Frequency counter

Also known as: frequency counter, freq counter, counter-timer

A frequency counter measures the frequency of a periodic electrical signal by counting cycles over a precisely known interval.¹ It is the instrument you use to verify that a transmitter, oscillator, or reference is exactly on frequency — and its own accuracy is set entirely by the quality of its internal timebase, which is why frequency-stability is the spec that matters most.

A frequency counter counts input cycles during a gate interval set by its timebase; the count divided by the gate time is the displayed frequency, so timebase accuracy bounds the whole measurement.

How it works

The classic direct (gated) counter opens a gate for a fixed, timebase-derived interval — the gate time, say 1 second — and counts how many input cycles pass through. Cycles divided by gate time is the frequency. Its resolution is one count per gate: a 1-second gate resolves 1 Hz, and a longer gate resolves finer but takes longer. At low input frequencies this is coarse, because few cycles occur per gate.

A reciprocal counter inverts the scheme: it measures the period by counting high-speed timebase clock ticks over an integer number of input cycles, then takes the reciprocal. This gives constant relative resolution — the same number of significant digits — regardless of input frequency, so it resolves a 100 Hz signal as finely as a 100 MHz one for a given gate. Nearly all modern counters are reciprocal, often with interpolation that adds several more digits.

Accuracy and the timebase

A counter can never be more accurate than its timebase reference, because every measurement is a ratio against it. The hierarchy runs:

TCXO (temperature-compensated crystal) — a few ppm; adequate for casual work.
OCXO (oven-controlled crystal) — tens of ppb, far more stable over temperature and time.
GPSDO (GPS-disciplined oscillator) — locked to atomic time from the GNSS constellation, giving parts in 10¹¹ or better long-term. Many counters accept an external 10 MHz reference precisely so they can be slaved to a GPSDO.

Aside from the timebase, the dominant short-term error is ±1 count quantization plus trigger noise, both of which shrink with a longer gate or reciprocal interpolation.

In practice

Use the longest gate you can tolerate for the finest resolution on a stable signal.
Feed the counter a clean signal at an appropriate level; a squaring/trigger stage needs a decent slew rate, and noise near the trigger point adds jitter.
For real accuracy, discipline the timebase to a 10 MHz GPSDO and let an OCXO warm up and settle before trusting the last digits.

Relevance to SDR

Every SDR carries a reference oscillator, and its accuracy directly offsets every tuned frequency — the reason GopherTrunk and other SDR software expose a ppm frequency correction. A frequency counter, its own timebase disciplined by a GPSDO, is the bench tool that lets you measure an SDR’s or transmitter’s true frequency and derive that correction, or verify that a control channel really sits where the system says it does. GopherTrunk does not drive a counter; it estimates and corrects residual frequency offset in software from the recovered carrier, and a bench counter is a complementary external reference rather than part of the decode chain.

Sources

Frequency counter — Wikipedia, on gated and reciprocal frequency counting and timebase-limited accuracy. ↩