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 × 10^{14} 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: quartzlock@quartzlock.com, Internet: www.quartzlock.com.**