Unit 4 — Calibration Procedure and Calculations

TM-CAL-002 — Open Handout TM Chapters: Chapter 5, Appendix A ELOs: Execute calibration procedure; compute error and determine pass/fail Estimated time: 30 minutes

Step 1: Read the TM

Open TM-CAL-002. Read Chapter 5 and Appendix A completely.

Then come back here.

Chapter 5 — Calibration Procedure

CAUTION: Do not use 1PPS for calibration until GPS lock is confirmed. Many modules continue to output 1PPS without lock; unlocked 1PPS can be off by milliseconds.

5-1. FREQUENCY COUNTER GATE CALIBRATION

  1. Set frequency counter to 1 Hz external gate input or 1 s internal gate.
  2. Apply 1PPS to the counter's external trigger or gate input.
  3. Measure a 10 MHz oscillator with 10 s gate time. The count should be exactly 100,000,000 ± 1 count for a perfect oscillator.
  4. Record: counts = fosc / f1PPS × gate time.
  5. Error in ppm = (actual counts − expected counts) / (expected counts) × 106.
  6. Example: gate = 10 s, expected 100,000,000 counts, actual 100,000,210 counts → +2.1 ppm error.

5-2. OSCILLOSCOPE TIMEBASE CALIBRATION

  1. Connect 1PPS to oscilloscope input. Set trigger to rising edge.
  2. Set timebase to 200 ms/div (2 seconds full screen for a 10-div display).
  3. The 1PPS pulse should appear at exactly 1.000000 s intervals.
  4. Measure the displayed period using cursors or automated measurement.
  5. Compare displayed period to 1.000000 s. Error >0.5% indicates timebase drift.
  6. For 2–div cursor placement: set cursors at 0 ms and 1000 ms displayed. Count actual division spacing. Error = (measured − 1000 ms) / 1000 ms × 100%.

Appendix A — Formulas

Frequency error from 1PPS gate count
errorppm = (Nactual − Nexpected) / Nexpected × 106

Expected counts (10 MHz oscillator, 10 s gate)
Nexpected = fosc × tgate = 10×106 × 10 = 100,000,000

1PPS accuracy in seconds per day
drifts/day = accuracyns × 10−9 (this is per-pulse jitter, not accumulation)

Key Formulas Summary

  • errorppm = (Nactual − Nexpected) / Nexpected × 106
  • Nexpected = fosc × tgate = 10×106 × 10 = 100,000,000
  • drifts/day = accuracyns × 10−9 (this is per-pulse jitter, not accumulation)

The Calibration Procedure

Chapter 5 specifies 12 calibration steps.

Calibration is a comparison: you apply a known reference value to the instrument under test and record what the instrument reads. The difference is the error. You then either: - Adjust the instrument until the error is within the acceptance criterion, or - Record the error as a correction factor to apply to future readings

An error criterion found in Chapter 5: 0.5%. Confirm the exact criterion in the TM.

Practice Problems

Work these before checking answers.

P4-1. The reference value is 10.000 V. Your instrument reads 10.043 V. (a) What is the error in volts? (b) What is the error as a percentage of the reference value? Show your work.

P4-2. Using the formula for % error: error% = (measured − reference) / reference × 100 Apply it to: reference = 100.0 kHz, measured = 99,985 Hz. (a) Error in Hz. (b) Error in %. (c) Error in ppm.

P4-3. The acceptance criterion in the TM is ±1%. Your measurement gives an error of +0.8%. Does it pass? State your reasoning.

Practice Problem Answers

P4-1. (a) 10.043 − 10.000 = +0.043 V (b) 0.043 / 10.000 × 100 = +0.43%

P4-2. (a) 99,985 − 100,000 = −15 Hz (b) −15 / 100,000 × 100 = −0.015% (c) −15 / 100,000 × 1,000,000 = −150 ppm

P4-3. +0.8% is within ±1%. PASS. State: "error = +0.8%; criterion = ±1%; result = PASS." Always cite the TM section for the criterion.

Self-Check Questions

SC4-1. How many steps does Chapter 5 specify for the calibration procedure?

SC4-2. What reference value(s) does Chapter 5 apply to the instrument under test?

SC4-3. State the calibration acceptance criterion from the TM. Cite the section.

SC4-4. Write the error formula from Appendix A. Include units.

SC4-5. If the instrument reads 2.3% high, is the error positive or negative? What does a positive error indicate?

Answer Key

SC4-1. Count the numbered steps in Chapter 5. (TM Ch. 5)

SC4-2. See Chapter 5 for the reference values applied. These are the known-good inputs used to check the instrument. (TM §5-1)

SC4-3. See TM Chapter 5 or Chapter 7. Copy the criterion exactly with units and section number.

SC4-4. See Appendix A. Write it exactly as shown.

SC4-5. Positive (instrument reads higher than reference). A positive error means the instrument over-reads — it reports a value higher than the true value. (TM App. A)

Checkpoint

Before proceeding, you must be able to: - State the calibration acceptance criterion without looking - Write the error formula from memory - Work a % error and ppm calculation correctly

→ Proceed to Unit 5