Kahn Envelope Elimination Research Articles

System Linearity-Based Characterization of High-Frequency Multilevel DC–DC Converters forS-Band EER Transmitters

In modern telecommunications systems, the requirements for the amount and speed of transmitted data are increasing rapidly. For better spectral efficiency, complex-type modulations [quadrature amplitude modulation (QAM) and orthogonal frequency-division multiplexing (OFDM)] are used, resulting in high peak-to-average power ratio (PAPR) signals. As a consequence, when conventional linear amplifiers (classes A and AB) are used in transmitters, both prominent linearity and poor efficiency are assured. The efficiency of the transmitters can be significantly improved using some of the popular techniques: Kahn envelope elimination and restoration (EER) principle, envelope tracking (ET) technique, Doherty amplifier, and Chireix’s amplifier. In this article, two different approaches in supplying a 2.4-GHz class-E power amplifier (PA) ( ${S}$ -band) based on the EER technique are analyzed and compared. The consistent part of the EER transmitters is a switched-capacitor-based voltage divider combined with a switching structure—analog multiplexer, in the envelope amplifiers (EAs) and a class-E PA. The analog multiplexer is assisted with a series linear regulator in the first system and a fourth-order ${LC}$ filter in the second system, providing supply modulation for the PA. The systems are compared in terms of efficiency and system linearity using the standard metrics [error vector magnitude (EVM) and adjacent channel power ratio (ACPR)]. Special consideration is devoted to the characterization of how the EAs affect the quality of the RF output signal. It is shown that the tested EER transmitters can reproduce 16-QAM and 64-QAM signals up to 2.5 MHz, manifesting an average efficiency of about 32% with the first transmitter, and 16-QAM signals up to 660 kHz RF bandwidth and average efficiency of 44% with the second transmitter. A software tool that accurately estimates the EA linearity is utilized and successfully tested on both EER transmitters.

Envelope Amplifier Based on Switching Capacitors for High-Efficiency RF Amplifiers

Modern transmitters usually have to amplify and transmit signals with simultaneous envelope and phase modulation. Due to this property of the transmitted signal, linear power amplifiers (class A, B, or AB) are usually used as a solution for the power amplifier stage. These amplifiers have high linearity, but suffer from low efficiency when the transmitted signal has high peak-to-average power ratio. The Kahn envelope elimination and restoration technique is used to enhance the efficiency of RF transmitters, by combining highly efficient, nonlinear RF amplifier (class E) with a highly efficient envelope amplifier in order to obtain a linear and highly efficient RF amplifier. This paper presents a solution for the envelope amplifier based on a multilevel converter in series with a linear regulator. The multilevel converter is implemented by employing voltage dividers based on switching capacitors. The implemented envelope amplifier can reproduce any signal with a maximum spectral component of 2 MHz and give instantaneous maximum power of 50 W. The efficiency measurements show that when the signals with low average value are transmitted, the implemented prototypes have up to 20% higher efficiency than linear regulators used as a conventional solution.

Multilevel Power Supply for High-Efficiency RF Amplifiers

In communication systems, linear power amplifiers (PAs) (class A, B, or AB) are usually used as a solution for the PA stage. These amplifiers have high linearity, but suffer from low efficiency when the transmitted signal has high peak-to-average power ratio. The Kahn envelope elimination and restoration technique is used to enhance the efficiency of RF transmitters by combining highly efficient, nonlinear RF amplifier (class D or E) with a highly efficient envelope amplifier in order to obtain linear and highly efficient RF amplifier. This paper presents a solution for the envelope amplifier based on a combination of multilevel converter and linear regulator. The proposed solution can reproduce any signal with maximum spectral component of 2 MHz and give instantaneous maximum power of 50 W. The efficiency measurements show that when the signals with low average value are transmitted, it has up to 49% higher efficiency than an ideal linear regulator that is used as a conventional solution. Additionally, the algorithm for the optimization of the voltage levels, for the multilevel converter, is explained as well.

