Testing and Measuring Clock Accuracy: ppm, Drift and Logging Methods

Designing a clock system is only half the job. To ensure reliable performance, it is important to measure and verify its accuracy over time. This involves understanding drift, calculating ppm (parts per million) and using proper logging methods.

This article explains how to evaluate the accuracy of your clock system in a practical and meaningful way.

What Does Clock Accuracy Mean?

Clock accuracy describes how closely a clock matches the true reference time.

• Perfect clock → no deviation over time
• Real clocks → always have some error

Accuracy is typically expressed in ppm.

Understanding ppm (Parts Per Million)

• 1 ppm = 1 microsecond per second
• ≈ 0.0864 seconds per day

Example:

• 10 ppm → ~0.86 seconds per day
• 50 ppm → ~4.3 seconds per day

Drift vs Offset

Offset

• Instant difference from correct time

Drift

• Rate at which error accumulates over time

Drift is the more important parameter for long-term accuracy.

Reference Time Sources

To measure accuracy, you need a reliable reference:

• NTP (internet time)
• GPS (very high accuracy)
• Atomic clock receiver

The better the reference, the more meaningful the measurement.

Simple Measurement Method

1. Set your clock to correct time
2. Wait a known period (e.g. 24 hours or several days)
3. Compare with reference time

Example:

• Clock loses 3 seconds in 3 days

Calculating Drift

Drift (ppm) can be calculated as:

• ppm = (time error / elapsed time) × 1,000,000

Example:

• 3 seconds error over 3 days
• 3 / (3 × 86400) × 1,000,000 ≈ 11.6 ppm

Long-Term Measurement

Short tests may not reveal true performance. Better results are obtained by:

• Measuring over several days or weeks
• Recording multiple data points

This averages out temporary variations.

Logging Methods

Manual Logging

• Record time difference periodically
• Simple but less precise

Automated Logging

• Use microcontroller to log timestamps
• Compare with reference automatically

Data Logging Example

• Store timestamp every hour
• Compare with NTP or GPS

This allows detailed analysis.

Graphing Drift

Plotting drift over time reveals patterns:

• Linear drift → stable oscillator error
• Non-linear drift → temperature effects

This helps identify underlying issues.

Temperature Testing

• Test at different temperatures
• Observe drift changes

This is especially important for crystal-based RTCs.

Comparing Clock Types

Improving Accuracy Based on Measurements

• Apply software compensation
• Adjust DS3231 aging register
• Add periodic synchronization

Common Mistakes

• Measuring over too short time
• Using inaccurate reference time
• Ignoring temperature effects

Best Practices

• Measure over long periods
• Use high-quality reference
• Log and analyze data

Conclusion

Measuring clock accuracy is essential for understanding and improving system performance. By calculating drift and using proper logging methods, you can identify issues and apply effective corrections.

With careful measurement and calibration, even low-cost systems can achieve surprisingly high accuracy.