UNCLASSIFIED
TM-INST-034
WWV/WWVH AND RADIO STANDARD CALIBRATION
NIST Time Signal Frequency Reference Calibration Using WWV, WWVH, and CHU
Prepared by: Mervyn Martin, KO6NNH  •  Merced, California  •  26 May 2026
Amateur Radio / Electronics — Not for commercial calibration use

Overview

This method uses free, over-the-air time standard broadcasts (WWV, CHU, MSF, DCF77, etc.) to calibrate the TinySA's 30MHz reference with 0.1-1 ppm accuracy.

Why Radio Time Standards Work

  • Transmitted from atomic clock-controlled transmitters
  • Frequencies are exact within ±1×10⁻¹² (0.00001 ppm)
  • Free, continuous broadcasts
  • No special equipment needed (just a receiver)
  • Worldwide coverage from multiple stations

What You'll Achieve

Frequency Error: < 3 Hz @ 30MHz (0.1 ppm)
                Accuracy: Limited by propagation and receiver quality
                Cost: $0-20 (if you have receiver/SDR)
                Time Required: 30-60 minutes

Available Time Standard Broadcasts

Worldwide Time Standard Stations

Station Country Frequencies Power Coverage
WWV USA (Colorado) 2.5, 5, 10, 15, 20, 25 MHz 2.5-10 kW North America, Pacific
WWVH USA (Hawaii) 2.5, 5, 10, 15 MHz 5-10 kW Pacific, Asia
CHU Canada 3.330, 7.850, 14.670 MHz 3-10 kW North America
MSF UK 60 kHz 60 kW Europe
DCF77 Germany 77.5 kHz 50 kW Europe
JJY Japan 40/60 kHz 50 kW Japan, East Asia
BPC China 68.5 kHz 90 kW China, Asia
RBU Russia 66.66 kHz 10 kW Russia

Note: WWV at 10 MHz was discontinued in 2023 to save costs. Only 2.5, 5, 15, 20, and 25 MHz remain operational.

Best Stations for Calibration

North America: - CHU (Canada) - 7.850 MHz or 14.670 MHz (best) - WWV - 5 MHz or 15 MHz - WWVH - 5 MHz or 15 MHz

Europe: - DCF77 - 77.5 kHz (strongest in Europe) - MSF - 60 kHz (UK and Western Europe)

Asia: - JJY - 40 or 60 kHz - BPC - 68.5 kHz

Worldwide: - Any station you can receive clearly - HF stations (3-20 MHz) work best for long distance - LF stations (60-77 kHz) work best locally

Required Materials

Minimum Setup (Free if you have equipment)

Item Cost Notes
Shortwave receiver $0-50 Or any HF-capable radio
OR RTL-SDR dongle $25-35 USB SDR receiver
Wire antenna $0-5 Random wire, 20-50 feet
TinySA $100-150 Owner supplied
3.5mm audio cable $2 Receiver to TinySA
Item Cost Purpose
RTL-SDR v3 $30 Clean, stable reception
Upconverter (for LF) $20 Receive 60-77 kHz stations
Antenna wire $5 50-100 ft wire
Audio interface $10 Clean signal connection

Tools

  • Computer (for SDR software)
  • Basic audio editing software (Audacity, free)
  • Calculator or spreadsheet

Theory of Operation

How Time Standard Broadcasts Work

  1. Master Clock: Atomic clock at transmitter site (cesium or hydrogen maser)
  2. Frequency Synthesis: Atomic clock generates exact carrier frequency
  3. Transmission: High-power transmitter broadcasts on precise frequency
  4. Reception: You receive the signal and use it as frequency reference

Carrier Frequency as Reference

The carrier frequency itself is the atomic clock reference: - WWV 5 MHz = exactly 5,000,000.00 Hz - CHU 7.850 MHz = exactly 7,850,000.00 Hz - DCF77 77.5 kHz = exactly 77,500.00 Hz

Zero-Beat Method

Concept: 1. Use TinySA to generate local reference frequency 2. Tune to match broadcast carrier 3. Listen for zero-beat (no audible tone = perfect match) 4. Measure frequency difference

Calibration Methods

Method 1: Direct Frequency Measurement (Simplest)

Use TinySA as spectrum analyzer to measure broadcast carrier.

