PC Frequency meter. 2005.2

This project is a frequency meter that consists of hardware that plugs into your PC soundcard, and software that provides the analysis and user interface.

Version 2005.2 fixes a small display bug and increases accuracy on short sample times.


The software records a stretch of audio from the sound card to memory, then analyses it and provides you with a calculated frequency.

Download (180KB) the PC software.


The range of the unit is limited by 2 factors.
1. The minimum frequency response of your sound card multiplied by the prescaler factor.
2. The maximum frequency response of your brand of 74HC4060.
In the past I have used this circuit with a Phillips 74hc4060 and obtained reliable results at up to 90MHz.


There are 2 versions of this project.
1. A simple 1 chip circuit, offering no isolation barrier. (because soundcards are cheap.)
2. A galvanic isolated circuit of about 3 times the complexity.

Hardware option 1.

A Simple one chip solution.

The diagram.

The simple circuit offers no isolation, but the simplicity of the circuit combined with cost of sound cards and the fact that diodes 1 and 2 add a good measure of protection makes this circuit ideal for the average user.

Notice the physical hardware includes a 20MHz crystal oscillator module. This is not required to make the circuit work, but is included for calibration purposes. ( Stop the doubt before it makes you go batty ).

The circuit is to be powered by a 5V source. ( I use a USB port ).

Hardware option 2.

Galvanic isolation.

This hardware design provides galvanic isolation to provide additional protection for your sound card and enable a floating common.

The diagram.

The circuit consists of 2 parts : the prescaler, and the isolated power supply.

The prescaler

The prescaler consists primarily of a down counter. AC coupling is provided together with a DC trigger adjustment. An opto coupler provides the signal isolation.

The power supply

The power supply could be a complete hybrid 5Volt to 5Volt isolation unit, or as in this case a component based design. The transformer used is a base drive transformer savaged from a 11W energy efficient lamp. If I remember correctly mine comes from a Phillips unit.

The circuit is not particularly efficient, and as a result a simple shunt regulator can be used on the output. I use R9 to detect the shunting current. In my case it sits at 12mA.

The next version

The next version of this design is aimed at including a 1.2 GHz pre prescaler with a fet based preamp.

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