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IEICE Transactions on Electronics

Design and Demonstration of a Single-Flux-Quantum Multi-Stop Time-to-Digital Converter for Time-of-Flight Mass Spectrometry

Kyosuke Sano, Yuki Yamanashi, Nobuyuki Yoshikawa

Published Jan 1, 2014• 10 citations
Publisher

Abstract

We have been developing a superconducting time-of-flight mass spectrometry (TOF-MS) system, which utilizes a superconductive strip ion detector (SSID) and a single-flux-quantum (SFQ) multi-stop time-to-digital converter (TDC). The SFQ multi-stop TDC can measure the time intervals between multiple input signals and directly convert them into binary data. In this study, we designed and implemented 24-bit SFQ multi-stop TDCs with a 3×24-bit FIFO buffer using the AIST Nb standard process (STP2), whose time resolution and dynamic range are 100ps and 1.6ms, respectively. The timing jitter of the TDC was investigated by comparing two types of TDCs: one uses an on-chip SFQ clock generator (CG) and the other uses a microwave oscillator at room temperature. We confirmed the correct operation of both TDCs and evaluated their timing jitter. The experimentally-obtained timing jitter is about 40ns and 700ps for the TDCs with and without the on-chip SFQ CG, respectively, for the measured time interval of 50µs, which linearly increases with increase of the measured time interval.

