AC Calorimeter Bridge; a new multi-pixel readout method for TES calorimeter arrays. T. Miyazaki*, M. Yamazaki^ K. Futamoto1^ K. Mitsuda1^ R. Fujimoto1^,.
AC Calorimeter Bridge; a new multi-pixel readout method for TES calorimeter arrays T. Miyazaki*, M. Yamazaki^ K. Futamoto1^ K. Mitsuda1^ R. Fujimoto1^, N. Iyomotof, T. Oshima1, D. Audley1", Y. Ishisaki", T. Kagei**, T. Ohashi**, N. Yamasaki**, S. Shoji*, H. Kudo* and Y. Yokoyama* *ISAS(Present address is Lawrence Livermore National Laboratory (LLNL), Livermore CA) ^The Institute of Space and Astronautical Science(ISAS), Kanagawa, Japan ** Tokyo Metropolitan University, Tokyo, Japan ^Waseda University, Tokyo, Japan Abstract In order to realize a large format (e.g. ~ 32 x 32 ) calorimeter array, it is essential to multiplex calorimeter signals at cryogenic temperatures without losing signal to noise ratio. For this purpose we propose a brand-new readout method, the CABBAGE (Calorimeter Bridge Biased by an AC Generator) where an AC biased calorimeters are placed in resistance bridges. In this paper we first describe the principles of CABBAGE and investigate its response and noise. We propose the large format calorimeter array readout using CABBAGEs, and discuss the new TES microcalorimeter readout method without using SQUIDs.
INTRODUCTION Calorimeters are promising cryogenic detectors which can achieve very high energy resolutions. It is essential to multiplex calorimeter signals to realize large format calorimeter array. The simplest way to do this is to sum the signals of multiple sensors. But in this way, we also sum the noise from all the sensors, and it is impossible to achieve good energy resolutions. Two ways are proposed to multiplex calorimeters without reducing the signal-tonoise ratio. One way is to multiplex signals in the time domain. To realize this, we need switching devices at the low temperature stage, and this may be difficult for TES microcalorimeters which require SQUIDs to read out. In this paper, we discuss the second way: to multiplex in the frequency domain.
Calorimeter multiplex in frequency domain Figure 1 [left] shows the conventional TES microcalorimeter readout circuit. The output spectrum of this circuit is shown in Figure 2 [left]. If we use an AC current source of angular frequency COM to bias this calorimeter, the spectrum of output signal shifts by COM (Figure 2 [right]). Thus, we can read out multiple calorimeters with a single SQUID by biasing each of the calorimeters with an AC current of a different frequency. But in this case, the output current contains bias current at frequency COM, which is larger
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than the calorimeter signal by factor 100. And it is difficult to readout this output with a SQUID because of the very limited dynamic range of SQUID readout systems [1]. To avoid this problem, we propose to use a calorimeter bridge and cancel large bias currents as in Figure 1 [right]. We named this technique CABBAGE(Calorimeter Bridge Biased by AC Generator). This technique was used in infrared astronomy to reduce the effect of 1/f noise [2]. But we are the first to propose this technique to multiplex microcalorimeter signals. Ri
i——VW-
DC Current Source
r
Calorimeter R2
R3
I——VW—i——VW——' ________^>,_______|
AC Current Source
SQUID
RiR3-R2R =
FIGURE 1. [Left] Conventional TES microcalorimeter readout circuit. Bias microcalorimeter by DC current source and readout the output current by SQUID. [Right] TES microcalorimeter readout circuit using CABBAGE. Bias bridge circuit which includes microcalorimeter by AC current source.
By summing the output currents from multiple CABBAGEs, we can multiplex calorimeter signals. In this way, signal and phonon noise spectrums are shifted in the frequency domain by bias frequencies. And, we can readout signals from multiple calorimeters without cross talk, if we choose proper bias frequencies. But the Johnson noise of each of calorimeters is summed. To reduce this noise, we should use bandpass filters for each pixel. Compared to non-bridge readout case, CABBAGEs have extra Johnson noises from RI and /?2- It is possbile to reduce contributions from these resistors by choosing R\ and #2 < #3. Calorimeter A
Calorimeter B
0
FA
FIGURE 2. [Left] Power Spectrum of signal and noises of 2 DC biased microcalorimeters. Signals and phonon(thermal) noises depends on frequency largely, and Johnson and amplifier noise depend on frequency less. [RightJPower Spectrum of 2 microcalorimeter multiplex in frequency domain. Frequencies of two microcalorimeters are FA and FB respectively. There are no cross talks of signals and phonon noises. But the Johnson noise contributions are summed.