Theoretical Analysis of Hole Punch Signal Conditioning for High Efficiency EER Power Amplifiers

High efficiency and linearity can be achieved using Kahn's envelope elimination and restoration (EER) technique. However, when used with modulation schemes such as code division multiple access (CDMA) and orthogonal frequency division multiplexing (OFDM), the resulting polar drive signals suffer from expanded bandwidth. Rudolph has shown that the bandwidth of the polar drive signals can be reduced by hole punching the signal in the phase plane (limiting the minimum envelop of the modulated signal). This paper theoretically analyses the hole punch method and quantifies adjacent channel interference (ACI) and the error vector magnitude (EVM) caused by the ensuing distortion. It also shows how the results differ using different shapes and lengths of window functions. Although the hole punch process manages to achieve spectrum improvement for polar components, its EVM will limit the data throughput in some wireless standards.

A new adaptive delay method for wideband wireless Kahn's RF power amplifiers

This paper presents an adaptive delay control method used in wideband Kahn's envelope elimination and restoration (EER) RF power amplifiers (PAs). Besides control circuits, envelope and phase signals are also implemented by baseband digital signal processing (DSP) technique. It avoids using not only nonlinear limiter and envelope detector, but also difficult RF delay module or complicated feedback loop, each of which is necessary in conventional EER PA's structure. With the help of applying phase detector, counter, and latch register, the delay varies adaptively with circuit parameters, and then the new PA maintains performance at all times. With the application of this method we can realize a PA applicable to IEEE 802.11b standard with 11 MHz signal bandwidth, of which adjacent channel power ratio (ACPR) is better than -40 dBc at the first side lobe

Kahn EER technique with single-carrier digital modulations

The Kahn envelope elimination and restoration (EER) technique allows for linear RF power amplification by combining nonlinear, but efficient, radio frequency (RF) and audio frequency (AF) power amplifiers. The EER technique requires a coordinate transform of the Cartesian digital in-phase and quadrature (I & Q) signals to polar amplitude (A) and phase-modulated RF (RF-P) signals. An ideal recombination of the A and RF-P signals in the output stage of the transmitter yields perfect amplified I & Q signals in the output of the transmitter. However, any small mismatch of the delay between A and RF-P paths or bandwidth restrictions within the A path reveal the dominant nonlinearity inherent with the coordinate transform, resulting in out-of-band (OOB) radiation. In this paper, the EER technique is investigated for single-carrier digital modulations. It is shown that the characteristics of the resulting OOB radiation merely depends on the size of the hole in the vector diagram of the digital modulation and is practically independent of other parameters of a digital modulation. Universal curves are given, which can be useful for developing and aligning EER digital transmitters.

Out-of-band emissions of digital transmissions using Kahn EER technique

The Kahn envelope elimination and restoration (EER) technique allows for linear RF power amplification by combining nonlinear, but efficient, RF and AF power amplifiers (PAs). In order to use the EER technique for digital signals, a coordinate transform from the original Cartesian in-phase and quadrature mode into a polar mode has to take place, yielding an envelope (or amplitude) and a PM RF signal. This coordinate transform is extremely nonlinear and thereby broadens the spectra of the original signals. In the final PA stage, both signals are recombined. However, since this recombination process is imperfect, out-of-band (OOB) emissions come up, also known as adjacent channel power or spectral regrowth. In this paper, the impact of the broadening of the amplitude and phase signals on OOB emissions is investigated with respect to imperfect restoration due to signal delays and limited bandwidth of the amplitude path. It is shown that the amount of OOB emissions can significantly be reduced if the modulation scheme shows a hole at the origin in its vector diagram.

Intermodulation distortion in Kahn-technique transmitters

The Kahn Envelope Elimination and Restoration (EER) technique implements a linear RF power amplifier (PA) by combining nonlinear, but efficient, RF and AF power amplifiers. For signals with high peak-to-average ratios, the average efficiency of a Kahn-technique transmitter can be three to four times that of a transmitter that employs conventional linear RP PAs. Since switching-mode amplifiers are employed, the linearity of an EER transmitter depends upon parameters such as the bandwidth of the envelope modulator and the differential delay between envelope and phase signals. This paper determines the relationship between these parameters and IMD levels and verifies the predictions by laboratory measurements. The results can be used to determine the requirements for the components of a Kahn-technique transmitter.