PC Frequency meter. 2005.2This 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.
SoftwareThe 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.
RangeThe 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.
HardwareThere 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 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 circuit consists of 2 parts : the prescaler, and the isolated power supply.
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 versionThe next version of this design is aimed at including a 1.2 GHz pre prescaler with a fet based preamp.