in the compensation system. The paper focuses on the mathematical model of the laser nonlinear distortion which is an important tool in developing the feed-forward laser transmitter system model. The nonlinearity of the laser diode is modeled using Volterra series analysis, and the magnitudes of the laser 3rd order intermodulation distortion products (IMD3) are computed in MATLAB. The analysis result from the mathematical model has shown considerably good agreement with the results from some of the previous works. Hence, the mathematical model is deemed as useful in the modeling of feed-forward transmitter system.

Improved large-signal GaN HEMT model suitable for intermodulation distortion analysis

In this article, a complete empirical large-signal model of GaN high electron-mobility transistors (HEMTs) is presented. The developed nonlinear model employs differentiable trigonometric function continuously to describe the drain-source current characteristic and its higher order derivatives, making itself suitable for the simulation of intermodulation distortion (IMD) in microwave circuits. Besides, an improved charge-conservative gate charge model is proposed to accurately trace the nonlinear gate-source and gate-drain capacitances. The model validity is demonstrated for different 0.25-µm gate-length GaN HEMTs. The simulation results of small-signal S-parameters, radio frequency (RF) large-signal power performances and two-tone IMD products show an excellent agreement with the measured data.

4-Channel, time-domain measurement system using track and hold amplifier for the characterization and linearization of high-power amplifiers

We propose in this paper a 4-channel time-domain test bench for the characterization and linearity enhancement of microwave power amplifiers (PAs). The proposed time-domain measurement system utilizes four track and hold amplifiers (THAs) for direct subsampling of radiofrequency (RF) signals. The use of wideband THAs to replace samplers or mixers enables reducing analog IF circuit complexity. It permits direct digitization of RF signals like CW, two-tone and pulsed modulated signals, bringing more flexibility in the receiver's performance by enhancing the dynamic range and bandwidth. This test bench is capable of completely extracting the phase, amplitude, and transfer characteristics of non-linear devices excited with CW or modulated RF signals. In this work, two-tone transfer characteristics of a 50 W GaN HEMT Nitronex PA were extracted and processed for applying digital pre-distortion linearization to enhance linearity performance. Time-domain envelope and carrier waveforms (voltage and current) along with third-order inter-modulation distortion (IMD) products with and without digital pre-distortion scheme are also presented.

Successive Regeneration and Adaptive Cancellation of Higher Order Intermodulation Products in RF Receivers

In this paper, a general framework for the adaptive feedforward cancellation of higher order intermodulation distortion (IMD) products is presented. By generating only second-order and principal-odd-order IMD reference products in the RF/analog domain and reproducing higher order IMD reference products at digital baseband, the proposed reference distortion scheme minimizes the analog hardware burden on the system. Inherent in this procedure is an approximation that the profile of blocking signals causing IMD is dominated by one very large blocker. The limitations imposed by this approximation are quantitatively examined and shown to permit cancellation ratios of nearly the square of the ratio between the dominant and nondominant blocking signal RMS amplitudes. An experimental receiver employing the proposed technique was constructed utilizing a wide-swing low-noise transconductance amplifier in order to accommodate a rail-to-rail (+12.4 dBm) out-of-band blocker and a -16.3-dBm nondominant blocker. The measured receiver out-of-band 1-dB desensitization point is +12.5 dBm and the peak uncorrected two-tone third-order intermodulation intercept point (IIP3) is +33.5 dBm. Utilizing the proposed IMD cancellation scheme in the presence of a modulated dominant blocker improves the total input-referred IMD error power by over 24 dB, resulting in an extrapolated IIP3 metric of +43.5 dBm.

