RF Leakage Research Articles

Hundreds megawatt‐level pulse transmission waveguide vacuum port using series inductive post‐wall

ABSTRACTA simple waveguide vacuum port for hundreds MW pulse transmission at S‐band is proposed. It uses the series inductive post‐wall to improve the characteristics of the RF leakage attenuation and maximize the unit area of the vacuum port related with effective pumping speed. This type of vacuum port significantly increases RF leakage attenuation above −50 dB without RF electrical breakdown phenomena and provides simple structure to enhance the conductance of the waveguide vacuum port found to be around for above −50 dB RF leakage attenuation. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:2894–2896, 2014

Intermodulation and Harmonic Distortion in Frequency Reconfigurable Slot Antenna Pairs

Due to the nonlinear nature of solid state devices, reconfigurable antennas with PIN diodes are capable of producing spurious frequency products through intermodulation and harmonic generation. These undesirable signals might act as an interference source to other electronic systems. In this paper, we investigate intermodulation and harmonic generation in frequency reconfigurable MIMO slot antennas. Our study has shown that intermodulation is more pronounced in the transmitting scenario than the receiving case. It is also found that an “off” diode needs to be appropriately reverse biased to avoid RF leakage and harmonic generation when the RF power is high. In addition, the effect of mutual coupling on intermodulation and harmonic generation has been studied experimentally. Our results show that the level of intermodulation could be lowered by reducing the mutual coupling between the antenna ports. Lastly, we demonstrate that a particular spurious frequency product may be suppressed by inserting harmonic traps at the antenna level.

A BiCMOS Ka‐band RF‐pulse former for short‐range high‐resolution radar and high‐data‐rate communication systems

ABSTRACTAn ultra‐low‐leakage RF‐pulse former capable of producing very narrow RF pulses with small rising and falling time for short‐range high‐resolution radar and high‐data‐rate communication systems is developed using 0.18‐µm BiCMOS technology. The narrow RF pulses are produced from a continuous‐wave RF signal by gating a high‐speed single‐pole single‐throw switch implemented with small gate‐resistors. The RF‐pulse former exhibits an extremely low RF leakage implementing the RF leaking suppression technique, in which the RF leaking signal is cancelled by combining with its replica using an active balun. In addition, the active balun also amplifies the gated RF pulses hence providing gain for the RF‐pulse former. A graphical analysis based on the insertion loss and isolation contours, enabling selection of the optimum sizes for transistors in the series‐shunt switches, is also presented. From 31 to 37.1 GHz, the designed RF‐pulse former exhibits −1.9 dB (loss) to 1.1 dB (gain) and 14.5–30‐dB input return loss. The output return loss is higher than 10 dB from 33 to 35.9 GHz. From 30 to 40 GHz, the isolation is higher than 40 dB and, especially at 34 GHz, the isolation reaches 70 dB. The rising and falling times were measured as 136 and 70 ps, respectively. Very narrow RF pulses of 200 ps have been generated. The RF‐pulse former consumes a 7.3 mA from a 1.8‐V power supply and occupies a chip area of 0.225 mm2. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:2773–2777, 2013

A Cascode Feedback Bias Technique for Linear CMOS Power Amplifiers in a Multistage Cascode Topology

A novel feedback bias technique for a multistage cascode topology is developed to improve the linearity and reliability of power amplifiers (PAs). Due to the large parasitic capacitance and low substrate resistivity of CMOS technology, signal swings are coupled between the ports of transistors. The proposed method utilized the RF leakage signals at the gate of common-gate (CG) transistor in a cascode topology for employing negative feedback, which not only enhances the linearity of the PA, but also alleviates the voltage stress between the gate and the drain of the CG device in a cascode topology from 4.5 to 1.9 V. This technique requires no additional components or space and is easily applicable to the multistage cascode topology, which is one of the most popular structures of CMOS PA designs. In order to prove the concept, a 1.95-GHz fully integrated linear PA was implemented in a 0.18- μm CMOS technology. With a 3.4-V power supply, the PA transmits a saturated output power of 26 dBm with a power-added efficiency (PAE) of 46.4%, and a linear output power of 23.5 dBm with a PAE of 40% using a 3 GPP WCDMA modulated signal. The PA occupies 1.60 × 0.52 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . This PA demonstrates the potential of the highly efficient CMOS PA design approach for wireless communication standards.

