5.4 Frequency Standards, Crystal Calibrators and Standard-Frequency Transmissions

5.4 Frequency Standards, Crystal Calibrators and Standard-Frequency Transmissions

ACMA Exam Focus: Standard syllabus item 11.1 asks you to recall the uses and limitations of crystal calibrators, digital frequency counters and standard frequency transmissions. Examiners favour the limitations half of that sentence. Know what a 100 kHz calibrator can and cannot tell you, why its accuracy is only as good as its own crystal, and why an off-air standard-frequency broadcast is never as good at your receiver as it was at the transmitter.

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1. Why Frequency Accuracy Matters to an Amateur

The Australian amateur class licence administered by the ACMA authorises you to operate within the amateur bands and requires that you do not cause harmful interference to other radiocommunications. Both of those obligations rest on one assumption: that you actually know your transmit frequency. Nothing in the licence mandates that you own any particular test instrument — measuring your frequency is simply how a competent operator keeps the promise the licence asks of them.

Three practical reasons to care:

1. Band edges are hard limits. The 20 m allocation runs 14.000–14.350 MHz. Your entire emitted signal must fit inside, not just the dial reading. An upper-sideband transmission occupies roughly 0.3–3 kHz above the suppressed carrier, so a dial showing 14.349 MHz already spills over the top edge. Good practice is to keep the carrier at least 3 kHz inside an edge on the sideband that runs towards it.
2. Band plans depend on it. The WIA band plan is a voluntary agreement, not law, but it is how CW, narrowband digital, beacon and SSB segments stay separated. Being 5 kHz out drops you into the wrong segment.
3. Netting and skeds. Answering a net on a nominated frequency, or landing inside the narrow FT8 watering hole, requires far better than dial-guess accuracy.

Calibration is not permanent. Crystals age (typically around 1 ppm in the first year), oscillators drift during warm-up, and mechanical dials suffer backlash. A radio calibrated a decade ago is not calibrated today.

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2. Crystal Calibrators and Marker Generators

A crystal calibrator, also called a marker generator, is a small crystal oscillator — classically 100 kHz, sometimes 1 MHz or 25 kHz — that is deliberately run into hard limiting so its output is a squarewave rich in harmonics. A 100 kHz oscillator therefore places a marker signal at every 100 kHz point right across HF: 3.500 MHz, 3.600 MHz, … 14.200 MHz, and so on. Loosely couple a few microvolts into the receiver's antenna socket and you hear a comb of steady carriers.

Zero-beating the dial

Tune the receiver towards the marker nearest your frequency of interest with the BFO or SSB detector switched in. As you approach, the beat note falls in pitch; at zero beat the receiver's tuned frequency equals the marker exactly. Read the dial, and the difference between the dial reading and the known marker frequency is your dial correction. In practice most operators tune for a low audible note rather than true silence, because the ear resolves a 100 Hz tone more reliably than the point at which the tone disappears.

Finer markers with a divider chain

A 1 MHz crystal followed by a divide-by-10 and a divide-by-4 stage yields 100 kHz and 25 kHz marker combs from a single, more easily trimmed crystal. Twenty-five kilohertz markers make the wide 10 m band far easier to navigate.

Limitations you must be able to recall

• It tells you only where a marker is. Between markers you are still interpolating on the dial, so it calibrates the dial rather than measuring an arbitrary signal.
• Harmonics weaken with order. By VHF the several-thousandth harmonic of a 100 kHz oscillator is buried in receiver noise, so calibrators are an HF technique.
• It is only as accurate as its own crystal, which must be trimmed against a known standard — and the error is multiplied by the harmonic number, as the worked example below shows.
• Radiated markers are interference. Keep the coupling weak and switch the calibrator off after use.

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3. Standard-Frequency and Time-Signal Transmissions

National metrology institutes broadcast carriers and time codes locked to caesium clocks. These are the reference against which you trim a calibrator, check a counter's timebase, or verify a receiver dial.

Australia no longer has a national HF standard-frequency station. VNG began at Lyndhurst, Victoria in 1964, moved to Llandilo, New South Wales in 1988, and shut down permanently at the end of 2002. If an exam answer tells you to listen to VNG, it is wrong.

Using one

Tune to 10.000 MHz, identify WWV or WWVH by voice, zero-beat the carrier and note the dial error. Then trim the calibrator so its 100th harmonic zero-beats the same carrier, and the whole marker comb inherits that accuracy.

The limitation is propagation

An HF standard signal reaches you via the ionosphere. Which frequency is usable depends on the time of day, the season and the solar cycle — 5 and 10 MHz typically work overnight from Australia, 15 MHz in daylight. Worse, the reflecting layer is moving, so the received carrier carries a small Doppler shift, and multipath smears the phase. A carrier transmitted to about 1 part in 10¹² is therefore only good to roughly 1 part in 10⁷ on a short off-air observation. Averaging over hours or days recovers much of the accuracy; a two-minute listen does not.

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4. Modern References

• GPSDO (GPS-disciplined oscillator). A one-pulse-per-second output from a GPS receiver steers an OCXO, producing a 10 MHz reference better than 1 part in 10¹¹ once locked. It needs sky view and a lock period, and falls back to plain OCXO performance if satellites are lost.
• 10 MHz reference input. Many current transceivers accept an external 10 MHz reference, so one GPSDO can discipline the whole shack.
• Network time is not a frequency standard. NTP disciplines your computer's clock to perhaps a few milliseconds. It does nothing to your radio's oscillator.
• Digital modes need time, not frequency. FT8 transmissions start on 15-second boundaries, so your PC clock must be right to within about a second or decodes collapse. The decoder searches a frequency window, so modest oscillator error is tolerated.

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5. Accuracy and Limitations Compared

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6. Worked Examples

Converting a ppm specification to hertz. Parts per million scale with frequency:

$\Delta f = f_0 \times \frac{\text{ppm}}{10^6}$

For a $\pm 2$ ppm TCXO at $f_0 = 14.200$ MHz:

$\Delta f = 14\,200\,000 \times \frac{2}{10^6} = \pm 28.4 \text{ Hz}$

Does ±28.4 Hz matter?

• SSB: no. A 28 Hz shift on speech is imperceptible and smaller than one click of most tuning knobs.
• CW: marginally. A 600 Hz note becomes 572 Hz or 628 Hz — audible, harmless in a 500 Hz filter, but enough to push a signal towards the skirt of a 50 Hz filter.
• Band edge: yes. Setting 14.349 MHz on USB already places emitted energy above 14.350 MHz before any timebase error is counted. Accuracy near an edge is a compliance question, not an audio-quality one.

Why a calibrator's error multiplies. Suppose a 100 kHz crystal runs 20 Hz high. That is $20/100,000 = 200$ ppm. The marker near 14.2 MHz is the 142nd harmonic, so its error is:

$142 \times 20 = 2\,840 \text{ Hz}$

The same 200 ppm, now worth almost 3 kHz. This is exactly why an untrimmed calibrator can leave you outside the band while the marker looks perfectly on frequency.

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Exam Traps

• A calibrator marks fixed points; it does not measure an unknown frequency.
• A counter's resolution is not its accuracy — its ageing timebase sets the real figure.
• Received standard-frequency accuracy is always worse than transmitted accuracy.
• VNG is gone; use WWV, WWVH or BPM.