I build a lot of RF circuits, and often it’s convenient to measure and log frequency with a computer. Previously I’ve built standalone frequency counters, frequency counters with a PC interface, and even hacked a classic frequency counter to add USB interface (twice, actually). My latest device uses only 2 microchips to provide a Raspberry Pi with RF frequency measurement capabilities. The RF signal clocks a 32-bit counter SN74LV8154 ($1.04 on Mouser) connected to a 16-bit IO expander MCP23017 ($1.26 on Mouser) accessable to the Raspberry Pi (via I²C) to provide real-time frequency measurements from a python script for $2.30 in components! Well, plus the cost of the Raspberry Pi. All files for this project are on my GitHub page.
The entire circuit is only two microchips! I have a few passives to clean up the RF signal (the RF input is loaded with a 1k resistor to ground, decoupled through a series 100 nF capacitor, and balanced at VCC/2 through a voltage divider of two 47k resistors), but if the measured signal is already a strong square wave they could be omitted. The circuit requires a gate pulse which typically will be 1 pulse per second (1PPS) and can be generated by dividing-down a 32.768kHz oscillator, a spare pin on a microcontroller, a fancy 1PPS time reference, or like in my case a GPS module (Neo-6M) with 1PPS output to provide an extremely accurate gate.
The connections are intuitive! The I2C address is 0x20 when A0, A1, and A2 are grounded. GPB(1-4) control the register select of the counter, and GPA(0-7) reads each bit of the selected register. The whole thing is controlled from Python, but could be trivially written in any language.
Read more: Raspberry Pi RF Frequency Counter
