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IEEE Transactions on Applied Superconductivity

On-chip clock oscillator for high precision RSFQ applications

V. K. Kaplunenko

Published Jun 1, 2003• 4 citations
Publisher

Abstract

This paper discusses the possible modifications of the flux-flow oscillator, which is a part of an integrated receiver, for use as a precise clock oscillator in RSFQ circuits. At the price of losing tunability, the flux-flow oscillator can be used at resonance modes. To get higher clock rates, a parallel array of 20 identical under-damped Josephson junctions connected by superconducting micro-strip lines can also be used with an SIS mixer to form a phase-locked loop. The array shows a sharp resonance at 600 GHz, which can be used to make a precise on-chip clock. The linewidth of the emitted power of 150 kHz was estimated from the step differential resistance (free-run stability of 2.5 /spl middot/ 10/sup -7/). To convert frequency down to the desired level the array was matched to an over-damped pair of junctions, which form a precise frequency divider. The divider almost does not add any time jitter, thus the linewidth of 15 kHz can be achieved at 60 GHz. The clock stability can be improved by using an on-chip phase-locked-loop (PLL). The main part of the PLL is a phase detector, which can be based on an RSFQ Josephson sampler combined with an RSFQ fast counter. The suggested circuit probably could not compete with an SIS mixer, but significantly simplifies the circuit, and enables an on-chip RSFQ PLL closed by RSFQ frequency meter and DAC.