An improved empirical large-signal model for the GaAs- and GaN-based HEMTs

A complete empirical large-signal model for the GaAs- and GaN-based HEMTs is presented. Three generalized drain current I–V models characterized by the multi-bias Pulsed I–V measurements are presented along with their dependence on temperature and quiescent bias state. The new I–V equations dedicated for different modeling cases are kept accurate enough to the higher-order derivatives of drain-current. Besides, an improved charge-conservative gate charge Q–V formulation is proposed to extract and model the nonlinear gate capacitances. The composite nonlinear model is shown to accurately predict the S-parameters, large-signal power performances as well as the two-tone intermodulation distortion products for various types of GaAs and GaN HEMTs. © 2011 Wiley Periodicals, Inc. Int J RF and Microwave CAE , 2011.

Enhanced prediction of GaN MISHEMT RF power performance using improved large-signal model

An empirical large-signal model for a high-power AlGaN/GaN metal–insulator–semiconductor HEMT (GaN MISHEMT) utilising improved drain current (I–V) and gate charge (Q–V) formulation is presented. The proposed modelling equations accurately model the asymmetric bell-shaped transconductance (Gm) and gate capacitance (C11) characteristics over a wide bias range of operation. The simulation results of RF large-signal harmonic power levels and two-tone intermodulation distortion products show excellent agreement with the measured data.

Predistortion of a 100‐W base station high‐power amplifiers

AbstractThis article presents the design of a fifth‐order RF predistorter for use in a base‐station high‐power amplifier (HPA).The predistorter uses two intermodulation (IM) signal generators, with identical configuration, to individually generate the third‐ and fifth‐order inband intermodulation (IM3 and IM5) signals for cancellation of the third‐ and fifth‐order intermodulation distortion products (IMDP3 and IMDP5) at the output of the HPA. The IM signal generators are constructed using two Schottky diodes as non‐linear devices to generate the inband IM3 and IM5 signals. The proposed predistorter is designed and optimized using computer simulation. A hardware prototype is built and tested in a practical 100‐W HPA using a standard IS‐95 CDMA signal at 2.2 GHz. Measured results show that the spectral regrowth of the signal at the HPA output can be significantly suppressed over a wide range of output power levels. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:861–867, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25855

An improved nonlinear model of HEMTs with independent transconductance tail-off fitting

We present an improved large-signal device model of GaAs/GaN HEMTs, amenable for use in commercial nonlinear simulators. The proposed model includes a new exponential function to independently control the transconductance compression/tail-off behaviors. The main advantage of this model is to provide a simple and coherent description of the bias-dependent drain current (I—V) that is valid in all regions of operation. All aspects of the model are validated for 0.25-μm gate-length GaAs and GaN HEMT processes. The simulation results of DC/pulsed I—V, RF large-signal power and intermodulation distortion products show excellent agreement with the measured data.

Nonlinear HEMT model formulated from the second-order derivative of the I–V and Q–V characteristics

In this paper, an empirical nonlinear model of high electron mobility transistors (HEMTs) suitable for a wide bias range is presented. Unlike the conventional large-signal models whose fitting parameters are coupled to the measured drain current and gate capacitance characteristics, the derived modeling equations are direct formulated from the second-order derivative of drain current (I–V) and gate charge (Q–V) with respect to gate voltage. As a consequence, the proposed nonlinear model is kept continuously differentiable and accurate enough to the higher-order I–V and Q–V derivatives. Besides, the thermal and trapping effects have been implemented in the large-signal model along with its dependence on temperature and quiescent-bias state. The composite nonlinear model is shown to accurately predict the S-parameters, large-signal power performances as well as the two-tone intermodulation distortion products for various types of GaAs and GaN HEMTs.

Inverted asymmetric Doherty power amplifier driven by two-stage symmetric Doherty amplifier

A new high efficiency Doherty structure for code division multiple access 2000 1×3-FA test signal is presented. The proposed Doherty structure consists of a two-stage symmetric Doherty drive amplifier and an inverted asymmetric Doherty power amplifier. This Doherty structure can improve the efficiency characteristic and reduce the amplifier's dimensions. Also, higher-order intermodulation distortion products can be self-compensated. For demonstration of the amplifier's performance, a 74 W Doherty amplifier has been simulated and measured.