RF Losses Research Articles

Design and Analysis of Serpentine Flexure Based RF MEMS Switch for High Isolation with Low Pull-in voltage

This paper presents the design and performance analysis of radio frequency micro electro mechanical system switches having serpentine flexure designs. In this paper, we have done, behavioral analysis by changing material, gap and thickness, increase in the actuation area using menders, impact of holes on effective working of the switch, stress analysis for low spring constant, electromechanical analysis for low pull-in voltage and high capacitance ratio, time dependent analysis for faster switching time, scattering parameter analysis for reducing RF losses are simulated and compared with the theoretical analysis. The significant improvements in this work are lesser air gap and beam thickness reduced the pull-in voltage to 2.75 V for 1.66 µm displacement, a rapid switching time of 0.9 µs obtained through simulation, an improved capacitance ratio of 157.25, isolation is 49.59 dB, insertion loss is 0.005 dB. Actuation area is increased, and switch area is reduced to achieve the optimum performance of the switch.

Optimum Electromagnetic Modelling of RF MEMS Switches

Radio frequency microelectromechanical systems (RF MEMS) switch technology may have the potential to replace semiconductor technology in future communication systems as well as communication satellites, wireless and mobile phones. RF MEMS switches are being developed for low insertion loss, high isolation loss and high linearity that are required over a broad frequency band application. In wireless mobile communication systems, microstrip transmission lines (t-line) have received attention for their attractive benefits such as low profile, light weight, and easy fabrication. The present work has thus been to explore the design and modelling of low loss RF MEMS switches implemented in a microstrip discontinuity t-line configuration. The electromagnetic modelling of an RF MEMS switch with a microstrip line is presented with a simple analytical model to determine the scattering parameters. This study shows that an RF MEMS switch with a microstrip t-line provided less than 0.01 dB–0.05 dB insertion loss for 50 GHz frequency. Moreover, there is an isolation loss of 50 dB over frequencies up to 50 GHz frequency. This low insertion and high isolation loss in the single beam RF MEMS switch with a microstrip discontinuity t-line contributes to configuring a low loss transmission line for RF communication.DOI: http://dx.doi.org/10.5755/j01.eie.24.5.21842

N-Doped Surfaces of Superconducting Niobium Cavities as a Disordered Composite

The Q-factor of superconducting accelerating cavities can be substantially improved by a special heat treatment under N2 atmosphere (N-doping). Recent experiments at Fermi National Laboratory investigated the dependence of Q on the RF frequency and showed, unexpectedly, both an increase and a decrease with the RF field amplitude. This paper shall explain this finding by extending a previously proposed model founded on the two fluid model of RF losses, percolation and the proximity effect in a disordered composite.

Cu/Co Multilayer-Based High Signal Integrity and Low RF Loss Conductors for 5G/Millimeter Wave Applications

This paper presents copper/cobalt (Cu/Co) multilayer-based low RF loss conductors and their usage in high signal integrity coplanar waveguide (CPW) transmission lines (TLs) and high Q-factor inductors for the applications of millimeter wave bands including a 28-GHz 5G communication band. Low conductor loss, reduced signal dispersion, and low noise figure of the fabricated devices are featured. CPW (TLs) employing 10 pairs of Cu (150 nm)/Co (25 nm) superlattice conductors, the so-called metaconductors, have been designed and fabricated. Their RF characterization has been performed. Experimental results show a broadband RF resistance reduction of up to 50% (max.) in 7–32 GHz (3.5 Octaves), a phase delay improvement of 25.5%, and a thermal noise voltage reduction of 33% compared to reference Cu-based CPW TLs. In comparison with the devices from the other literature, the presented TLs show the best signal integrity performance in Ku, K, and Ka bands. In addition, a high Q-factor planar inductor on a glass substrate has been designed, fabricated, and characterized as an example of high-performance RF passive components, showing record high Q-factors of 54 at 28 GHz and 63.5 (max.) at 29 GHz.