RF leakage current in electrosurgical units: Influence of the layout in taking measurements

The RF leakage current in electrosurgical units is a critical parameter to measure, because it may cause accidental burns in patients. The particular standard for electrosurgical units IEC 60601-2-2 indicates the maximum RF leakage levels and defines the elements and their layout to do these measurements. On this paper we show the RF leakage current values of 6 electrosurgical units. We did these measurements in two different ways: in the first one we measured in normal conditions of use in the operating rooms and in the second we followed the mentioned Standard. The results shows differences between one group and the other, observing higher RF leakage current values in the measurements that we did without following the standard's layout.

Microwave leakage from field modulation slots in TE011 electron paramagnetic resonance cavities

This article considers a Q-band cylindrical TE011 electron paramagnetic resonance cavity with slots cut parallel to wall currents to allow penetration of magnetic field modulation and with mode suppression gaps in the end regions of zero wall currents to reduce interference from nearby modes. Origins of the small amount of rf leakage generally observed from modulation slots have been studied using a finite element computer program and ways to eliminate this leakage have been found. The leakage is caused by field distortions from the waveguide coupling iris, which is centered on the cylindrical sidewall. The distortion is of two distinct types, which result in comparable leakage levels. One is from iris fields that have components perpendicular to the fields of the mode of interest. The iris near fields couple to a nonresonant circumferential radial mode in the modulation slots, like that of a sectorial waveguide horn. The other type of rf leakage is caused by coupling of the aperture to nearby cavity modes that leak from the modulation slots if they have currents across the slots. End gaps have been used to “suppress” the TM modes that, without the gaps, are degenerate with the TE011. We find that the TM modes are not suppressed, but rather altered, as are most modes. The modes are combinations of coaxial modes that are excited in the end-gap regions coupled to normal cavity modes. The most desirable way to reduce leakage is the combination of a thin slotted iris, thin modulation slots and sufficiently thick cavity walls. Electric discharge machining techniques can be used to produce slots of the required dimensions. A less desirable way to reduce the rf leakage is to place a floating conducting shield just outside the modulation slots. If resistive material is placed in the end gaps, rf leakage from the modulation slots decreases because the amount of energy coupled to an adjacent cavity mode is reduced, which increases the Q of the TE011 mode.

DC offset and IM2 removal in direct conversion receivers

DC offsets cause interference in homodyne receivers, caused by nonlinearity and imbalances in the receiving amplifier and mixing circuits, as well as local oscillator (LO) leakage into the RF input port and RF leakage into the LO port. The latter causes a squaring action (hence, second order intermodulation distortion (IM2)) in the mixer which generates a DC offset (low frequency) that varies with the amplitude of any high power RF interfering (jamming) signal, whatever its applied frequency. A method for removing this DC offset is described. The signal immediately after the mixer is squared to obtain a distortion estimate of the jamming signal, and then subtracted from the main signal path to remove the interference from the desired signal. The experimental results indicate an 11.5 dB improvement in jamming margin for systems with 30 kHz RF bandwidth. In the experiment an adaptive equaliser is used to equalise the two paths and improve the dynamic response.

An evaluation of radio frequency exposure from therapeutic diathermy equipment.

To assess the physiotherapist's exposure to radio frequency electromagnetic fields (RF) leaking from short wave diathermy equipment, we conducted on-site measurements of stray electric and magnetic fields (27.12 MHz) close to continuous wave (CW) short wave equipment. The results show that the operator's knees may have the highest exposure level for both electric field (E-field) and magnetic field (H-field) in the normal operating position, i.e., behind the device console. The whole-body E-field exposure normally does not exceed the 1992 IEEE recommended limit during a normal treatment session. On the other hand, the operator's whole-body exposure to H-field was barely below the recommended limit. Our data suggest little chance of immediate harmful effects of RF leakage from the diathermy. Nonetheless, physiotherapists should still be advised to remain at a distance of at least 20 cm from the electrodes and cables to avoid possible overexposure.

