Mikko Kiviranta

SQUID-based readout schemes for microcalorimeter arrays

Microcalorimeter arrays with superconducting phase-transition thermometers are an attractive alternative for construction of imaging X-ray spectrometers. The low source impedance and low operating temperatures make dc SQUIDs good candidates for their preamplifiers. In large arrays, the circuit complexity as well as heat leakage through wiring make some sort of a multiplexing scheme desirable. Some circuit topologies for time-domain and frequency-domain multiplexing schemes are compared. One particular frequency-domain multiplexing circuit, being developed for the XEUS space mission, is reviewed in more detail. The design issues related with dynamic range, bandwidth and noise folding are discussed.

UHF RFID Reader With Reflected Power Canceller

Reflected power imposes severe linearity problems to the receiver in radio frequency identification (RFID) readers. In this letter, the design and realization of a ultra high frequency RFID reader with a reflected power canceller circuit based on quadrature feedback are presented. Theory and measurements of the signal and noise in the receiver are presented as a function of incident carrier power. The receiver sensitivity is even better than without the canceller with high incident carrier power: A noise spectral density at the data band is -140 dBm/Hz at +12 dBm incident carrier power. At the same time, input compression point of +15 dBm is achieved. The dynamic range of the receiver is improved by 10 dB.

Use of SiGe bipolar transistors for cryogenic readout of SQUIDs

We have measured characteristics and noise of four commercially available SiGe heterojunction bipolar transistors at 4.2 K. One device exhibits 75 pV Hz−1/2 white noise and a 1/f noise corner of 20 kHz at 9 mW power dissipation, proving suitable for readout of SQUIDs immersed in liquid He. Another device exhibits 370 pV Hz−1/2 white noise and a 1/f noise corner of 5 kHz at 50 µW dissipation, and still gives useable gain at 7 µW dissipation. This device is useful for SQUID multiplexers intended for closed-cycle refrigerators, in which the cooling power is limited. The achievable dissipation is even compatible for operation in the low-temperature stages of dilution refrigerators or ADRs. Furthermore, SiGe-based dc SQUID readout with 0.35 μΦ0 Hz−1/2 flux noise was demonstrated. Applying negative flux feedback yielded a slew rate exceeding 50Φ0 μs−1.

A coupled DC SQUID with low 1/f noise

A low-noise coupled DC superconducting quantum interference device (SQUID) especially optimized for low frequency is discussed. Using large Josephson junctions and a low loop inductance the contribution of the critical current fluctuation to 1/f noise can be minimized. To minimize the inductance the Josephson junctions are placed in the center of the washer close to the SQUID loop. A scaled-up copper model of the SQUID and the signal coil is used to analyze the impedance of the SQUID loop affected by the signal coil. An equivalent circuit model describing the effective inductance of the SQUID loop as a function of the frequency is used to design appropriate damping of the resonances. The DC SQUID characteristics are smooth and the noise performance of the SQUID does not markedly suffer from resonances. The contribution of the 1/f noise at 1 Hz is found to be about 5*10/sup -7/ Phi /sub 0// square root Hz.<<ETX>>

All-planar SQUIDs and pickup coils for combined MEG and MRI

Flux trapping and random flux movement are common problems in superconducting thin-film devices. Ultrasensitive magnetic field sensors based on superconducting quantum interference devices (SQUIDs) coupled to large pickup coils are especially vulnerable to strong external fields. The issue has become particularly relevant with the introduction of SQUID-based ultra-low-field (ULF) nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) techniques. In this paper, we study the constraints of thin-film-based magnetometers and gradiometers as exposed to magnetic field sequences of ULF MRI. In particular, we address issues such as response recovery, transient noise, magnetization and behaviour under shielded room conditions after prepolarization. As a result, we demonstrate sensors that are suitable for a combined multi-channel magnetoencephalography (MEG) and MRI imaging system.

