Member Only Explanation

Portable Frequency Counter: Auto-Calibrating Via GPS

The main drawbacks of a low-cost frequency counter are its limited medium- to long-term stability and the absence of an internal reference for calibration. This project addresses these issues by incorporating an ultra-stable TCXO and a highly precise satellite-based reference source.

Affordable frequency counters often lack precision and stability. Here, a frequency counter is presented with a guaranteed eight‑digit accuracy and of acceptable cost. An OLED display provides excellent readability. Calibration is automatic by use of the GPS signal. A built-in rechargeable battery makes the instrument portable.

Principle of Operation

The so-called reciprocal counter method is used. A number of complete cycles of the input signal is counted. During the period this takes place, the cycles of a reference signal with a precisely known frequency, the reference frequency (f_ref), are counted. With the formula

the frequency of the input signal (f_in) is calculated. For acquiring the number of input cycles (n_in) and the number of cycles of our reference frequency signal, n_ref counts are required. If we want a result with an accuracy of eight digits n_ref should be greater than 10⁸.

Thus, the value of the reference frequency defines the duration of the period du...