A frequency counter measures how often a signal repeats, to a precision far finer than a radio dial. A signal generator does the opposite: it makes a clean signal of known frequency and level to feed into something under test. Together they let you check a transmitter's frequency, align a receiver, and test filters and amplifiers.
How a counter works
Toy example at 25.4 Hz so the pulses can be seen. A real counter does the same with millions of pulses per second.
Resolution (1 ÷ gate time)1.0 Hz
A counter opens a gate for a precisely timed interval and counts the pulses that arrive. Count ÷ gate time is the frequency. A longer gate gives finer resolution: 0.1 s gives 10 Hz, 1 s gives 1 Hz, 10 s gives 0.1 Hz. Because the gate can open at any moment relative to the signal, the count can be one too high or low: the "±1 count" uncertainty, which is exactly the resolution.
Above the counter's own limit, a prescaler divides the signal down first, and the display multiplies the result back:
The prescaler reduces the signal frequency to within the counter's operating range.
Resolution is not accuracy
A display of 1 Hz resolution means little if the gate is the wrong length. The gate is timed by the counter's time base, a crystal oscillator, and any error in it scales with the frequency being measured. Error = frequency × (ppm ÷ 1,000,000), so at 144.2 MHz:
Time base (typical)
Error in ppm
Error at 144.2 MHz
Basic crystal
about 10
about 1.4 kHz
Temperature-compensated (TCXO)
about 1
about 144 Hz
Oven-controlled (OCXO)
about 0.01
about 1.4 Hz
GPS-disciplined
far better than 0.001
negligible for amateur use
Check a counter's time base against a known standard such as the WWV time signals on 10 MHz, or a GPS-disciplined source. The same limits apply to the frequency display on a radio: it is only as good as its reference crystal. See Crystals and resonators.
Signal generators
RF signal generator. A calibrated frequency and a known, adjustable output level, often with AM or FM modulation. It is how you measure a receiver's sensitivity and check an S-meter. By the common HF convention S9 is 50 µV across 50 Ω, which is −73 dBm: (50 × 10⁻⁶)² ÷ 50 = 5 × 10⁻¹¹ W.
Audio and function generators. Sine, square and triangle waves at audio frequency. Two audio tones fed to an SSB transmitter make the two-tone test of linearity, read with a spectrum analyzer.
Sweep generators step the frequency across a range, for plotting a filter's response with a scope or analyzer.
Modern generators are built around a DDS (direct digital synthesizer): a counter-like accumulator steps through a sine table at the crystal's clock rate, and a DAC makes the wave.
One crystal, many frequencies: the tuning word sets the output, the crystal sets the accuracy.
Output frequency2× lowest step
With a 32-bit accumulator on a 100 MHz clock the frequency step is 100 MHz ÷ 2³² = about 0.023 Hz, and the crystal sets the accuracy. See PLLs and frequency synthesizers.