DOI
10.1587/transele.e97.c.182

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We have been developing a superconducting time-of-flight mass spectrometry (TOF-MS) system, which utilizes a superconductive strip ion detector (SSID) and a single-flux-quantum (SFQ) multi-stop time-to-digital converter (TDC). The SFQ multi-stop TDC can measure the time intervals between multiple input signals and directly convert them into binary data. In this study, we designed and implemented 24-bit SFQ multi-stop TDCs with a 3×24-bit FIFO buffer using the AIST Nb standard process (STP2), whose time resolution and dynamic range are 100ps and 1.6ms, respectively. The timing jitter of the TDC was investigated by comparing two types of TDCs: one uses an on-chip SFQ clock generator (CG) and the other uses a microwave oscillator at room temperature. We confirmed the correct operation of both TDCs and evaluated their timing jitter. The experimentally-obtained timing jitter is about 40ns and 700ps for the TDCs with and without the on-chip SFQ CG, respectively, for the measured time interval of 50µs, which linearly increases with increase of the measured time interval.
We have been developing superconducting time-of-flight mass spectrometry systems that utilize superconducting strip particle detectors and single-flux-quantum (SFQ) time-to-digital converters (TDCs). We previously demonstrated a 24-bit SFQ TDC with a 3 × 24-bit first-in first-out (FIFO) buffer using the AIST Nb standard process (STP2) with a time resolution of 100 ps. In this study, we improved the SFQ TDC by using a current recycling technique. The technique enables us to increase the FIFO buffer capacity without increasing the total supply current, and the result is an improvement of ion count rates. We designed and demonstrated the SFQ TDC with a 12 × 24-bit FIFO buffer, whose total supply current was reduced by 71% with the current recycling technique.
We have been developing a time-to-digital converter (TDC) using single-flux-quantum (SFQ) circuits in order to realize an imaging system based on a superconducting strip single-photon detector (SSPD). In this system, the absorption position of a single photon in a meander-shaped SSPD is determined by measuring the delay time of the detected signal propagating through an SSPD delay line. Increasing the time resolution of the TDC is the primary concern for improving the spatial resolution of the system. Here we focused on the double-oscillator SFQ TDC, where the difference of oscillation periods between two SFQ ring oscillators determines the time resolution. In the previous study, a time resolution of about 4 ps was demonstrated by using the Nb standard Josephson integrated circuit (IC) process with a critical current density of 2.5 kA/cm2. In this study, we improved the time resolution by adopting the Nb advanced Josephson IC process with a critical current density of 10 kA/cm2and by optimizing the circuit parameters of the coincidence detector. The histogram and the linearity of the delay measurements show that the time resolution is improved to up to 2 ps.
近年、飛行時間型質量分析法 (TOF-MS, Time-OfFlight Mass Spectrometry) において、SFQ 回路を用 いた飛行時間測定回路 (TDC, Time-to-Digital Converter) で構成される高分解能 TOF-MS システ ムの開発が行われている [1]。しかし、TDC の測 定で得られるヒストグラムの半値幅が想定よりも 大きくなってしまうことが問題であった。この原 因として考えられているのは、TDC 内の Clock Generator (CG)を構成している SFQ リングオシレ ータの jitterが挙げられる。そこで我々は、SFQ リ ングオシレータの jitterを低減させ、オンチップ上 での TDCの動作の安定化を目指している。SFQ 回 路における jitterは、Josephson Junction (JJ)のスイ ッチング時間のばらつきであり、JJ を流れるバイ アス電流に重畳する熱雑音に起因する [2]。今回、 パルス間斥力を用いた jitter の低減効果について、 シミュレーションにより検討したので報告する。 Fig. 1 にシミュレーションに用いた回路を示す が、リング内に磁気結合によって磁束を複数箇所 から印加することで、複数の SFQ パルスを発生さ せている。Jsim によって得られた磁束の印加点数 と周波数の関係性を Fig. 2 に示す。計算において 2.5kA/cm 2 Nbプロセスを仮定した。通常、リング 内の磁束印加点数が増加するとリング内を伝搬す る SFQ 数が増加するため、発振周波数も比例して 増加すると考えられる。しかしながら、Fig. 2より リング内の磁束印加点数と発振周波数は比例関係 にないことが分かる。我々は、この関係性からリ ング内にパルス間斥力が働いていると考え、jitter との関係性についてシミュレーションを行った。 4.2Kの熱雑音を考慮したシミュレーションによ って得られた磁束の印加数と jitterの関係性を Fig. 3 に示す。図における jitter とは、リングオシレー タ内の JJがスイッチしてから次にスイッチするま での時間間隔の標準偏差を表している。今回、JJ1 接合当たりの jitterに規格化したものを縦軸とした。 Fig. 3 より、磁束の印加数が増加することによって JJ1 接合当たりの jitter が減少していることが確認 出来る。今後は、アンシャント JJ を用いることで 更なる jitterの低減を目指す。 Fig. 1. A test circuit to calculate the timing jitter of SFQ ring oscillators.
In order to control the state of qubits by a microwave pulse, the irradiation time and the amplitude have to be controlled precisely. We have developed a single-flux-quantum (SFQ) microwave chopper for high-speed switching of microwave pulses. The proposed chopper is composed of a DC/SFQ convertor, an SFQ switch, a PTL driver, and a superconducting low-pass filter (LPF). The chopper converts an input microwave, which is generated by an external microwave generator at the room temperature, into microwave pulses by using start/stop SFQ control signals. We designed and implemented a microwave chopper module, which can be attached to dilution refrigerators. SFQ chips were fabricated using the ISTEC 2.5 kA/cm2 Nb process. We tested the microwave chopper module at 4.2 K, and demonstrated that a 5-GHz microwave whose amplitude ranging from 0 μV to 150 μV can be chopped by the SFQ control signals.
A new generation of the Josephson digital circuits-rapid single flux quantum (RSFQ) devices-based on overdamped externally shunted high quality Nb-AlO x -Nb tunnel junctions have been designed, fabricated and experimentally investigated. All-refractory material 11-layer 5 mu m integrated technology has been successfully employed for the preparation of the RSFQ microcircuits. The T flip-flop has been demonstrated to operate properly up to clock frequencies of 100 GHz at 25% margins. The proper operation of the buffer stage for a one-directional SFQ pulse transmission at clock frequencies up to 125 GHz has been achieved. The possibilities to convert the information from the usual DC form to SFQ and vice versa have been demonstrated. An output signal in an SFQ/DC converter as high as 220 mu V has been measured. A new counter type A/D converter which includes a comparator and reversible binary counter with DC outputs has been fabricated and tested. The proposed ADC seems to be very promising for high-accuracy measurement of medium-bandwidth signals. The possibilities of creating a number of SFQ devices with outstanding parameters are discussed.
We have designed, fabricated, and successfully tested a Time-to-Digital converter (TDC). The TDC circuit consists of superconductive counters based on toggle flip-flops with destructive readout, shift registers, parallel-to-serial converters, and pulse detectors. The value of the counter is latched while the counter is operating at full speed. The TDC resolution is directly determined by the maximum counter speed, or 50 ps for an available 20 GHz clock rate. The circuits are implemented in HYPRES' standard Nb process with a critical current density of 1.0 kA/cm/sup 2/ using HYPRES' library of standard RSFQ cells.