A 20 GHz microwave sampler

A microwave sampler circuit which operates over the frequency band of 1-20 GHz and has a number of novel features is described. These features include a wideband microstrip-to-slot balun and a wideband active isolator the function of which is to reduce the local oscillator to RF leakage from the input port of the sampler. The signal-to-noise ratio over the input bandwidth is greater than 20 dB at an input power level of -32 dBm. This signal-to-noise ratio was measured in an IF bandwidth of 175 MHz and includes the contribution from the IF amplifier. The sampler, which is made on alumina using MIC techniques, has an integrated impulse generator driven with a sinusoidal local oscillator of only 20 dBm over the frequency band of 250-350 MHz. The IF signal is in the 10-175-MHz band. The RF input VSWR is better than 2:1 up to 20 GHz, and the oscillator to RF breakthrough is better than -58 dBm (-78 dBc) when driven with a local oscillator of 20 dBm. This unusually low leakage was achieved by using the active isolator prior to the sampling circuit. >

Plunger design and rf leakage measurement of choke stub for JT-60 ICRF heating system

It was planned to move a stub rf terminator during rf injection for producing a good impedance matching of an antenna with a few plasma shots in JT-60 ICRF heating experiment. A choke stub was adopted to effect this operation. Rf leakage and high power performance were the major subject in the research and development of the choke stub. For the design of the choke plunger, a simple analytical model was developed to calculate the rf leakage of the plunger. The validity of the model was demonstrated by rf leakage measurements.

A compact and movable king-type probe for surface field measurements

The direct measurements of surface currents using King-type probes are valuable diagnostic tools in calibration studies. A new design and the corresponding calibration curves of a compact movable probe is introduced. It has been successfully used for surface current measurements in open scatters with thin walls and for the detection of RF leakage on shielding enclosures.

Peripheral Cable-Shield Termination: The System EMC Kernel

This paper presents experimental data that show an over-whelming dependence of system electromagnetic compatibility (EMC) on system cable effects, rather than leakage through cracks, seams, and other box (LRU and WRA) openings. A laboratory experiment compares cable-shield RF leakage from a pigtail-terminated cable with that of one that has been terminated peripherally. Theoretical justification for these empirical observations is presented. That a shielded conductor carrying an RF current i induces its own return current -i on the inside surface of the shield is proved. By confining this return current to the inside of the shield, external emissions are attenuated by the intrinsic shielding effectiveness (which can be quite a large number) of the shield.

An inexpensive r.m.s. ammeter for use with electrosurgical units.

A true r.m.s. reading rf ammeter utilizing low-cost integrated circuits has been developed for use in measurements of the power output and rf leakage of electrosurgical units. The meter is sufficiently accurate for most clinical applications without the drawbacks of the thermal ammeter.

Model Study on the Reduction of RF Leakage in the TRIUMF Cyclotron

Since the 1 MW RF cavity of the TRIUMF cyclotron is contained completely inside the vacuum chamber, RF leakage from the resonator cavity into the tank vacuum chamber can cause undesirable heating and damage to uncooled structures and diagnostic equipment. The cavity traversed by the beam outside the resonators has been successfully simulated with the two dimensional code SUPERFISH and the quasi-three-dimensional code RFQ3D. Numerical calculations using these codes and precise 1:10 model measurements show that the power coupling between the operating mode of the resonant cavity and the parasitic modes of the beam cavity depends on the voltage distribution along the dee gap, as well as on the frequencies and the field patterns of the parasitic modes. These parameters are quite sensitive to the geometries and imperfections of the centre post, the flux guides an the cantilevered hot arms. Adjustments of these geometries have achieved a significant reduction in the RF leakage in the model.