Frequency Domain Multiplexed Readout of TES Detector Arrays With Baseband Feedback

SRON is developing an electronic read-out system for an array of Transition-Edge Sensors (TES) which combines the techniques of Frequency Domain Multiplexing (FDM) with Base-Band FeedBack (BBFB). Its potential astronomical applications are in the read-out of soft X-ray microcalorimeters of the XMS instrument on the International X-ray Observatory (IXO) and the far-IR bolometers of the SAFARI instrument on the Japanese-European mission SPICA. In this paper we focus on the experimental verification of the system, demonstrating for 16 pixels that simultaneous read-out does not degrade the noise performance, and that crosstalk between pixels is close to the requirements for IXO. A detailed analysis of the BBFB system is presented, identifying its limitations and routes for improvement to the levels required for implementation on both missions.

Performance of an x-ray microcalorimeter under ac biasing

Frequency domain multiplexing (FDM) is an attractive option for the readout of imaging arrays of microcalorimeters. Implementation of FDM requires ac biasing of the individual microcalorimeters. In this letter we present a small signal model for the behavior of a microcalorimeter under ac bias. Moreover, we have measured the behavior of the same microcalorimeter under ac (at 46 kHz) and dc bias. These experiments show that the performance of the device is very similar in terms of energy resolution, pulse shapes, and current–voltage characteristics. The measured energy resolution at 5.89 keV is 6.9 eV for ac bias and 5.5 eV for dc bias. The effective time constant in both cases is 100 μs.

EURECA - A European-Japanese micro-calorimeter array

The EURECA (EURopean-JapanEse Calorimeter Array) project aims to demonstrate the science performance and technological readiness of an imaging X-ray spectrometer based on a micro-calorimeter array for application in future X-ray astronomy missions, like Constellation-X and XEUS. The prototype instrument consists of a 5 × 5 pixel array of TES-based micro-calorimeters read out by by two SQUID-amplifier channels using frequency-domain-multiplexing (FDM). The SQUID-amplifiers are linearized by digital base-band feedback. The detector array is cooled in a cryogenfree cryostat consisting of a pulse tube cooler and a two stage ADR. A European-Japanese consortium designs, fabricates, and tests this prototype instrument. This paper describes the instrument concept, and shows the design and status of the various sub-units, like the TES detector array, LC-filters, SQUID-amplifiers, AC-bias sources, digital electronics, etc. Initial tests of the system at the PTB beam line of the BESSY synchrotron showed stable performance and an X-ray energy resolution of 1.58 eV at 250 eV and 2.5 eV @ 5.9 keV for the read-out of one TES-pixel only. Next step is deployment of FDM to read-out the full array. Full performance demonstration is expected mid 2009.

Design and performance of multiloop and washer SQUIDs intended for sub-kelvin operation

We have developed a set of dc SQUIDs intended for frequency-domain multiplexed readout of transition edge sensor (TES) arrays. The first design is based on the Ketchen-style washer, and has exhibited a 1.2 × 10−7 Φ0 Hz1/2 flux noise both at 4.2 and 0.43 K. The second design is based on the multiloop structure and utilizes the spoke-terminating resistors which double as junction shunts in order to minimize the excess noise due to the resonance damping. The multiloop device has exhibited a 3.5 × 10−7 Φ0 Hz1/2 flux noise level at 4.2 K. The mutual inductances are chosen such that a typical ac-biased TES-based x-ray calorimeter would need only a moderate amount of negative feedback in order to handle the dynamic range of the signal.

Baseband Feedback for Frequency‐Domain‐Multiplexed Readout of TES X‐ray Detectors

The linear dynamic range available for Frequency‐Domain‐Multiplexed (FDM) read‐out of TES X‐ray detectors is seriously limited by the SQUID current amplifiers used for the read‐out of TES‐detectors. Baseband feedback is one of the ways to increase the linearity and dynamic range of SQUIDs for the TES signals. Baseband feedback is realized by demodulation, and low‐pass filtering of the AM‐signals at the amplified summing point, thereby retrieving the signals for each detector, and subsequent remodulation and summing of the individual detector signals with phase compensation for the delay and phase rotation at each carrier frequency. This algorithm creates sufficient gain‐bandwidth at and around each carrier frequency (1–10 MHz) to reduce the error signal at the input of the SQUID amplifier for both the AC‐carriers and the signals modulated onto them. The paper presents the principle, modeling, and initial results.