Procedure

  1. Tune TinySA to time standard:
  2. WWV: 5 MHz or 15 MHz
  3. CHU: 7.850 MHz or 14.670 MHz

  4. DCF77: 77.5 kHz

  5. Connect antenna to TinySA

  6. Find carrier peak on spectrum display

  7. Read frequency using TinySA's marker

  8. Compare to known frequency: Known CHU: 7,850,000.00 Hz TinySA reads: 7,850,050 Hz Error: +50 Hz = +6.4 ppm

  9. Calculate correction: ppm_error = (measured - actual) / actual × 10^6 ppm_error = (7,850,050 - 7,850,000) / 7,850,000 × 10^6 ppm_error = +6.37 ppm

  10. Apply to 30MHz reference: ``` If TinySA is 6.37 ppm fast at 7.850 MHz, it's also 6.37 ppm fast at 30 MHz

30 MHz error = 30,000,000 × 6.37/10^6 = 191 Hz Actual frequency = 30,000,191 Hz ```

  1. Enter correction in TinySA config: -6.37 ppm

Advantages

  • Simplest method
  • Direct measurement
  • No additional hardware

Disadvantages

  • Limited by TinySA's uncalibrated reference (circular problem)
  • Need good signal strength
  • Propagation effects can cause error

Method 2: Receiver + TinySA Beat Frequency (More Accurate)

Use a receiver to heterodyne against TinySA's tracking generator.

Setup

Antenna → Receiver → Audio Out → Computer/Oscilloscope
                                ↑
                            TinySA → Signal Generator Mode

Procedure

  1. Configure TinySA as signal generator:
  2. Set frequency to CHU: 7.850 MHz
  3. Set output power: -10 dBm

  4. Connect to receiver antenna input (via attenuator or loose coupling)

  5. Tune receiver to CHU 7.850 MHz

  6. Adjust TinySA frequency until you hear zero-beat:

  7. High tone: TinySA frequency too high
  8. Low tone: TinySA frequency too low

  9. Silence (zero-beat): Perfect match

  10. Fine-tune for null:

  11. Adjust in 1 Hz steps
  12. Listen in SSB or CW mode

  13. Find quietest point

  14. Read TinySA frequency when zero-beat achieved

  15. Calculate error: CHU actual: 7,850,000 Hz TinySA setting at zero-beat: 7,850,045 Hz Error: +45 Hz = +5.73 ppm

  16. Apply correction to 30 MHz reference

Advantages

  • More accurate than direct measurement
  • Less affected by propagation
  • Works with weak signals

Disadvantages

  • Requires receiver
  • More setup complexity
  • Skill needed to find zero-beat

Method 3: Audio Tone Method (Most Accurate)

Use WWV/CHU's audio tones as secondary reference.

Background

WWV and CHU broadcast: - 1000 Hz audio tone (except top of minute) - 500 Hz tone (first hour tone, WWV only) - Tone frequency accurate to ±0.001 Hz

Procedure

  1. Record 1000 Hz tone from WWV or CHU:
  2. Use SDR software
  3. Or connect receiver audio to computer

  4. Record 30-60 seconds of clean tone

  5. Analyze in audio software:

  6. Use Audacity: Analyze → Plot Spectrum
  7. Find peak frequency

  8. Should be exactly 1000.000 Hz

  9. Measure actual frequency:

  10. If you read 1000.5 Hz, your soundcard is 0.5 Hz fast
  11. This doesn't help us directly (soundcard not connected to TinySA)

Alternative: Tone Beat Method

  1. Generate 1000 Hz with TinySA (if capable)
  2. Mix with WWV 1000 Hz tone
  3. Listen for beat frequency
  4. Adjust TinySA tone generator for zero-beat
  5. Calculate error

Limitation: This calibrates the tone generator, not the 30 MHz reference. Only useful if tone generator uses same reference.

Method 4: Harmonic Multiplication (Advanced)

Use harmonics of lower frequency to reach 30 MHz.

Concept

If you can accurately measure a lower frequency, multiply to 30 MHz:

CHU 7.850 MHz × 4 = 31.40 MHz (close to 30 MHz)
                WWV 5 MHz × 6 = 30 MHz (perfect!)

Procedure (WWV 5 MHz Example)

Problem: WWV 10 MHz was discontinued, and 5 MHz × 6 = 30 MHz.