RF Small- and Large-Signal Characteristics of CPW and TFMS Lines on Trap-Rich HR-SOI Substrates

The main objective of this paper is to evaluate RF losses and nonlinear behavior of coplanar wave- guide (CPW) and thin-film microstrip (TFMS) lines from room temperature up to 175 °C fabricated on two different types of high resistivity (HR) silicon-on-insulator (SOI) substrates. One standard high-resistivity and one trap-rich (TR) substrates, Soitec RFeSI90 product, are used. It is also shown that by using stacked CPW lines on the TR HR-SOI wafer, transmission lines with performances as high as TFMS lines are achievable, with low losses and low-harmonic distortion, while providing much more flexibility in their design. Through measurements, it is shown that the TR HR SOI substrate shows good stability in terms of losses and linearity up to 120 °C.

Rf losses in a high gradient cryogenic copper cavity

The development of high brightness electron sources can enable an increase in performance and reduction in size of extreme X-ray sources such as free electron lasers (FELs). A promising path to high brightness is through larger electric fields in radio-frequency (rf) photoinjectors. Recent experiments with 11.4 GHz copper accelerating cavities at cryogenic temperatures have demonstrated $500\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ surface electric fields with low rf breakdown rates. However, when the surface electric fields are larger than $300\text{ }\text{ }\mathrm{MV}/\mathrm{m}$, the measured cavity quality factor, ${Q}_{0}$, decreases during the input rf pulse by up to 30%, recovering before the next rf pulse. In this paper, we present an experimental study of the rf losses, manifested as degradation of ${Q}_{0}$, in a copper cavity operated at cryogenic temperatures and high gradients. The experimental conditions range from temperatures of 10--77 K and rf pulse lengths of 100--800 ns, using surface electric fields up to $400\text{ }\text{ }\mathrm{MV}/\mathrm{m}$. We developed a model for the change in ${Q}_{0}$ using measured field emission currents and rf signals. We find that the ${Q}_{0}$ degradation is consistent with the rf power being absorbed by strong field emission currents accelerated inside the cavity.

Full EM Design Method For HTS MMIC Josephson Mixers

We report the full electromagnetic (EM) design and simulation method, and applied it to develop a 34-GHz high-temperature superconducting (HTS) microwave monolithic integrated circuit Josephson mixer. The mixer is modeled in EM simulation software, high-frequency simulation structural simulator, with the junction area modeled as an excitation port with frequency-dependent impedance. Impedance matching between the junction and RF/IF ports is then optimized accordingly. Module design is carried out for the optimized HTS Josephson mixer, and the cavity resonance issue is investigated and eliminated. The HTS mixer module was experimentally developed and measured to verify the simulation. The measured frequency response of the conversion gain agrees with the simulation results of combined RF and IF transmission loss.

RF transparent vanadium oxide based single and bi-layer thin films as passive thermal control element for satellite antenna application

Germanium coated black polyimide (GBP) is often used as passive thermal control element for sunshield membrane of satellite antenna. However, GBP degrades fast while there is no protective coating applied. The present work is aimed towards the development of protective films which could hinder the degradation of GBP. At the same time, the basic characteristic of GBP membrane that is RF transparency should not be trade off. Here, RF transparent and protective vanadium oxide (VO) and vanadium oxide/silicon dioxide (VO/SO) thin films are developed by RF magnetron sputtering technique on GBP at a constant 400 W. Field emission scanning electron microscopy (FESEM) and X-ray photoelectron spectroscopy (XPS) techniques are utilized to investigate microstructural and oxide characteristics, respectively. Thermo-optical properties such as, solar absorptance (αs) and IR emittance (εir) are also evaluated. Introduction of SiO2 layer over vanadium oxide layer leads to decrease in αs to about 12%. RF losses e.g. insertion loss and return loss are measured in Ka band (27–39 GHz) and they found to be RF transparent characteristic. Thermal stability of the oxide films are also studied by differential scanning calorimetry (DSC) technique.

RF Characteristics of Two Generations of RFeSI HR-SOI Substrates Over Temperature

RF losses and nonlinearities of the commercially available Soitec radio frequency enhanced signal integrity (RFeSI) high-resistivity silicon-on-insulator substrates are investigated through investigation of 50- $\Omega $ coplanar waveguide lines manufactured on them. It is shown that the losses of the RFeSI substrates are very small. They have a temperature minimum value at around 70 °C. The generated second- and third-harmonic powers, due to the nonlinearities of the substrates, also have a similar temperature minimum value. The results indicate that the RFeSI substrates present a good RF performance over a temperature range from 0 °C to 115 °C. The RF performance of the two generations of RFeSI substrates is also compared. The losses of the two RFeSI substrates remain the same over the explored temperature range, whereas the harmonic performance of generation 2 (RFeSI 90) is better than that of generation 1 (RFeSI 80). This stems from the increased effective resistivity.

