Abstract Body

Our research group developed several types of SFQ digital-to-analog converters (DACs) of SFQ pulse-frequency modulation (PFM), where the SFQ pulse repetition frequency was modulated to generate various values of quantum voltages [1,2]. Since those DACs were intended to be applied to the AC voltage standards, higher resolution was given priority over the synthesized waveform frequencies. One of our DACs fabricated using a 2.5 kA/cm² Nb/AlO_x/Nb integration technology (referred to as the AIST-STP2) synthesized a 47 kHz sinusoidal voltage waveform of 9-bit resolution [1].

In this work, we increased the synthesized waveform frequency up to 1 GHz at the sacrifice of resolution down to 4 bits. In addition, to feed the input digital signal of 16 GHz, we designed an on-chip digital signal generator for a sinusoidal waveform. That is to say, we designed an SFQ-based microwave generator. The configuration is based on a scheme of the direct digital synthesizer (DDS) [3] which is shown in Fig. 1(a). It is composed of a phase register (PR), a phase/amplitude converter (PAC), and a 4-bit DAC. For the 4-bit DAC, we employed our previously-developed DAC of the “sum of selected bit sequence” configuration [4,5].

For circuit designing, we used the RSFQ digital cell library “CONNECT” updated for the 10 kA/cm² Nb/AlO_x/Nb integration process (referred to as the AIST-HSTP) [6]. The CAD layout of the microwave generator excluding a bipolar output voltage multiplier is shown in Fig. 1(b). The total number of Nb/AlO_x/Nb Josephson junctions was 1635. Numerical circuit operation is shown in Fig. 1(c). The number of SFQ pulses for each PR clock period was changed as the sequence of (0, 3, 5, 6, 7, 6, 5, 3, 0, -3, -5, -6, -7, -6, -3, 0), which corresponded to a 1 GHz sinusoidal voltage waveform.

References

[1] Y. Mizugaki, et al., Electronics Lett. 50 (2014) 1637–1639.
[2] Y. Mizugaki, et al., IEICE Electronics Express 13 (2016) 20160242.
[3] V. S. Reinhardt, Proc. IEEE Int. Frequency Cont. Symp., Salt Lake City, 1993, pp. 230–241.
[4] Y. Mizugaki, et al., IEEE Trans. Appl. Supercond. 21 (2011) 3604.
[5] Y. Mizugaki, et al., IEICE Electronics Express 19 (2022) 20220194.
[6] N. Takeuchi, et al., Supercond. Sci. Technol. 30 (2017) 035002.

Acknowledgment

This work was partly supported by JSPS KAKENHI Grant Number JP20H02201. It was also supported through the activities of VDEC, The University of Tokyo, in collaboration with Cadence Design Systems.

Figure 1. (a) Configuration of a microwave generator based on a scheme of the direct digital synthesizer. (b) CAD layout of a designed microwave generator. (c) Numerical results for the circuit operation. The number of SFQ pulses for each PR clock period was changed as the sequence of (0, 3, 5, 6, 7, 6, 5, 3, 0, -3, -5, -6, -7, -6, -3, 0), which corresponded to a 1 GHz sinusoidal voltage waveform.

Keywords: digital-to-analog converter (DAC), RSFQ, direct digital synthesizer (DDS)