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Frequency Domain Multiplexed Readout of TES Detector Arrays With Baseband Feedback Roland den Hartog, Jörn Beyer, Dick Boersma, Marcel Bruijn, Luciano Gottardi, Henk Hoevers, Rui Hou, Mikko Kiviranta, Piet de Korte, Jan van der Kuur, Bert-Joost van Leeuwen, Mark Lindeman, and Ad Nieuwenhuizen
Abstract—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 BaseBand 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.
TABLE I REQUIREMENTS FROM MISSIONS ON READ-OUT
Index Terms—Bolometers, FDM, IXO, microcalorimeters, SPICA, SQUID read-out electronics, TES.
I. INTRODUCTION
F
DM is based on the amplitude modulation of an AC bias carrier with the signal detected by the TES. It allows in principle the read-out of multiple TES pixels in a row using a single SQUID-based pre-amplifier, thus significantly reducing the heat-load on the array via the external wiring. Read-out systems based on FDM are under development at SRON [1], but also at UCB, LLNL and ISAS [2]–[5], and deployed at Polar Bear, a South Pole Telescope with 1275 bolometer pixels (150 GHz), and the APEX-SZ experiment in Chile. The development at SRON is currently concentrating on two instruments: the SAFARI TES-based bolometer instrument on the SPICA mission, and the XMS TES micro-calorimeter instrument on the IXO mission. BBFB attempts to cancel the error signal at the SQUID sumpoint at and around each carrier frequency, thereby creating sufficient dynamic range over a broad range of carrier frequencies [6]. For both missions BBFB allows an extension of the SQUID Manuscript received August 03, 2010; accepted November 02, 2010. Date of publication January 28, 2011; date of current version May 27, 2011. This work was supported in part by ESA under Contract TRP 5417. R. H. den Hartog, D. Boersma, M. P. Bruijn, L. Gottardi, H. Hoevers, R. Hou, P. de Korte, J. van der Kuur, B. van Leeuwen, M. Lindeman, and A. Nieuwenhuizen are with the SRON National Institute for Space Research, Sorbonnelaan 2, 3584CA Utrecht, The Netherlands (e-mail:
[email protected]). J. Beyer is with PTB, Abbestrasse 2-12, Berlin D-10587, Germany. M. Kivviranta is with VTT, The Technology Research Centre of Finland, Tietotie 3, Espoo 02110, Finland. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2010.2101998
linear dynamic range, typically , which is used for SAFARI to multiplex more pixels per SQUID channel, and for XMS to match the larger dynamic range density of the TES detectors to the SQUID pre-amp. II. MISSION REQUIREMENTS The requirements on the read-out system for both the XMS and the SAFARI instrument are summarized in Table I. They were derived during instrument studies for ESA, in close collaboration with JAXA (XMS, SAFARI) and NASA (XMS). It is clear that the requirements for both instruments are generally quite comparable, which allows a large synergy in the development of the read-out systems for the two missions. III. EXPERIMENTAL SET-UP The experimental set-up at SRON in which the measurements described in this paper have been carried out is summarized in Fig. 1. The coldhead, shown in Fig. 2, contains a compact assembly consisting of a 5 5 TES array, of which 15 TES’s are connected, an interconnection chip with two test resistors, two filter chips (allowing for redundancy) with 18 LC filters each, a second interconnection chip and a PTB 16 SQUID array with Output Current Feedback (OCF) [7]. The warm electronics consist of an analog Front-End with a Low-Noise Amplifier (LNA) and a digital Demux board designed around a Xilinx Virtex 5 FPGA, which presently runs at a clock speed of 40 MHz. An ADC samples the signal from the amplifier chain, while one DAC provides the AC bias signals for the TES detectors and
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Fig. 1. FDM/BBFB system block diagram for one signal chain. The systems consists of three sections, i.e.: the cold electronics in the cryostat, the analog front-end electronics, and the digital electronics. The input and output lines of the cold and FEE sections are filtered. Fig. 3. Open-loop scan of the response measured on the ADC as a function of frequency with input signal at AC-bias carrier (top) and input signal at SQUIDfeedback (bottom), showing 17 resonances. The TES’s are in superconducting state. The dips directly following each resonance peak are resp. due to resonance between the LC-filter and the capacitive bias voltage divider (top) and the screening of the SQUID FB coil by the input coil (bottom).