(Invited) Substrate and Device Engineering for Iot and Automotive

The main challenges of the semiconductor industry have long revolved around density scaling and performance improvement of ICs. However, a growing proportion of semiconductor consumption is now driven by factors other than pure performance and density. The Internet Of Things (IoT) & Automotive markets are perfect examples of this, where increased energy efficiency combined with enhanced reliability and heterogeneous functions integration capabilities have become major hurdles to overcome. Advanced substrates such as Silicon-On-Insulator (SOI) are progressively emerging as key platforms to address these pain points. In fact, the choice of advanced substrates results from a tradeoff between device and substrate complexity with regards to system level challenges. Bringing all the complexity of device implementation and integration on the device manufacturing process - rather than the substrate - turns out to be less and less relevant to address the challenges of heterogeneous functions integration, reliability and power efficiency. In this talk, we will discuss how three engineered substrate platforms, namely Power-SOI, RF-SOI and FD-SOI, are currently reshaping IoT and Automotive with new industry standards. Power-SOI Power-SOI substrates are based on thick BOX and top silicon (typically 1µm or more). They address high voltage device integration challenges. They allow for the replacement of all leaky and area consuming junction isolation techniques by oxide isolation, bringing significant advantages in terms of device characteristics (higher RDSON, lower leakage, lower area), but also in terms of reliability (latch-up free operation) and integration with high voltage power devices up to 600V integrated with digital functions on the same substrate. Power-SOI offers therefore tremendous benefits for power management ICs and is particularly advantageous for automotive applications. RF-SOI RF-SOI is based on a high resistivity base material often combined with a low mobility layer just beneath the BOX. RF-SOI allows the integration of ultra high linear and low loss RF components within the same substrate. This unique capability has made RF-SOI the de facto technology for a growing number of smartphone Front-End-Modules (FEM) components (switch, LNA, PA, duplexers…), i.e. all the RF components linking the antenna and the transceiver. RF-SOI will continue to expand into new applications like automotive where high throughput and low latency connectivity will be key. RF-SOI will also bring major value in the IoT space, as its integration capability offers a single FEM chip solutionwhich is particularly beneficialat system level. FD-SOI FD-SOI is the natural technology for the continuation of Moore’s law for battery-powered and cost-sensitive applications. FD-SOI is actually the simplest platform enabling fully depleted operation of transistors, which is a fundamental advantage to go below the 28nm node. FD-SOI substrates havean ultra thin top layer and BOX. The benefits of FD-SOI are numerous: simple device manufacturing, high Fmax compatible with mmWave spectrum, ultra low voltage operation down to 0.4V, All these characteristics have made FD-SOI a key technology for all applications not purely driven by density and digital performance. In automotive, it brings huge differentiation in terms of reliability and power consumption, enabling the possibility of highly reliable fanless solutions. In IoT, significant battery life gains can be expected, while providing the best cost per transistor of all CMOS technologies. Conclusion Advanced substrates such as SOI appear more and more as key platforms to ease integration, while exacerbating device advantages such as power and reliability for a growing number of applications not purely driven by density and performance. These key features will probably impose SOI in the future as a new industry standard for IoT and automotive.

Resistivity and Radio-Frequency Properties of Two-Generation Trap-Rich Silicon-on-Insulator Substrates**Supported by the National Natural Science Foundation of China under Grant Nos 61376021 and 61674159, and the Program of Shanghai Academic/Technology Research Leader under Grant No 17XD1424500.

Crystal morphologies and resistivity of polysilicon trap-rich layers of two-generation trap-rich silicon-on-insulator (TR-SOI) substrates are studied. It is found that the resistivity of the trap-rich layer of generation 2 (TR-G2) is higher than that of generation 1 (TR-G1), although the crystal morphologies of the trap rich layers are the same. In addition, the rf performance of two-generation TR-SOI substrates is investigated by coplanar waveguide lines and inductors. The results show that both the rf loss and the second harmonic distortion of TR-G2 are smaller than those of TR-G1. These results can be attributed to the higher resistivity values of both the trap-rich layer and the high-resistivity silicon (HR-Si) substrate of TR-G2. Moreover, the rf performance of the TR-SOI substrate with thicker buried oxide is slightly better. The second harmonics of various TR-SOI substrates are simulated and evaluated with the harmonic quality factor model as well. It can be predicted that the TR-SOI substrate will see further improvement in rf performance if the resistivities of both the trap-rich layer and HR-Si substrate increase.

