Paul Theilmann

Enhanced Electromagnetic Interference Shielding Through the Use of Functionalized Carbon-Nanotube-Reactive Polymer Composites

We report on a new principle yielding enhanced electromagnetic shielding, using as an example a composite comprised of carbon nanotubes (CNTs) integrated with a reactive ethylene terpolymer (RET). Such composites were synthesized through the chemical reaction of the functional groups on the CNT with the epoxy linkage of the RET polymer. The main advantages of these composites include good dispersion with low electrical percolation volume fractions (~0.1 volume%), yielding outstanding microwave shielding efficiency for electromagnetic interference applications. The shielding effectiveness was characterized for both single-walled and multiwalled CNT-based composites and was much enhanced in the former. The specific roles of absorption and reflection in determining the total shielding, as a function of the nanotube filling fraction, is also discussed.

An Analytical Model for Inductively Coupled Implantable Biomedical Devices With Ferrite Rods

Using approximations applicable to near field coupled implants simplified expressions for the complex mutual inductance of coaxial aligned coils with and without a cylindrical ferrite rod are derived. Experimental results for ferrite rods of various sizes and permeabilities are presented to verify the accuracy of this expression. An equivalent circuit model for the inductive link between an implant and power coil is then presented and used to investigate how ferrite size, permeability and loss affect the power available to the implant device. Enhancements in coupling provided by high frequency, low permeability nickel zinc rods are compared with low frequency high permeability manganese zinc rods.

Near zero turn-on voltage high-efficiency UHF RFID rectifier in silicon-on-sapphire CMOS

A UHF RFID rectifier which turns on at near zero input voltage is demonstrated. The rectifier is fabricated in 0.25-µm silicon-on-sapphire (SOS) CMOS technology using intrinsic, near zero threshold devices. A novel improved cross-coupled bridge topology is used to minimize the leakage incurred through the use of intrinsic devices while maintaining their low power turn on characteristics. The fabricated rectifier demonstrates a peak power conversion efficiency (PCE) of 71.5% at 915MHz with a RF input of −4 dBm and a 30 kΩ load. More importantly, a PCE ≫ 30% was measured for all RF input powers between −28 and −4 dBm demonstrating state-of-the-art efficiency across a wide range of input powers.

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An inherent shortcoming of rectifiers designed using standard CMOS devices is poor low input power performance. It is shown that this can be overcome through the use of intrinsic devices with close to zero-threshold voltage available in a 0.25 μm silicon-on-sapphire (SOS) CMOS process. A novel complementary bridge rectifier structure based on a combination of cross-connected and diode bridge rectifier topologies is introduced to avoid the excessive leakage current incurred through the use of intrinsic devices. A design strategy which maximizes efficiency and produces an input impedance which will interface well with the inductive coil type antennas used in biomedical implants is presented for this new rectifier type. The fabricated rectifier achieves a 1 μW DC output power for an input power of -26.5 dBm at 100 MHz. A peak measured power conversion efficiency of 67% is achieved at 100 MHz, but more importantly >;30% PCE is attained for a wide output power range which reaches as low as -40 dBm. At the target 1 μW output power a PCE of 44% was achieved.

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We report through a comparison of the electromagnetic properties of polymer composites constituted of linear and nonlinear (helically coiled) carbon nanotubes (CNTs) that the electromagnetic interference (EMI) shielding efficiency could be much increased in the latter. A higher ac conductivity and relative dielectric permittivity (both e′ and e″) was recorded when coiled structures were used, and was ascribed to enhanced capacitive and electric field depolarization effects. The EMI shielding was related to the extended length/diameter aspect ratio of the CNTs. Our study has implications in the design of materials for EMI shielding, where nanostructure geometry could play a major role.