Fig. 2. 16 channel TES FDM cold head. LC chip size is 20
2 14 mm
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a second the BBFB signal which drives the error signal in the SQUID to zero. To keep multiple TES pixels locked in a closed read-out loop, three conditions must be met: a. The loop Gain-Bandwidth , where product (GBW) must be smaller than is the spacing between the carrier frequencies [8]; b. the maximum excursions of the error signal in the SQUID coil must ; c. a stable Electro-Thermal Feedback remain below (ETF) regime must be established. The choice of SQUID was driven by the need to integrate TES, LC filters and SQUID on a coldhead at the 50 mK level. In previous FDM experiments [8] the SQUID had a high , at the expense of a power dissipation of 1 , which limited its application to the 1K level of the cryostat. The PTB C4X16FM SQUID array we use now has a more modest power dissipation of 15 nW, making it suitable for application at the 50 mK level, but the transfer is consequently lower as well: . The array available for these experiments still consisted of low-C TES detectors designed according to previous XEUS specifications rather than the IXO specifications on which the requirements in Table I are based. In the current set-up, the TES
in its optimal bias set point has a dynamic range more than 4 . The times that of the SQUID, and a pulse risetime of GBW of the BBFB loop is limited to 25 kHz, which implies that gain is lacking at the highest frequencies (risetime) of the signal bandwidth. So extra measures were needed to ensure stable operation: a. To increase the pulse risetimes, the inductance in the LC filters rather than the 400 nH. b. The TES was designed to be 1 array was operated at a higher bath temperature of 90 mK, which reduces the modulation depth of the pulses. c. A magnetic field of 0.1 G was applied to further reduce the TES responsivity. Although the above choices allowed a stable read-out of multiple pixels, they prevented excellent X-ray or baseline resolutions. The high-resolution demonstration of FDM is discussed by Gottardi et al. [9]. IV. LC FILTERS FDM requires noise-blocking filters to prevent contribution of wide-band Johnson noise from the resistive TES thermometers to the neighboring frequency channels. The LC filters used for this task are implemented in series with each TES as shown in Fig. 1. The quality or factor of those filters is driven by the requirement to operate the TES in voltage bias, i.e.: the equivamust be considerably lent series resistance of the LC-filter . smaller than the set-point resistance Fig. 3 illustrates the open-loop, in-situ characterization of the LC filters, which were on one of the first chips from the successful production process [10]. Using the redundancy on two LC filter chips, we found 17 filters working, for 15 TES pixels and 2 resistors (at 1.4 and 2.7 MHz). The filter at 4.0 MHz appeared to have a high series resistance, but to function correctly when the TES was put in a set point. The yield of the production process, tested at room temperature was found to be 97% for this limited sample, but several LC filters were lost due to
DEN HARTOG et al.: FDM READOUT OF TES DETECTOR ARRAYS WITH BASEBAND FEEDBACK
Fig. 4. Statistics of the difference between target and measured resonance frequency for all the LC filters (top) and the quality factor of the LC filters. Light bars indicate filters on the left chip in Fig. 2 (U2B) and dark bars LC filters on the right chip (U2C).There are 4 frequencies with low Q factors: 1406 and 2761 kHz are test resistors and have by design a Q of 130, 3960 kHz has a failed LC filter due to ESD and in the circuit at 4050 kHz the serial resistance dominates.