RF Loss Characteristics of Coplanar Waveguide Employing Chemically Modified Graphene on Flexible Substrates

In this work, a coplanar waveguide employing chemically modified graphene was fabricated on a polyethylene terephthalate (PET) substrate and its RF loss characteristics were investigated. According to the results, the coplanar waveguide showed much lower loss characteristics than conventional graphene-based transmission line. The insertion loss of conventional graphene/Si structure was – 24 to − 33 dB at 1 GHz, but that of the graphene/PET structure was − 0.773 dB at the same frequency. However, the graphene/PET structure showed a comparatively higher loss than the gold–Ti/PES structure. The higher loss of the graphene/PET structure originated from the comparatively lower conductivity of graphene. The results of the present study indicate that the graphene/PET structure is prospective for application in flexible RF devices.

Effect of cooldown and residual magnetic field on the performance of niobium–copper clad superconducting radio-frequency cavity

We present the results of rf measurements on a niobium–copper clad superconducting radio-frequency cavity with different cooldown conditions and residual magnetic field in a vertical test Dewar in order to explore the effect of thermal current induced magnetic field and its trapping on the performance of the cavity. The residual resistance, extracted from the Q0(T) curves in the temperature range 4.3–1.5 K, showed no dependence on a temperature gradient along the cavity during the cooldown across the critical temperature up to ∼50 K m−1. The rf losses due to the trapping of residual magnetic field during the cavity cooldown were found to be ∼4.3 nΩ μT−1, comparable to the values measured in bulk niobium cavities. An increase of residual resistance following multiple cavity quenches was observed along with evidence of trapping of magnetic flux generated by thermoelectric currents.

Effects of Amerindian Genetic Ancestry on Clinical Variables and Therapy in Patients with Rheumatoid Arthritis.

To define whether Amerindian genetic ancestry correlates with clinical and therapeutic variables in admixed individuals with rheumatoid arthritis (RA) from Latin America. Patients with RA (n = 1347) and healthy controls (n = 1012) from Argentina, Mexico, Chile, and Peru were included. Samples were genotyped for the Immunochip v1 using the Illumina platform. Clinical data were obtained through interviews or the clinical history. Percentage of Amerindian ancestry was comparable between cases and controls. Morning stiffness (p < 0.0001, OR 0.05), rheumatoid factor (RF; p < 0.0001, OR 0.22), radiographic changes (p < 0.0001, OR 0.05), and higher number of criteria were associated with lower Amerindian ancestry after Bonferroni correction. Higher Amerindian ancestry correlated only with weight loss (pBonferroni < 0.0001, OR 2.85). Increased Amerindian ancestry correlated with higher doses of azathioprine (p < 0.0001, OR 163.6) and sulfasalazine (p < 0.0001, OR 48.6), and inversely with methotrexate (p = 0.001, OR 0.35), leflunomide (p = 0.001, OR 0.16), and nonsteroidal antiinflammatory drugs (pBonferroni = 0.001, OR 0.37). Only the presence of RF and weight loss were modified after confounders adjustment. Amerindian ancestry protects against most major clinical criteria of RA, but regarding the association of RF with increased European ancestry, age, sex, and smoking are modifiers. Ancestry also correlates with the therapeutic profiles.

Wafer-level micropackaging in thin film technology for RF MEMS applications

In this work, a thin-film packaging was developed to be used for radio-frequency microelectromechanical system configurations. The fabricated packages are suspended membranes in the multilayer SixNy/aSi/SixNy on conductive coplanar waveguides (CPWs) of different length. Several geometric parameters of the membranes, which are the length, the curvature radius at the vertices of the rectangular base, the density and the diameter of holes on the capping surface, were also varied. The mechanical properties of the suspended membranes were investigated by mechanical simulations and surface profilometry measurements as a function of the geometric parameters. RF characterization was performed to evaluate the impact of the package on the CPW performance. Finally, network analysis was carried out, allowing to clarify the origin of the RF losses measured for the fabricated microdevices.

