Atomic frequency standards, high-stability oscillators and GPS-disciplined oscillators are widely used in systems that require synchronisation or accurate and stable frequency references, such as telecommunications networks, navigation satellite systems and some broadcast applications. The manufacturer may provide standard specifications, but how can the user of precision frequency standards and clocks be confident that their behaviour is fit for their purpose?

The NPL characterisation service evaluates the frequency and time output signals of oscillators, frequency standards or GPS disciplined oscillators.  To be suitable for characterisation by the service, the device under test must produce either or both of the following output signals:

• a 5 MHz or 10 MHz sine or square wave output
• a 1 pulse per second (1PPS) output

The procedure is described as characterisation rather than calibration because the output of the device is measured and analysed, but not adjusted to the nominal value.The characterisation measurements are normally carried out with the device located in one of NPL’s temperature-controlled laboratories, although measurements against a portable caesium clock taken to the customer’s site may be feasible under certain circumstances. The device will if possible be set up in the laboratory and operated for 24 hours prior to the start of the measurements, to allow it to stabilise and to verify correct operation.

A 5 MHz or 10 MHz frequency signal is typically measured using a high-performance phase comparator against a reference frequency derived from one of the active hydrogen maser atomic frequency standards at NPL, which are traceable to the UK national time scale UTC(NPL).

A 1PPS output signal is similarly measured against a reference signal with a known offset from UTC(NPL), using a time interval counter. In a typical characterisation, the device under test will be measured continuously over a 10-day period, with measurements taken at intervals of 1 second or longer.

The measurement results are analysed using standard time domain statistics such as Allan deviation (ADEV), modified Allan deviation (MDEV) and time deviation (TDEV), according to the customer’s requirements. A 10-day data sequence allows these variances to be calculated accurately out to an averaging time of 1 day, and also allows any diurnal variations to be observed.

The results of the characterisation are presented in a report, usually within 3 weeks of the completion of the measurements. A formal calibration certificate is not usually issued because of the nature of the devices covered by the service.

If required, the results can be processing further using the BIPM Circular T to provide traceability to UTC. However, because Circular T is only published monthly, this process may introduce a delay in the submission of the report, or alternatively the offset from UTC may be reported later in an addendum to the report.

The NPL oscillator characterisation service is UKAS Accredited.

Contact

Customer Service tel: +44 20 8943 6796
E-mail: timefreq_enquiries@npl.co.uk