AVR Gated GPS Locked Frequency Counter/Standard

What is it?

The Gated GPS Locked Frequency Counter / Frequency Standard is a standalone unit that uses the 1PPS signal from a GPS module to calibrate its internal frequency and lock to it. It can then operate in two modes:

Frequency Counter Mode – measure external frequencies with GPS-calibrated accuracy

Frequency Standard Mode – provide a precise reference frequency

An Si5351 module generates the reference frequency standard, while the GPS lock ensures exceptional long-term accuracy.

Why did I make it?

My old frequency counter was always a few Hz off. This project solves that problem using a dual 16-bit counter configured as a single 32-bit counter. The frequency to be measured is converted to a square wave, fed to the counter chip, and represented as a 32-bit binary number. An ATmega328 reads this value and converts it to the displayed measured frequency.

Key Hardware Features:

Input transistor amplifier on board

Frequency standard output

GPS connection via side SMA connector

20x4 LCD display (included, driven by I2C adapter or onboard PCF8574 expander)

Board accommodates Si5351 with TCXO (optional Si5351 can also be used)

What makes it special?

GPS-locked calibration for lab-grade accuracy

Displays current GPS time and date

Serial output for remote monitoring

Powered by 9V DC with onboard voltage regulator

Gate switch with selectable 1, 2, 5, or 10 second gate times

10 MHz (or any frequency set in code) output

1PPS output

What's Included

Assembled board with GPS module, Si5351,

20x4 LCD. power supply, and enclosure not included.