Power efficient, low loss and ultra-high isolation RF MEMS switch dedicated for antenna switch applications

We present a new ultra-high isolation RF MEMS latching switch with ultra-low power consumption dedicated to antenna switch applications in multiband and multi-standard transceivers. Power management of the switch is done by integrating latching actuators in switch structure. Actuation voltage of the switch is 5V and the switch consumes a power of around 131mW for 50 µs while transition to ON or Deep-OFF states. The switch is modeled and its operation is comprehensively analyzed. The switch has an IIP3 of 127dBm and power handling of up-to 6W without causing stiction. The insertion loss of the switch is −0.5dB up to 20GHz. When the switch is OFF, the input and output are isolated with −50dB for up to 20GHz. At the Deep-OFF state, the switch is connected to the GND and isolation reaches −130dB up to 20GHz. These results are from FEM simulations and has not been verified by experiment.

Investigation of Wafer Level Packaging schemes for 3D RF interposer multi-chip module

Abstract Very small RF modules can be realized through heterogenous integration of GaAs MMIC (monolithic microwave integrated circuit) onto a low loss Si sub-mount, with high density routing lines realized by advanced patterning. In this paper we investigate how to integrate MMIC active devices on GaAs with the RF passives produced on an interposer, using Si wafer process technology. High resistive Silicon substrates are required to minimize RF losses. The interposer is thinned below 100 μm to reveal Cu TSVs from the back of the interposer, while the front side is covered entirely with a silicon capping wafer for shielding the device. We compare different wafer level packaging approaches for producing the low RF-loss interposers, and populating them using die-to-die (D2D) or die-to-wafer (D2W) bonding of the MMIC components, followed by wafer level encapsulation. Two D2W approaches are compared, in the first approach the D2W mounting and the encapsulation happens before the Si interposer is thinned for TSV reveal. To avoid damage during thinning of the wafer, thicker substrates with deeper TSV of 150 μm or more are required. In a second approach, the thinning of the interposer is done prior to the mounting. Initial electrical data showed that the approach yielded proper RF performance, but further yield optimization is required.

Experimental Assessment of Microwave Loss Caused by Non-Hexavalent Chromium-Based Conversion Coatings

Abstract Conversion coatings are used to inhibit corrosion on aluminum structures while maintaining electrical conductivity. The most common type of conversion coatings in aerospace applications (MIL-DTL-5541 Type I), contain hexavalent chromium compounds as the corrosion-inhibiting additive. These Type I conversion coatings have a long pedigree and are highly effective in preventing corrosion; however, the hexavalent chromium compounds in these coatings are carcinogenic and water-soluble. Therefore, the use of these compounds is highly regulated in order to protect both workers and the public leading to high cost in both use and disposal. In addition to these regulations, use of these materials on new designs for DOD is prohibited by DFARS 48 CFR Parts 223 and 252, and is scheduled to be prohibited in Europe in September 2017 by REACH regulations. In response, new more environmentally friendly non-hexavalent chromium-based processes are becoming available. Coatings resulting from these types of processes are referred to as MIL-DTL-5541 Type II conversion coatings. The long term reliability and performance impacts resulting from the use of these coatings are not fully understood and there currently is an effort in the aerospace industry organized by NASA to fully define these impacts while hardware is still in the design stage. While significant work has been performed to define the corrosion performance of various type-II conversion coatings, there has been minimal work performed to quantify the impact a type-II conversion coating would have on RF electrical assemblies. Of particular interest is the impact a conversion coating can have on microwave loss at higher frequencies. Many RF electrical assemblies use aluminum radiator and waveguide structures to transfer energy between components and radiate into freespace. If microwave losses increase due to a change in conversion coating, there could be negative impacts to key performance parameters such as system sensitivity, dynamic range, noise figure, and radiated power. Understanding this impact is critical in determining whether the design change impact is isolated only to the conversion coating or whether it propagates to other subcomponents to compensate for the loss in performance. The standard way to quantify the electrical resistance of conversion coatings is defined by MIL-DTL-81706B. The test involves collecting a DC resistance measurement on a processed panel using a two-probe measurement with 200 psi of pressure applied to the probes. The resulting value is averaged from 10 samples of data collected across the panel. While this test in MIL-DTL-81706B is well defined, it has significant limitations that caused this research to seek another way to quantify this value. First, the repeatability of the two-point probe is not consistent across the panel. Some of the conversion coatings can be brittle and can easily be disturbed by the force applied by the probes. The poor repeatability is exacerbated when the test articles are environmentally exposed, leaving a non-uniform surface. Finally, this test methodology is performed at DC, which does not directly quantify the impact of the coating at microwave frequencies due to phenomena such as skin effect and potential plasmonic response. This talk discusses an experiment performed to assess the impact of the use of type-II conversion coatings on microwave loss. In order to assess this impact, a set of precision machined waveguide structures were used as test articles in the experiment. The advantage of using this waveguide-based approach is that it provides a distributed surface to assess the average impact of conversion coatings on surface resistivity. This average resistivity more closely maps to the RF losses seen by microwave systems. In addition, testing the waveguide test article provides a very repeatable test methodology; waveguide technology is very mature from a manufacturing perspective. Also, the waveguide flanges provide a repeatable way to connect to the test article so long as they are masked or cleaned after any potential environmental exposure. Finally, the rectangular shape of the waveguide can be canonically described by a closed form expression, improving understanding of the specific mechanisms leading to the loss. This talk discusses an experiment where multiple 3-foot pieces of WR-28 were used as test articles. The WR-28 test articles were chosen to assess the impact to performance at Ka-band. The 3-foot sections are convenient articles because they can easily be measured on a workbench while at the same time being electrically long at Ka-band (on the order of 100 wavelengths). This talk discusses three different populations of test articles, each coated with a different type of conversion coating. This talk also discusses how an initial measurement of these test articles before environmental exposure showed little difference between these populations. Finally, this talk will discuss plans for environmental testing and in-process RF measurements to be captured during these tests.

