Comparator Measures Frequency And Phase

June 29, 2004
This precision frequency/phase comparator now incorporates a high-speed time-interval counter with generous memory and battery backup for demanding measurements.

Measurement of fractional differences in frequency and phase is often useful for evaluating sets of oscillators, precise lengths of cables, and other sets of high-frequency components. The model A7-X is an improved version of a proven frequency/phase comparator (the A7) from Quartzlock, with internal time-interval-counter card for handling measurements on devices with frequencies other than 5 or 10 MHz. The frequency/phase comparator can be used for testing minute differences in phase and frequency between frequency multipliers, dividers, standards, and other components.

Basically, the A7-X (see figure) employs a moving coil meter for rapid, unambiguous display of fractional frequency difference or relative phase difference between two sources. The comparator also provides outputs for connection to an external time-interval counter for high-resolution analysis of a source's time-domain stability (to 100 ps).

The major improvement in the A7-X is the inclusion of an internal model A7-X TIC time-interval-counter card, which eliminates the need for an external counter. Supported by Windows-based software, the counter's resolution is around 50 fs for a single measurement with filter off, and around 15 fs with 10-Hz filter.

The counter acquires data at a rate of 1000 readings/s, with data analysis performed by the Stable 32 PC software. Data can be saved in the form of text files for further analysis. The rapid data-acquisition rate supports phase-noise calculations to a Fourier frequency of 500 Hz from the carrier. By extending the data acquisition to several thousand seconds, phase noise can be analyzed within 1 mHz of the carrier.

The frequency/phase comparator has two modes of operation: frequency-measurement mode and phase-difference mode. In the first, the moving coil meter indicates fractional frequency difference and the counter operates like a frequency counter, with full-scale meter ranges from ±10−7 to ±10−12. The root-mean-square (RMS) resolution is better than 5 × 1014 for a 1-s gate. In the second mode, the meter reads the relative phase difference between the reference and the measurement inputs, with a full-scale range selectable between ±10 µs and ±10 ps.

Two versions of the A7-X frequency/phase comparator are currently available: the analog model A7-A offers 1 × 10−13 measurement resolution while the metrology grade model A7-MX) provides resolution to 1 × 10−16. Quartzlock UK Ltd., Gothic, Plymouth Rd., Totnes, Devon TQ9 5LH, England; (44)(0)1803 862 062, FAX: (44)(0)1803 867 962, e-mail: [email protected], Internet:

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