David Seebacher

Reduction of Aliasing Effects of RF PWM Modulated Signals by Cross Point Estimation

The trend in transmitter systems is to move the digital domain closer toward the antenna using digital modulators and drivers to reduce circuit complexity and to save power. One promising approach is the use of RF pulse width modulation (RF PWM). Unfortunately purely digital discrete time RF PWM suffers from aliasing problems which limit the achievable resolution. For a 40 MHz bandwidth signal at 2.6 GHz carrier frequency for example the achievable signal quality is limited to ~ 43 dBc. This paper describes the root cause of this effect, an error in the determination of the cross points, due to the sampled nature of the signal and proposes a method to compensate for it. It is shown that by interpolating the signal and estimating the cross points the signal quality can be significantly improved. The interpolation is simplified by interpolating the decomposed outphasing signals instead of the full signal. This has the advantage that a constant instead of a phase modulated reference function can be used. It is shown that by simple cross point estimation the signal quality can already be improved to 65 dBc. When either considering a second modulator or when using a delta sigma like noise shaping architecture the signal quality can be further enhanced to 75 dBc.

Measurement instrument selection for very high bit rate contactless transponder evaluation

In this paper we discuss properties of the digital sampling oscilloscope versus signal analyzer as instrument in the test bench to characterize ISO/IEC14443 compliant contactless communication devices. An extension to the existing near-field communication (NFC) standard specifies very high bit rates (VHBR) in the range of 10 Mbit/s for the Reader to Transponder communication link. One draft specification takes advantage of sophisticated signal processing and clever coding, to allow an energy-efficient contactless system, compliant to existing frequency regulation and to typical Reader hardware, which can also be used in mobile applications such as NFC in Smartphones. Considerations for the test bench to characterize this communication air interface are of general interest also for existing RFID standards in the 13.56 MHz frequency band.

Predistortion of Digital RF PWM Signals Considering Conditional Memory

The trend in transmitter systems is to move the digital domain closer towards the antenna using digital modulators and drivers to reduce circuit complexity and to save power. A common assumption made is that they are capable of generating ideal pulses and thus do not suffer from analog imperfections. But the output signals of real drivers for high frequency operation are not perfectly rectangular anymore, which leads to distortion lowering the signal quality. In this paper the general properties of high frequency digital driver circuits operating at 2.6 GHz are analyzed and the impact of the different effects is presented. The predistortion of such drivers in the context of digital discrete time RF PWM modulators is studied. It has been found that conventional sample based predistortion can only correct the driver nonlinearity from -29 dBc to -49 dBc for the example considered using a 40 MHz bandwidth signal at 2.6 GHz. Therefore a special predistortion scheme considering the impact of pulses adjacent to the other samples is proposed. The mitigation of effects due to the discrete time nature of the signal is considered and discussed in detail. The capabilities of the proposed predistortion scheme are verified by extensive simulations as well as by measurements. By applying the proposed predistortion concept the spectral quality can be further improved to -66 dBc. In addition different scenarios with limited resolution and a carrier frequency offset are analyzed.

A common drain parallel amplifier in GaN using baseband PWM and direct filter connection for efficiency enhancement

To cope with the increasing demand for bandwidth in wireless communications coding schemes with high crest factors are employed. As a result the power amplifier (PA) is operated far below maximum output power for most of the time leading to low average efficiency in traditional designs. Therefore PA concepts providing efficiency enhancement in back off are key for efficient transmitters. Baseband PWM operated PAs in combination with direct filter connection are possible candidates. Due to the constant current of the used filters special PA structures are required. The proposed parallel common drain amplifier offers this properties. Its general operational principle is explained and an implementation study in a GaN MMIC process is presented. The designed PA operates at 2.65 GHz delivering a maximum output power of 3 W. To get optimum performance of the direct filter connection a codesign of the PA and the cavity filter included in the output matching network was done, resulting in a compact solution.

Silicon technologies and circuits for RF and mm-wave applications

The presentation will start with an overview of the features and capabilities of state-of-the-art SiGe-BiCMOS and RF-amplifier technologies for applications such as high-data-rate communications, phased-arrays and pro-active safety systems like identification and e-safety. The capabilities offered by SiGe-BiCMOS and microwave packaging enable the integration of complete multichannel transceivers on a chip or in a package even including the antennas. The criteria and trade-offs for the technology selection and system partitioning will be discussed in part two of the presentation. In addition to the electrical components performance, major criteria such as high reliability, long lifetime and high yield fabrication will be addressed. Advanced packaging technologies will be presented as well, including embedded passive components and package co-design. Finally existing circuit design examples and future solutions for pro-active safety systems will be presented, followed by power-amlifiers and high-speed transceivers for communications and point-to-point links.

The impact of the Q-factor of the parasitic capacitances of RF transistors on their load modulation capabilities

With the high number of users the wireless spectrum has become a valuable resource. In order to use it efficiently sophisticated coding schemes with high crest factors are used. As a result the power amplifier is operated far below maximum output power for most of the time, which results in rather low efficiency of traditional designs. Therefore efficiency enhancement methods such as the Doherty or Chireix combiner are experiencing a revival. Their performance heavily depends on the load modulation properties of the used RF transistors and unfortunately often falls behind the predicted theory, especially for high frequency operation. This paper analyses the impact of the different loss mechanisms of RF power transistors on their load modulation capabilities. Special attention was paid to the frequency dependent losses due to the limited Q-factor of the parasitic capacitance. Based on the example of a Doherty amplifier the importance of the load modulation properties for the efficiency in back off is highlighted.

A source modulated amplifier in GaN designed for baseband PWM and direct filter connection

To cope with the increasing demand for bandwidth in wireless communications coding schemes with high crest factors are employed. As a result the power amplifier (PA) is operated far below maximum output power for most of the time, leading to low average efficiency in traditional designs. Therefore PA concepts providing efficiency enhancement in back off are key for efficient transmitters. Baseband PWM operated PAs in combination with direct filter connection are possible candidates. Due to the constant current of the used filters special PA structures are required. The proposed source modulated amplifier offers these properties. Its general operational principle is presented and an implementation study in a GaN MMIC process was carried out. The designed PA operates at 2.65 GHz delivering a maximum output power of 3W. To get optimum performance of the direct filter connection a codesign of the PA and the cavity filter included in the output matching network was performed, resulting in a compact solution.

High frequency Class-DE push/pull power amplifier utilizing a parallel compensation inductance

In this paper the operational principle of a Push-Pull Class-DE PA utilizing a parallel compensation inductance is proposed and corresponding design equations are derived. The Push-Pull configuration has the advantage of having two low side switches, but it requires a transformer with its inherent parasitic parallel inductance. This parallel inductance is used as an integral part of the matching network, mitigating the demand for the conventional series compensation inductance and enabling efficient load modulation over a wide dynamic range. The derived design equations are verified by simulation and it is shown that the Push-Pull Class-DE PA in combination with Chireix outphasing is a suitable solution for achieving high efficiency also in back off.

Efficiency enhancement of burst mode transmitters by RF energy recovery

Modern communication standards have high peak to average power ratios (PAPR) to achieve high spectral efficiency, trading of power amplifier (PA) efficiency. In this paper a method to directly reuse the RF energy reflected from carrier bursting PAs to improve the efficiency will be introduced and compared to conventional operation with and without DC energy recovery. Efficiency enhancement utilizing DC energy recovery is limited by the rectifier and PA efficiency. Therefore a method to reuse the RF energy directly, which does not suffer from these limitations is presented. It will be shown that it is possible to maintain high efficiency in back off with this method.