A High Power Helicon Antenna Design for DIII-D

A new antenna design for driving current in high beta tokamaks using electromagnetic waves, called Helicons, will be experimentally tested for the first time at power approaching 1 megawatt (MW) in the DIII-D Tokamak. This method is expected to be more efficient than current drive using electron cyclotron waves or neutral beam injection, and may be well suited to reactor-like configurations. A low power (100 watt (W)) 476 megahertz (MHz) “comb-line” antenna, consisting of 12 inductively coupled electrostatically shielded, modular resonators, was tested in DIII-D and showed strong coupling to the plasma without disturbing its characteristics or introducing metal impurities.The high power antenna consists of 30 modules affixed to back-plates and mounted on the outer wall of the vacuum vessel above the mid-plane. The antenna design follows a similar low power antenna design modified to minimize RF loss. Heat removal is provided by water cooling and a novel heat conducting path using pyrolytic graphite sheet. The CuCrZr antenna modules are designed to handle high eddy current forces. The modules use molybdenum Faraday shields that have the plasma side coated with boron carbide to enhance thermal resistance and minimize high Z impurities. A RF strip-line feed routes the RF power from coaxial vacuum feed-throughs to the antenna. Multipactor analysis of the antenna, strip line, and feed-through will be performed. A 1.2 MW, 476 MHz klystron system, provided by the Stanford Linear Accelerator (SLAC) will provide RF power to the new antenna. A description of the design of the high power antenna, the RF strip-line feeds, and the vessel installation will be presented.

Impact of Remanent Magnetic Field on the Heat Load of Original CEBAF Cryomodule

The heat load of the original cryomodules for the continuous electron beam accelerator facility is ~50% higher than the target value of 100 W at 2.07 K for refurbished cavities operating at an accelerating gradient of 12.5 MV/m. This issue is due to the quality factor of the cavities being ~50% lower in the cryomodule than when tested in a vertical cryostat, even at low RF field. Previous studies were not conclusive about the origin of the additional losses. We present the results of a systematic study of the additional losses in a five-cell cavity from a decommissioned cryomodule after attaching components, which are part of the cryomodule, such as the cold tuner, the He tank, and the cold magnetic shield, prior to cryogenic testing in a vertical cryostat. Flux-gate magnetometers and temperature sensors are used as diagnostic elements. Different cool-down procedures and tests in different residual magnetic fields were investigated during the study. Three flux-gate magnetometers attached to one of the cavities installed in the refurbished cryomodule C50-12 confirmed the hypothesis of high residual magnetic field as a major cause for the increased RF losses.