Time Interval and Resolution Specifications in Counters

In basic direct-counting architectures, timing resolution is limited by the period of the internal reference oscillator. For instance, a basic 100 MHz reference clock provides a fundamental timing resolution of 10 ns. To break past this limitation, universal counters employ interpolator circuits. These circuits measure the fractional clock cycles between the input trigger events and the next available clock edge.

Using these resolution-enhancement techniques, the Keysight Technologies (Agilent HP) 5334B achieves a 2 ns single-shot time interval resolution. The Keysight Technologies (Agilent HP) 53220A improves this limit to 100 ps single-shot resolution, while the high-performance Stanford Research Systems SR620 achieves a 25 ps rms single-shot resolution. High single-shot resolution is crucial for transient analysis, clock jitter evaluation, and propagation delay measurements where averaging is not possible.

Frequency resolution is typically expressed in digits per second of gate time. Unlike time interval measurements which depend on discrete event edges, frequency counters count multiple cycles over an integrated period. The Keysight Technologies (Agilent HP) 53220A offers up to 12 digits/second of frequency resolution, whereas the Stanford Research Systems SR620 provides up to 11 digits of frequency resolution with a 1-second gate time.

While lengthening the gate time increases frequency resolution, it does not improve the counter's ability to measure single-shot timing variations. For characterizing transient anomalies, phase jitter, or single-cycle drift, the single-shot time interval specification remains the primary metric of measurement accuracy.

Capturing high-resolution data is only the first step in precise timing analysis. Processing these measurements through statistical functions allows engineers to extract jitter characteristics and drift trends. The Stanford Research Systems SR620 features a sample size range of 1 to 1,000,000 and calculates standard deviations, mean, minimum, maximum, and Allan variance directly on the instrument. It also supports histogram generation up to 250 bins and strip chart displays to track jitter trends.

Similarly, the Keysight Technologies (Agilent HP) 53220A includes built-in math and statistical analysis, Allan deviation calculations, and an internal memory capable of holding 1,000,000 readings. For older architectures like the Keysight Technologies (Agilent HP) 5334B, averaging multiple intervals allows users to achieve an average resolution of 200 ps, which is an order of magnitude improvement over its 2 ns single-shot timing limit.