Measuring Frequency

One line of the syllabus — “measurement of frequency and use of simple frequency meters” — gets a whole module because it sits directly on top of a licence condition. You are required by law to have a frequency-measuring device at your station and to use it every time you change frequency. Transmitting outside your authorised band is not an embarrassment to be apologised for on the air; it is the one operating mistake that reliably ends with a licence being suspended or cancelled. This lesson covers the instruments the exam names, in ascending order of accuracy, and then the rest of the test bench.

The absorption wavemeter

The simplest instrument that measures frequency at all is a tuned circuit with a detector on it. A coil, a variable capacitor calibrated in frequency, a diode and a microammeter — nothing else, and no power supply. Hold the coil near the transmitter's tank or feeder, turn the capacitor, and the meter peaks when the tuned circuit is resonant at the frequency present. The energy that moves the needle is absorbed from the transmitter, which is where the name comes from.

Its virtues are that it costs almost nothing, cannot itself radiate, and works on a dead transmitter's field without any connection. Its vice is accuracy: the tuned circuit has a modest Q, so the peak is broad and the reading is good to a few per cent at best. On 7 MHz, three per cent is 210 kHz — hopelessly coarse for checking a band edge, but perfectly adequate for its real job, which is harmonic hunting. If your transmitter is on 7 MHz and the wavemeter also peaks strongly at 14 and 21 MHz when held near the feeder, you have a harmonic problem and the low-pass filter needs attention.

The heterodyne frequency meter

Far better accuracy comes from comparing the unknown against a known. A heterodyne frequency meter contains a calibrated variable oscillator. Its output is mixed with the signal being measured and the difference is fed to headphones. Tune the internal oscillator towards the unknown frequency and you hear a beat note falling in pitch; when the two frequencies coincide the note falls to nothing. That is zero beat, and the frequency is then read off the calibrated dial.

The ear is a remarkably good null detector — a trained operator can judge zero beat to within a few tens of hertz — so the method is orders of magnitude better than an absorption wavemeter. Its accuracy is limited entirely by how good the internal oscillator's calibration is, which is why such a meter carries a built-in crystal reference for checking itself. Modern equivalents use the receiver you already own: tune your rig to the transmitted signal, and its calibrated dial does the same job.

The grid-dip meter

A dip meter is the opposite arrangement: a small variable-frequency oscillator with its coil exposed, and a meter reading the oscillator's own current. Hold the coil close to a tuned circuit and sweep. When the oscillator reaches the resonant frequency of the circuit under test, that circuit absorbs energy from it, the oscillator's current falls, and the meter dips. The name comes from valve days, when the meter read grid current; the modern transistor version is a gate-dip meter, and the technique is identical.

Its value is that the circuit under test is not powered. That makes it the instrument of choice for:

• finding the resonant frequency of a tank circuit or an IF transformer on the bench;
• checking where an antenna or a trap actually resonates before any RF is applied;
• measuring an unknown coil or capacitor, by resonating it against a known one;
• finding the frequency of a dead oscillator, which is how you diagnose one that will not start.

Switched to passive mode, with its own oscillator off, a dip meter also works as an absorption wavemeter.

The digital frequency counter

A counter does exactly what the name says: it opens a gate for a precisely known interval, counts the input cycles that arrive during it, and divides. Open the gate for one second and the count is the frequency in hertz.

Everything therefore depends on the gate being exactly as long as it claims, and the gate is derived from a crystal oscillator — the timebase. The accuracy of a counter is set entirely by the accuracy of its timebase, not by how many digits it displays. An eight-digit counter with a drifting crystal displays eight digits of nonsense.

Worked example — what the timebase costs you

A counter's timebase is specified as accurate to ±1 part per million. You measure a 2 m signal at 145.000 MHz. How much might the reading be out?

145 000 000 × 0.000001 = 145 Hz

So the true frequency is somewhere within ±145 Hz of the display, whatever the last digits say. A cheap counter specified at ±10 ppm would be ±1450 Hz on the same signal — still far better than any wavemeter, and easily good enough to prove you are inside 144–146 MHz.

The gate-time trade-off

A longer gate counts more cycles and so resolves the frequency more finely, but you wait longer for each reading. A one-second gate gives 1 Hz resolution and one reading per second. A 0.1-second gate gives 10 Hz resolution and ten readings per second, which is what you want when you are turning a trimmer and need to see the effect immediately. There is also an unavoidable ±1 count uncertainty on the last digit, because the gate does not open in step with the incoming cycles. Short gate for adjusting, long gate for the final reading.

The crystal calibrator and marker generator

A crystal calibrator is a crystal oscillator, usually at 100 kHz, deliberately run so that its output is a distorted waveform rich in harmonics. Those harmonics appear as markers every 100 kHz all the way up through HF. Tune the receiver until you hear a marker, and you have a point of known frequency against which to check the dial. Many units divide down to give 25 kHz or 10 kHz markers as well.

The practical use is the one the licence condition demands: before transmitting near the edge of a band, find the marker that sits on the band edge and confirm which side of it you are. On an older analogue transceiver this was the standard pre-transmission check.

Calibrating against a standard

All of the above are only as good as their own calibration, so at some point every station needs an external reference. Standard frequency and time signal stations exist for exactly this purpose. They transmit on 5, 10 and 15 MHz among others, are controlled by atomic standards, and are the ultimate check available to an amateur: zero-beat your calibrator against one and its accuracy becomes yours. A broadcast station's carrier is well controlled but is not published to any guaranteed tolerance, and the mains frequency wanders by a fraction of a hertz all day, so neither is a substitute. The time signals lesson covers the stations themselves and how their time codes are read.

The rest of the bench, briefly

The multimeter

The heart of an analogue multimeter is a moving-coil movement: a coil suspended in a magnetic field, deflecting against a spring by an amount proportional to the current through it. It is fundamentally a sensitive current meter, typically full-scale at 50 µA, and everything else is built around it.

A voltmeter must have a high resistance, because whatever current it draws is current taken from the circuit being measured. Connect a low-resistance voltmeter across a high-resistance potential divider and it loads the divider, changes the very voltage it is there to read, and gives you a confidently wrong answer. The old figure of merit for this is ohms per volt: 20 000 Ω/V was a good analogue meter; a modern digital multimeter presents 10 MΩ on every range and the problem largely goes away.

SWR meter, power meter, dummy load and oscilloscope

Two points worth carrying away. An SWR meter tells you nothing whatever about frequency, however prominent it is on the desk — that is a favourite distractor. And the dummy load is not optional equipment: the licence conditions permit tests of not more than 30 seconds on the air, and require an artificial aerial beyond that. The bench tools on this site do the arithmetic that goes with these instruments.