  1. Receive WWV 5 MHz

  2. Generate harmonics:

  3. Feed into frequency multiplier (×6)
  4. Or: Use non-linear device (diode, saturated amplifier)

  5. Output contains 5, 10, 15, 20, 25, 30 MHz harmonics

  6. Filter for 30 MHz:

  7. Use bandpass filter

  8. Or just select with TinySA spectrum analyzer

  9. Compare 30 MHz harmonic to TinySA's 30 MHz reference

  10. Measure beat frequency or phase

Circuit for Harmonic Generation

WWV 5 MHz → 1N4148 diode → 30 MHz bandpass filter → TinySA input
                                ↓
                              Ground via resistor
                
                Diode generates harmonics due to non-linearity
                Filter passes only 6th harmonic (30 MHz)

Advantages

  • Direct comparison at 30 MHz
  • High accuracy possible
  • Avoids frequency conversion math

Disadvantages

  • Requires building hardware
  • Needs signal processing
  • Harmonic signal is weak

Station Selection Guide

Which Station Should You Use?

North America:

Location Best Station Frequency When
Western USA/Canada WWV 5, 15 MHz Daytime
Eastern USA/Canada CHU 7.850 MHz Anytime
East Coast WWV 5 MHz Night
Pacific WWVH 5, 15 MHz Daytime

Europe: - DCF77 77.5 kHz (strongest, day/night) - MSF 60 kHz (UK, Western Europe)

Asia: - JJY 40 or 60 kHz - BPC 68.5 kHz

Propagation Considerations

HF Stations (3-20 MHz):

Frequency Daytime Nighttime Range
2.5 MHz Poor Good 0-1000 mi
5 MHz Good Excellent 0-2000 mi
7.85 MHz (CHU) Excellent Good 0-3000 mi
10-15 MHz Excellent Fair 0-5000 mi
20-25 MHz Fair Poor 0-3000 mi

LF Stations (60-77 kHz): - Day and night: Consistent - Range: 500-1000 miles (ground wave) - Advantage: Stable, minimal fading - Disadvantage: Need upconverter for RTL-SDR

Signal Quality Check

Good signal: - S-meter reads S7 or higher - Minimal fading - Clean carrier, no distortion - Stable over 5+ minutes

Poor signal (don't use): - Weak (< S5) - Rapid fading - Noise/static - Interference from other stations

Propagation Error Correction

Ionospheric Delay

Radio waves traveling through ionosphere are delayed:

Effect on frequency: - Doppler shift from moving ionosphere: ±0.1 Hz typical - Multipath causes fading, not frequency shift - Average over 5-10 minutes to eliminate

Correction: - Take multiple measurements (10+) - Average results - Discard outliers - Prefer stable, strong signals

Multipath Interference

Multiple paths cause fading:

Symptoms: - Signal strength varies - Apparent frequency wobble - Beat notes in audio

Solutions: - Use directional antenna - Wait for stable propagation - Choose higher frequency (less multipath) - Measure during quiet ionosphere (noon, summer)

Step-by-Step Calibration Example (CHU Method)

Equipment

  • RTL-SDR dongle
  • 50-foot wire antenna
  • Computer running SDR# or GQRX
  • TinySA

Procedure

1. Setup Receiver (15 minutes)

  • Connect RTL-SDR to computer
  • Connect antenna to RTL-SDR
  • Launch SDR software
  • Set frequency: 7.850 MHz
  • Set mode: AM or USB
  • Adjust RF gain for strong but not overloaded signal

2. Find CHU Signal (5 minutes)

  • Look for carrier on waterfall
  • Should see time code modulation (digital pulses)
  • Listen for voice announcements (top of hour)
  • Verify it's CHU (not another station)

3. Measure Carrier Frequency (10 minutes)

  • Switch to CW or narrow filter mode
  • Center on carrier
  • Use frequency counter in SDR software
  • Record frequency: _____

4. Calculate Error

Known CHU frequency: 7,850,000.00 Hz
                Measured frequency: 7,850,XXX.XX Hz  (fill in your reading)
                
                Error (Hz) = Measured - Known
                Error (ppm) = (Error / Known) × 10^6
                
                Example:
                Measured: 7,850,062 Hz
                Error = 7,850,062 - 7,850,000 = +62 Hz
                Error (ppm) = 62 / 7,850,000 × 10^6 = +7.90 ppm

5. Apply to 30 MHz

30 MHz error = 30,000,000 × (ppm_error / 10^6)
                
                Example:
                30 MHz error = 30,000,000 × 7.90 / 10^6 = 237 Hz
                Actual 30 MHz = 30,000,237 Hz