ESD problems in the wirebonding process. These problems are now under control. The reader is referred to a companion paper [10] for further details and development. The Q factors and frequency accuracy are summarized in Fig. 4. For the realization of accurate frequency combs, the random frequency offsets are more important than the systematic offsets. After removal of the systematic offset (equal to 2.4% for 5 filters on the U2B chip and 2.1% for 9 filters on the U2C chip), the random offsets in Fig. 4 are equal to 0.3% rms. Thus, the current LC filter chips have already a systematic accuracy of 3 kHz @ 1 MHz, and therefore both the Q factors and frequency accuracy are compliant with IXO specifications. V. CROSSTALK The analysis of crosstalk in our FDM read-out system is still in progress, so the following conclusions have a tentative character. Based on detailed crosstalk measurements such as the one shown in Fig. 5, we observe that the crosstalk signals show two components: a. A ‘fast’ component with a typical amplitude of 0.5 to 2 of the peak value, or of the initial X-ray energy. Its shape is differential with respect to the initiating pulse and its amplitude correlates with the size of error signal. This component is also present on the test resistor, which points to an electrical origin. But there are no obvious correlations with the position in the array or the frequency difference with respect to the initiating pixel, as expected from common impedance. Given its correlation with the error signal, we expect that a SQUID with a larger linear dynamic range should give a strong reduction of this component. of the b. A ‘slow’ component, with an amplitude of initial energy and a shape that follows the initial pulse.
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Fig. 5. Crosstalk measurements for a run in which 6 TES pixels and one resistor are simultaneously locked. The data shown are obtained by using about 2000 X-ray pulses in pixel 13 (top) and averaging the resulting crosstalk pulses in the other pixels, using the trigger on the X-ray pulse in pixel 13 as a time stamp. Note the difference in scale between the top and subsequent panels. The pixels are numbered and the diagram on the right indicates their relative locations in the array. The array support beams run horizontal between rows.
Fig. 6. The blue curve is noise of SQUID and LNA combined, the red curve is the LNA only. The open-loop noise level at the carrier freq. of the test resistor, 2.76 MHz is 172:5 dB V= Hz 53 pA= Hz at the SQUID input for a transimpedance of 44.5 . This number is consistent with the noise at component level (Table II) and confirmed in closed-loop noise measurements.
0
p
p
This component is absent on the test resistor and its amplitude correlates with the relative position in array: this crosstalk component seems to decrease in the vertical direction, as would be expected when it is dissipated in the horizontal support beams. Since its magnitude is the same as thermal crosstalk found in previous DC measurements, the thermal origin of this component seems obvious. For Safari both components are expected to be much smaller: the fast component because the signal frequencies and modulation amplitudes are much smaller, and the slower component
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Fig. 7. Baseline resolutions derived from the integration of NEP spectra ((a)–(c)) and from matched filtering of test pulses ((d)–(f)). These data have been taken for the 125 resistor biased at 2.76 MHz for resp. 1, 7, and 16 pixels operated in BBFB. Each histogram contains about 2000 events.
m
because the bolometer pixel are thermally much better isolated. Concerning IXO, we make the observation that crosstalk levels are approaching the requirements in Table I.
TABLE II PROJECTION OF IMPROVED NOISE PERFORMANCE
VI. NOISE PERFORMANCE The current noise performance is dominated by the combination of the LNA and the SQUID, and by the DAC reconstruction filters. As explained above, the choice for the SQUID was a trade between power dissipation and transfer coefficient, resulting in relative high noise levels for the SQUID and LNA combination. The SQUID noise suffered from an ESD accident during bonding, resulting in the noise behavior illustrated in Fig. 6. Nevertheless, we were able to obtain an essential result for the demonstration of FDM as a viable read-out system: frequency domain multiplexing does not degrade resolution. This is illustrated in Fig. 7, which shows three subsequent measurements of the baseline resolution, both as an NEP and a FWHM, for 1, 7 and 16 pixels simultaneously multiplexed. The baseline resolution stays in all measurements below 6 eV FWHM for 6 keV photons, or equivalently, below a NEP of . These values can be fully understood in terms of component performance as shown in the first row of Table II. Under the right circumstances, the X-ray resolutions should be identical to these values [9]. The current system Technological Readiness Level (TRL) is in the range 3 to 4. This has to be improved to 4 to 5 by the end of 2011. The subsequent rows in Table II show the expected noise figures from component improvements which have already been demonstrated in separate bench tests. One important step forward is the implementation of a new 2-stage PTB or VTT SQUID with larger dynamic range but low power dissipation at 50 mK, and the use of an input transformer to optimize the match between the SQUID and the primary circuit. In combination with an halving of the LNA voltage noise
Per row the steps to improve the noise performance of the FDM read-out system, further described in the text. Last row refers to the expected Safari performance, the two rows before to expected IXO performance. Per column the four main component noise sources, the total current noise and the resulting resolution expressed in eV FWHM for a 6 keV photon, or a NEP.