W Complementary Bridge Rectifier With Near Zero Turn-on Voltage in SOS CMOS for Wireless Power Supplies

A high efficiency GaN envelope tracking power amplifier (ETPA) operated at 780 MHz, corresponding to LTE band class 14, is presented. The ETPA was tested with both a 6.6 dB PAPR WCDMA signal and a 7.5 dB PAPR 10 MHz 16 QAM LTE signal. Under the WCDMA signal, a drain efficiency of ~70% with 13.5 dB of gain and 28.6 W of output power was measured. The corresponding NRMSE, ACPR1, and ACPR2 were 1.51%, -45 dBc, and -51 dBc, respectively. For the 10 MHz LTE signal, a drain efficiency of 60% with 13.6 dB of gain and 23W of output power was measured. The corresponding EVM, ACPR1, and ACPR2 were 3%, -45 dBc, and -46 dBc, respectively. Using DPD that corrects for memory effects, results that meet the standards specifications were obtained. To the best of the authors knowledge, the efficiency values presented here set a new record for power amplifier performance under high PAPR signals for both 5 MHz and 10 MHz modulation bandwidth signals.

The influence of coiled nanostructure on the enhancement of dielectric constants and electromagnetic shielding efficiency in polymer composites

An analytical model which predicts the attenuation of ultrawide-band (UWB) signals as they traverse various inhomogeneous tissues is presented. The model provides a computationally e-cient method of determining the frequency-dependent losses encountered by electromagnetic radio frequency (RF) signals used to communicate with biomedical implants. Classic transmission line theory is employed to generate an analytical representation which models the inhomogeneous tissue using layers of homogeneous material. The proposed model was verifled experimentally with tests of both single and multilayer samples. A realistic abdominal implant scenario was also modeled and the predictions were verifled using a commercially available 3D electromagnetic (EM) simulator. The results of this study indicate that for deep implants the higher frequency portion of the UWB spectrum is attenuated much more strongly than the lower end of the band. This implies that for robust communication UWB signals targeting biomedical implants should be limited to the lower portion of the spectrum.

A High Efficiency 780 MHz GaN Envelope Tracking Power Amplifier

This paper presents a high efficiency X-band envelope-tracking power amplifier (PA) that achieves a 60MHz modulation bandwidth. The system consists of a wideband envelope modulator and a single-chip gallium- nitride (GaN) MMIC X-band PA. The envelope modulator achieves >70% efficiency while delivering >7W into an 8Ω resistive load for a 60MHz LTE-A modulated signal with a 6.6dB PAPR. The envelope tracking system attains a PAE of 35.3% (including the power dissipation of the envelope modulator) with 1.1W of average output power at 9.23GHz using a 20MHz LTE signal. The X-band PA itself was measured to achieve a drain efficiency of 81.6% with a gain of 7.6dB under these conditions.

Computationally Efficient Model for UWB Signal Attenuation Due to Propagation in Tissue for Biomedical Implants

A multi-band power amplifier (PA) with electrically tunable output matching network is reported, whose output power and efficiency can be optimized over multiple frequencies (750MHz, 880MHz and 940MHz). It is incorporated within an envelope tracking (ET) amplifier using a high efficiency dynamic envelope voltage supply. High efficiency (50%) for all the three bands has been demonstrated with good output linearity, using a memory digital predistortion (DPD) method.

A 60MHz Bandwidth High Efficiency X-Band Envelope Tracking Power Amplifier

A linearization algorithm for the envelope supply voltage waveform is proposed for an open-loop multi-switcher envelope tracking power amplifier (ETPA). To maintain the fidelity of the supply voltage, we develop a two-step method comprising “non-linear pre-emphasis” and “envelope memory polynomial digital predistortion”, where the captured supply voltage of an RFPA is used. An ETPA is demonstrated using a 45 V CMOS multi-switcher envelope modulator and a 2.14 GHz 30 W GaN RFPA. The measured envelope absolute RMS error and ACLR are improved to 1.3% and -45 dBc from 6.4% and - 36dBc, respectively, compared with conventional pre-emphasis.