DOI
10.1109/tasc.2003.813950

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IEEE Transactions on Applied Superconductivity•Jun 1, 2005•14 citations
This paper discusses the possible modifications of the flux-flow oscillator, which is a part of an integrated receiver, for use as a precise clock oscillator in RSFQ circuits. At the price of losing tunability, the flux-flow oscillator can be used at resonance modes. To get higher clock rates, a parallel array of 20 identical under-damped Josephson junctions connected by superconducting micro-strip lines can also be used with an SIS mixer to form a phase-locked loop. The array shows a sharp resonance at 600 GHz, which can be used to make a precise on-chip clock. The linewidth of the emitted power of 150 kHz was estimated from the step differential resistance (free-run stability of 2.5 /spl middot/ 10/sup -7/). To convert frequency down to the desired level the array was matched to an over-damped pair of junctions, which form a precise frequency divider. The divider almost does not add any time jitter, thus the linewidth of 15 kHz can be achieved at 60 GHz. The clock stability can be improved by using an on-chip phase-locked-loop (PLL). The main part of the PLL is a phase detector, which can be based on an RSFQ Josephson sampler combined with an RSFQ fast counter. The suggested circuit probably could not compete with an SIS mixer, but significantly simplifies the circuit, and enables an on-chip RSFQ PLL closed by RSFQ frequency meter and DAC.
Ring oscillators are the traditional on-chip clock source for RSFQ mixed-signal and digital circuits. Their long-term frequency stability is essential for certain signal processing applications such as the sampling clock for an A/D converter or the output clock of a D/A converter. However conventional bias schemes lead to excessive drift in the frequency. Embedding the ring oscillator in a phase-locked loop (PLL) circuit allows it to inherit the frequency stability of an external reference source. An RSFQ circuit comprised of a ring oscillator, a divide-by-2 20 prescaler and a resynchronizer D flip-flop was fabricated using the standard 1 kA cm -2 niobium foundry service from Hypres. The circuit was stabilized using a low-frequency PLL with a bandwidth of 6 Hz. The output of the phase detector in the PLL was used to produce a feedback current that stabilized the frequency of the ring oscillator at around 8 GHz. The frequency spectrum of the RSFQ scaled oscillator output was measured in open- and closed-loop configurations. Compared to the open-loop phase noise spectrum the closed-loop spectrum showed a decrease of 30 dB of the oscillator phase noise at an offset of 1 Hz from the divided-down carrier frequency of 4.1 kHz. This agreed with the classical phase-locked loop model for the system. The long-term frequency drift of the RSFQ clock was determined by the stability of the oven-controlled quartz oscillator used as the reference in the PLL.
The Josephson flux flow oscillator (FFO) has proven to be one of the best on-chip local oscillators for heterodyne detection in integrated sub-mm receivers based on SIS mixers. Nb–AlO x –Nb FFOs have been successfully tested from about 120 to 700 GHz (gap frequency of Nb) providing enough power to pump an SIS mixer (about 1 μW at 450 GHz). Both the frequency and the power of the FFO can be dc-tuned. Extensive measurements of the dependence of the free-running FFO linewidth on the differential resistances associated with both the bias current and the control-line current (applied magnetic field) have been performed. The FFO line is Lorentzian both in the resonant regime, on Fiske steps (FSs), and on the flux flow step (FFS). This indicates that internal wide-band noise is dominant. A phenomenological noise model can account for the FFO linewidth dependence on experimental parameters. The narrow free-running FFO linewidth achieved, in combination with the construction of a wide-band phase-locked loop (PLL) system, enable us to phase lock the FFO in the frequency range 490–712 GHz where continuous frequency tuning is possible.
We present a technique for linewidth measurement and phase-locking of Josephson oscillators using digital rapid single-flux-quantum (RSFQ) circuits. The oscillator consists of a resistively shunted 6 /spl mu/m/spl times/6 /spl mu/m Nb/AlO/sub x//Nb Josephson tunnel junction that is integrated with RSFQ input and output circuits. A cascade of RSFQ T flip-flops is used to directly monitor the output of the Josephson oscillator. Spectral characteristics have been measured directly for oscillator frequencies ranging from 10-50 GHz. The linewidth can be reduced by over 100 times by phase-locking the oscillator to an RSFQ pulse train generated by an external sinusoidal signal. These Josephson oscillators can be used as on-chip stable high frequency clocks for RSFQ circuits.
The combination of narrow linewidth and wide band tunability makes the Josephson flux flow oscillator (FFO) a perfect on-chip local oscillator for integrated sub-mm wave receivers for, e.g., spectral radio astronomy. The feasibility of phase locking the FFO to an external reference oscillator is demonstrated experimentally. A FFO linewidth as low as 1 Hz (determined by the resolution bandwidth of the spectrum analyzer) has been measured in the frequency range 270–440 GHz relative to a reference oscillator. This linewidth is far below the fundamental level given by shot and thermal noise of the free-running tunnel junction. The results of residual FFO phase noise measurements are also presented. Finally, we propose a single-chip fully superconductive receiver with two superconductor–insulator–superconductor mixers and an integrated phase-locked loop.
A Josephson flux flow oscillator (FFO) is the most developed superconducting local oscillator for integration with an SIS mixer in a single-chip submm-wave receiver. Recently, using a new FFO design, a free-running linewidth /spl les/10 MHz has been measured in the frequency range up to 712 GHz, limited only by the gap frequency of Nb. This enabled us to phase lock the FFO in the frequency range 500-712 GHz where continuous frequency tuning is possible; resulting in an absolute FFO phase noise as low as -80 dBc at 707 GHz. Comprehensive measurements of the FFO radiation linewidth have been performed using an integrated SIS harmonic mixer. The influence of FFO parameters on radiation linewidth, particularly the effect of the differential resistances associated both with the bias current and the applied magnetic field has been studied in order to further optimize the FFO design. A new approach with a self-shielded FFO has been developed and experimentally tested.
We have developed clock sources for rapid single flux quantum (RSFQ) digital circuits using high-quality long Josephson junction (LJJ) resonant oscillators of annular geometry. Being a topologically closed system and unperturbed by reflections from boundaries and collisions among the fluxons (flux quanta), the annular LJJ oscillator has demonstrated a high quality factor. The novel design of an annular junction clock that allows easier interface with RSFQ circuitry has been realized. Magnetic fluxons are inserted into an annular LJJ by local injection of current into the control line of the junction. This technique resolves the long-standing problem of achieving superior stability of the fluxon state for application as a clock source. The linewidth of the phase-locked clock source is measured as low as 24 Hz relative to a reference oscillator at 40 GHz. The cycle-to-cycle jitter was measured to be 9 fs at a frequency of 36 GHz, or 0.04% of the clock period which is in good agreement with theoretical estimation.