Modelling the TRIUMF RF Cavity Using the Code RFQ3D

Development work on the TRIUMF cyclotron RF cavity requires an efficient code to study the effects on voltage distributions of resoaator geometry nonuniformity, including misalignment of the cantilevered cavity walls, the end flux guides and the centre post. A quasi-three-dimensional code, RFQ3D, although originally written for radio frequency quadrupole cavity studies, has been successfully adapted to simulate the TRIUMF cavity. The code uses four one-dimensional coupled transmission line equations to calculate the voltage distributions along the dee gap and allows for local variations in the transmission line inductance and capacitance. The effects of individual resonator components on the accelerating voltage profile and the RF leakage for both the fundamental and third harmonic frequencies are calculated and compared with precise 1:10 model measurements.

A New and Improved RF Resonator Segment for the TRIUMF Cyclotron

Programs at TRIUMF directed toward higher currents, separated turns and extraction of H- beam require a resonator structure of greater mechanical stability. Studies have led to the design and installation of a new resonator structure with increased stiffness and reduced vibration amplitude. Measurements with a newly designed segment installed in the TRIUMF cyclotron have led to more accurate determination of the RF leakage power (excitation of RF modes in the beam space).

Determination of the RF Leakage Field in the Vacuum Tank of the TRIUMF Cyclotron

In the TRIUMF cyclotron the dees are partially defined by the two halves of the quasi-circular vacuum chamber, which completely contains the 1 MW resonating cavity. A stray electric field or RF exists inside the dees and has occasionally caused problems to probes or other tank equipment. In order to understand the nature of this field a precise 1:10 scale metal model of the entire tank and resonator system has been built and is being investigated. Various resonances have been identified: the push-pull mode at 226 MHz and the push-push mode at 233 MHz; cross modes along the dee gap resonate at 243 MHz and 253 MHz. In the quasicircular tank region outside of the main resonating cavity the Tm310 and Tm410 modes are mainly responsible for the configuration of the leakage field since they are closer to the operating frequency. The measurements are in reasonable agreement with the results of numerical relaxation calculation and with measurements in the cyclotron tank.

A radio-frequency monitor for protection against overexposure from RF heaters.

Extensive investigation on the various potentially polluting RF sources has led to the conclusion that the largest proportion of problems arising from possible RF hazards originates from industrial heating machines (short-wave). In particular, these turn out to require checks due to their intrinsic and induced instabilities. Through the analysis of RF leakage characteristics of these machines, the problem is simplified and the solution is presented in terms of an RF hazard monitor which can be used by non-specialized personnel. The monitor is also suitable for several other types of potentially dangerous apparatus in the medium- and short-wave range.

Radiofrequency electromagnetic fields in Swedish radio stations and tall FM/TV towers.

Radiofrequency electric and magnetic fields have been measured around 11 large broadcast stations and tall FM/TV towers in Sweden. The results show that operating personnel may be exposed to fields exceeding by several times the present standard for occupational exposure to RF radiation. Maintenance personnel are especially vulnerable to exposure when climbing energized towers. The present study indicates that the transmitters should be switched off during the performance of certain tasks on the tower. Safe passage of maintenance personnel near energized antennas of certain types is impossible without a drastic reduction of the transmitted power. In the stations, the RF leakage radiation was generally low, but when work was done with the cabinet doors of the transmitter open, high strengths were found in the vicinity of the transmitter even though it was switched off. It was found that the transmitter acted as a tuned receiver of energy from the other transmitters.

RF Impedance of the Accelerating Beam Gap and Its Significance to the TRIUMF RF System

The RF system at TRIUMF is now operating with the highest Q, the lowest RF leakage into the beam gap, the best voltage stability, and the lowest resonator strongback temperatures ever measured since it was first put into operation. This paper describes the calculation of the RF impedance of the beam gap and its correlation to the RF problems encountered, which eventually led to modifications to the flux guides and resonator tips to accomplish the improved operation of the RF system.