6. Enter Correction in TinySA

  • CONFIG → XTAL/Reference
  • Enter: -7.90 ppm (opposite sign)
  • SAVE

7. Verification (10 minutes)

  • Re-measure CHU frequency
  • Should now read 7,850,000 Hz (within ±10 Hz)
  • If not, iterate

Accuracy Limitations

Factors Affecting Accuracy

Factor Typical Error Mitigation
Transmitter accuracy ±0.00001 ppm None needed (perfect)
Ionospheric Doppler ±0.1 Hz Average multiple readings
Receiver stability ±1 ppm Use GPS-locked SDR
Multipath fading ±1 Hz Use strong, stable signal
Measurement resolution ±1 Hz Use FFT or counter

Achievable Accuracy: - Single measurement: ±1-10 ppm - Averaged (10 readings): ±0.5-2 ppm - Ideal conditions: ±0.1-0.5 ppm

Troubleshooting

Can't Receive Station

No signal at all: - Check antenna connection - Verify SDR is working (try FM broadcast) - Try different time of day - Try different frequency/station

Weak signal: - Improve antenna (longer wire, outdoors) - Try different frequency - Wait for better propagation - Check for local interference

Frequency Unstable

Reading jumps around: - Increase averaging time - Use narrower filter - Wait for stable conditions - Improve antenna

Slow drift: - Normal (ionosphere moving) - Average over longer time - Take multiple measurements

SDR Frequency Offset

SDR itself uncalibrated: - This is the problem we're trying to solve! - Use "PPM correction" in SDR software - Measure error against CHU - Enter correction - Iterate

Alternative: Using RTL-SDR's Built-in Calibration

Many SDR programs allow you to calibrate the RTL-SDR against known frequency:

SDR# Procedure:

  1. Tune to CHU 7.850 MHz
  2. Note frequency error (e.g., reads 7.850.045 MHz)
  3. Click Configure (gear icon)
  4. Adjust "Frequency correction (ppm)" slider
  5. Adjust until it reads exactly 7.850.000 MHz
  6. Record ppm value
  7. This ppm applies to TinySA's reference (if using same oscillator)

Limitation: This calibrates the SDR, not the TinySA. They're separate devices with separate oscillators.

Advanced: Building a WWV-Locked 30MHz Reference

If you want continuous calibration:

Concept

Build a PLL (Phase-Locked Loop) that: 1. Receives WWV 5 MHz 2. Multiplies by 6 to get 30 MHz 3. Locks crystal oscillator to result 4. Provides clean 30 MHz locked to WWV atomic clock

Block Diagram

Antenna → Receiver → 5 MHz IF → PLL (×6) → 30 MHz output
                                                   ↑
                                              Local TCXO ←→ Feedback

Components Needed

  • WWV receiver or downconverter
  • PLL IC (e.g., ADF4002, ADF4351)
  • TCXO (30 MHz or 10 MHz)
  • Loop filter components
  • Power supply

Complexity: High (PLL design is advanced) Cost: $50-100 Result: Continuous 30 MHz locked to WWV

Comparison to GPS Method

Aspect GPS WWV/CHU Winner
Accuracy 0.01 ppm 0.1-1 ppm GPS
Cost $15-25 $0-30 WWV (if you have SDR)
Setup time 2-4 hours 30-60 min WWV
Availability Worldwide Regional GPS
Learning value GPS tech Radio propagation Tie
Cool factor Modern Classic WWV (nostalgia!)

Recommendation: Use GPS for best accuracy, WWV/CHU for quick check or learning experience.

Summary

What We Accomplished

✓ Used free atomic clock broadcasts for calibration ✓ Achieved 0.1-1 ppm accuracy with no cost ✓ Learned about HF propagation and time standards ✓ Verified TinySA reference against multiple sources

Key Takeaways

  1. Time standard broadcasts are free atomic clock references
  2. Carrier frequency is exact, derived from atomic clock
  3. Propagation effects limit accuracy to ~0.1-1 ppm
  4. Averaging multiple measurements improves accuracy
  5. CHU at 7.850 MHz is best for North America
  6. DCF77 at 77.5 kHz is best for Europe

When to Use This Method

  • You already have SDR or shortwave receiver
  • You want to verify GPS calibration
  • You enjoy learning about radio propagation
  • Cost is more important than ultimate accuracy
  • You live near a time standard transmitter

References

Happy calibration using 100-year-old technology!

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