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EXPERIMENT To research the properties of CABBAGEs experimentally, we used two Ti-Au TES microcalorimeters to readout X-ray pulses under AC bias. Transition temperatures of both TESes are about 0.5 K, a ~ 10 and the energy resolutions of each sensors are about 500 eV for DC bias operation. In order to sum the signals from two CABBAGEs, we used a multi-input SQUID Amplifier [3]. The bandwidth of the SQUID is about 760 kHz, and input noise level is about 3 pArms/v/Hz. We configured two calorimeter bridges and placed them in a 3 He Cryostat and operated the CABBAGEs using 10kHz and 20kHz AC current respectively. To demodulate the output signals, we used synchronous detection, and succeeded in discriminating the X-ray event pixel for each pulses. The energy resolutions(Table 1) are the same to the predictions within the error for 10kHz biased pixel. For 20kHz biased pixel, the incorrect setting of the bias current made the signal smaller and which is the cause of discrepancy of the energy resolution. TABLE 1. Summary of energy resolutions. Italic numbers are estimation from DC bias measurement. All values do not include the effect of thermalization fluctuations. Resolution Noise Contribution [eV] [eV] Johnson SQUID Phonon c Result Prediction A A C
10kHz (Ch.l) 20kHz (Ch.2)
DC CABBAGE DC CABBAGE
880 ± 150 840 ± 150 580 ± 100 3700 ± 500
800 500
460 || 546
277 308
244 277
442 442
442 -
278 308
168 202
147 164
303 -
303 303
FUTURE Large format(32x32 calorimeter array) We concluded from the results, that the CABBAGE method is promising for readout of microcalorimeter arrays. And we designed a 32 x 32 pixels calorimeter array(Figure 3). Multiplexing 32 calorimeters in a row, and bias 32 columns by common current sources, we can drive and readout 1024 calorimeters by only about 200 lines. This number is comparable to ASTRO-E XRS and far more realistic than readout 1000 pixels without multiplex: requires thousands of cables.
TES Calorimeter readout without SQUIDs If we bias the calorimeter with AC current, the output signal is confined in a relatively narrow frequency range compared to the carrier frequency, and it will be easier to use some impedance matching circuit like transformers, L-networks to step up the signal impedance without reducing the signal. In this case, it is better to amplify the signal at
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FIGURE 3. A design of 32 x 32 calorimeter array. Bias each column by same frequencies and readout summed signals of each row.
or below liquid helium temperature to avoid Johnson noise of cables or noise pick up. For this purpose, we can use GaAs MES FETs which are reported to work under liquid helium temperature[4] [5]. And if we take carrier frequency over 1MHz, we do not have to worry about the large 1/f noise of MES FETs. Input inductance of a transformer may slow the signal and reduce the signal-to-noise ratio. But L-network matching readout does not have this problem as far as it is operated at the resonant frequency. And narrow bandwidth of L-network limits the Johnson noises from other pixels. Signal
Signal
FIGURE 4. Multiple signal summation using gate-common circuit
To sum signals from multiple microcalorimeters, we can use summing loops [6]. But we must feedback output current to the summing loop in order to prevent cross talk between channels. Another way to sum the signals is to use virtual ground of gatecommon amplifier or Operational amplifier(Figure 4). In this case, we must place fast amplifier as close as possible to microcalorimeters. But we do not need to feedback signals from the room temperature electronics, because amplifier can feedback itself. And this simplicity is important for array readout.
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T. Miyazaki, Ph.D. thesis, University of Tokyo, 2001 A. E. Lange, et al , 1994, ApJ, 428, 384. K. Mitsuda, et al , 1999, NIMA, 436, 252. A. T. Lee, 1989, Rev. ScL lustrum., 60, 3315. S. L. Pohlen, et al. , 1999, Appl. Phys. Lett., 74, 2884. J. Yoon, etal. , 2001, Appl. Phys. Lett., 78, 371.
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