, this should improve the noise performance by to 0.4 a factor 2. A second important step is to improve the noise of the DAC reconstruction filter, and, to a lesser extent the noise of the anti-aliasing filter of the ADC, to values below the quantization noise. Per row the steps to improve the noise performance of the FDM read-out system, further described in the text. Last row refers to the expected Safari performance, the two rows before to expected IXO performance. Per column the four main component noise sources, the total current noise and the resulting resolution expressed in eV FWHM for a 6 keV photon, or a NEP. In the last three rows of Table II a projection is given of the read-out noise performance expected for the full number of pixels. For Safari several additional, non-critical modifications are necessary to process 160 pixels in a single FPGA: in particular, a lower sampling rate per pixel and a lower effective number of bits on the DAC.
DEN HARTOG et al.: FDM READOUT OF TES DETECTOR ARRAYS WITH BASEBAND FEEDBACK
For both XMS and Safari instruments the required factor 3 current noise improvement can be realized at the component level with the demonstrated improvement of components.
REFERENCES [1] J. van der Kuur et al., “Progress on frequency domain multiplexing for high countrate x-ray microcalorimeters,” in Proc. Low Temperature Detectors LTD 13, B. Cabrera, A. Miller, and B. Young, Eds., 2009, vol. 245, CP1185. [2] T. M. Lanting et al., “Frequency-domain multiplexed readout of transition-edge sensor arrays with a superconducting quantum interference device,” APL, vol. 86, p. 112511, 2005. [3] A. T. Lee, “SQUID readout multiplexers for transition-edge sensor arrays,” Nucl. Instr. and Meth., vol. A559, pp. 786–789, 2006.
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[4] M. Lueker et al., “A frequency domain multiplexed receiver for the south pole telescope,” in Proc. Low Temperature Detectors LTD 13, B. Cabrera, A. Miller, and B. Young, Eds., 2009, vol. 241, CP1185. [5] Y. Takei et al., Supercond. Sci. Technol., vol. 22, p. 114008, 2009. [6] M. Kiviranta, H. Seppä, J. van der Kuur, and P. de Korte, in AIP Conf. Proc., 2002, vol. 605, pp. 295–300. [7] D. Drung, J. Beyer, M. Peters, J.-H. Storm, and T. Schurig, “Novel SQUID current sensors with high linearity at high frequencies,” IEEE Trans. Appl. Supercond., vol. 19, no. 3, pt. 1, pp. 772–777, 2009. [8] R. den Hartog et al., “Baseband feedback for frequency-domain-multiplexed readout of TES x-ray detectors,” in Proc. Low Temperature Detectors LTD 13, B. Cabrera, A. Miller, and B. Young, Eds., 2009, vol. 249, CP1185. [9] L. Gottardi et al., “AC read-out circuits for single pixel characterization tes microcalorimeters and bolometers,” in 2010 Applied Superconductivity Conference. [10] M. P. Bruijn et al., “Superconducting LC filter circuits for frequency division multiplexed readout of TES detectors,” in 2010 Applied Superconductivity Conference.
