Dale Henderson

Evaluation of the primary frequency standard NPL-CsF1

A new caesium fountain frequency standard (NPL-CsF1) at the National Physical Laboratory is described. Procedures for evaluation of the systematic frequency shifts are presented. The NPL-CsF1 has a short-term stability σy(τ) = 1.4 × 10−13τ−1/2, predominantly due to the local oscillator phase noise. The accuracy of 1 part in 1015 is limited by the uncertainty of the frequency shift due to collisions between cold atoms.

Development of a 60 Hz Power Standard Using SNS Programmable Josephson Voltage Standards

We are implementing a new standard for 60 Hz power measurements based on precision sinusoidal reference voltages from two independent programmable Josephson voltage standards (PJVS): one for voltage and one for current. The National Institute of Standards and Technology PJVS systems use series arrays of Josephson junctions to produce accurate quantum-based DC voltages. Using stepwise-approximation synthesis, the PJVS systems produce sinewaves with precisely calculable RMS voltage and spectral content. We present measurements and calculations that elucidate the sources of error in the RMS voltage that are intrinsic to the digital-synthesis technique and that are due to the finite rise times and transients that occur when switching between the discrete voltages. Our goal is to reduce all error sources and uncertainty contributions from the PJVS synthesized waveforms to a few parts in 10 7 so that the overall uncertainty in the AC-power standard is a few parts in 106

Achieving Sub-100-ns Switching of Programmable Josephson Arrays

The accuracy of voltage waveforms, generated using programmable Josephson junction arrays, is limited by the time taken to switch between quantized levels. Relevant parameters are the time constants within the array chip, the characteristics of the cables, and the bandwidth of the bias electronics. A drive system for a 1-V SINIS array with 8192 junctions has been optimized, and a cable scheme has been devised which compensates for cable reflections. The measurements of the arrays temporal response, made with this cable system, are compared to an RC model, both with 3- and 7-ns single time constants

Frequency stability and phase noise of a pair of X-band cryogenic sapphire oscillators

Oscillators based on cryogenic sapphire resonators can supply the levels of microwave phase noise and frequency stability required for advanced time-and-frequency applications. NPL has realized two such oscillators of identical design, which operate at 9.204 GHz and incorporate both Pound-loop and power stabilization. Running both simultaneously, and comparing their outputs both to each other and to H-maser-referenced frequency sources, the phase noise and absolute frequency stability of these oscillators have been measured for the first time, based on both FFT-spectrum analyser and frequency-counter data. We report a double-sided phase noise of −87.5 dBc at 1 Hz, scaling as 1/f3, and a modified Allan deviation of less than 5 × 10−15 at 1 s; the fractional frequency drifts of the two oscillators with respect to the H-maser were −2.9 × 10−11 and −6.0 × 10−12 per day, respectively. Scope for further improvement is assessed.

Essen and the National Physical Laboratory's atomic clock

To commemorate the fiftieth anniversary of the development of the first atomic frequency standard, we present some notes about the work of Louis Essen at the National Physical Laboratory. In addition, we publish below some personal recollections of Essen on his work, which have previously been available only on the Internet (http://www.btinternet.com/~time.lord/TheAtomicClock.htm).

Application of a Josephson quantum voltage source to the measurement of microsecond timescale settling time on the Agilent 3458A 8

The voltage waveform measurement characteristics due to finite settling time on the microsecond timescale are investigated for the Agilent 3458A 8 digit voltmeter in DCV mode. A quantum-accurate voltage synthesizer based on the Josephson effect produces a change in test voltage in less than 1 μs and the voltage reported by the voltmeter is measured as a function of two timing parameters, aperture delay and duration. A model is presented and the level of agreement is discussed in the context of optimizing the configuration of the instrument for both high measurement throughput and accuracy.

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In this paper the extension of the AC Quantum Voltmeter is described with the aim of measuring waveforms at audio frequencies. A difference amplifier has been developed to improve the accuracy over the mHz to kHz range.

digit voltmeter

The optoelectronic coupling for a pulse-driven Josephson junction array (JJA) for use as a quantum voltage synthesizer has been designed. This system uses optoelectronic components to drive a group of JJAs in parallel. Since the drive system is electrically isolated from the JJAs, they can be connected in series, leading to a higher output voltage. A thin film circuit, including interdigitated capacitors to couple a photodiode to the JJAs has been designed and tested and shown to meet the design goals.

Operation of the AC Quantum Voltmeter at kHz frequency

This paper describes the development of a quantum waveform synthesizer operating at a sampling frequency of 100 kHz with the aim of generating sine waves directly traceable to the Josesphson quantum voltage standard. The quantum reference is based on arrays of 65 536 Josephson junctions, fabricated using Nb/TiN/Nb technology and operating at a bias frequency of 16 GHz corresponding to an output voltage of approximately 2.2 V. The synthesizer contains a specially constructed difference amplifier, digital source and anti-imaging filter. The type A uncertainty is estimated to be 40 nV for a 1s observation time.

AN OPTOELECTRONIC COUPLING FOR PULSE-DRIVEN JOSEPHSON JUNCTION ARRAYS

In response to the comment by Jefferts and Levi on page L11 of this issue, we review the uncertainty budget quoted in our earlier paper on the evaluation of the caesium primary frequency standard NPL-CsF1. As a result of this re-evaluation, performed in the light of new analyses and experimental data on the microwave leakage shift, we expand the fractional frequency uncertainty for this standard to 2.0 × 10−15 (1σ).