Dick Cappels' project pages http://www.projects.cappels.org

return to HOME

LC Determination by  Resonant Frequency Measurement
A well known L/C measurement circuit is pressed into service to make a bare bones measurement circuit.
Download the AVRStudio assembly source: lgm031227I.asm
Download the AVRStudio hex file: lgm031227.hex

Left-to-right: The 5 volt regulator, the LM393 oscillator (a 0.047 uf capacitor is mounted on the LC circuit connector), the AT90S2313, and the serial connector to the LCD module.

Recently, I have needed to measure inductances in the hundreds of microhenries to several millihenry range. Though I have a pretty good LRC meter and an excellent bridge on my workbench in Mesa, Arizona, I wanted to make these measurements in my home in Thailand, thus I decided to put something together. The RF Inductance meter on this website is good for low value rf inductors, but because of the way it works - putting a sharp-edged square wave through the inductor -its not suitable for inductors made with high permeability ferrites (Because of pulse shape distortion that results from high frequency losses in the core.) This meter operates at lower frequencies, and by careful selection of the resonating capacitor, the oscillator can be made to run anywhere from 100 kHz on down. This makes it possible to test near standard frequencies like 1 kHz and 400 Hz, to compare results with precision bridges.

The AT90S2313 frequency meter drives a serial terminal with a 0 to 5 volt signal. The firmware was adapted from the RS-232 Freq. Meter/Pulse Generator project on this site, and then tailored to work with the a two-line LCD (See the serial interface for Truly LCD also on this site). The serial connector has switched +8 volts to power the LCD and its interface.

The schematic. Just an oscillator and an AT90S2313 used as a frequency meter. The output is CMOS level serial. The inverting 2N2907 serves to isolate the AT90S2313 from electrostatic discharges. A double-row connector has room for both the resonating capacitor and the inductor under test.

 Circuit Description

This is basically just an oscillator based on a comparitor and a frequency meter. The oscillator oscillates at the resonant frequency of an LC parallel tuned circuit. A really nice version of this was created by Chris Krah using an AT90S1200, including floating point math, etc. to display the L and C readings directly. I'll post a link to Chris' version once it is published on the web (it was posted on the AVRFreaks board).

The LM393 comparitor only seems to work accurately a little past 100 kHz, but it is good enough for my purposes. Faster comparitors like the LM311 can work well at higher frequencies.  The second comparitor in the package was left floating. A dual-row connector is used to connect the resonant circuit under test to the oscillator. It has two rows so there will be one pair of contacts for the reference capacitor, and a second pair of contacts to connect for inductor under test. Or, if one wants to measure capacitors, a reference inductor can be soldered to the connector and the other set of contacts used for the capacitor under test.

Building it

I built this on a phenolic board with one pad per hole. As shown in the photograph above, the board is laid out pretty much the same as the circuit diagram. I made sure the oscillator components were on the opposite side of the LM393 from the AT90S2313 and its crystal oscillator to minimize coupling as a precaution because I was concerned that energy from the AT90S2313 oscillator could cause affect the frequency of the LM393 oscillator.  


Contents ©2003, 2005 Richard Cappels All Rights Reserved. http://www.projects.cappels.org/
Dick Cappels' web version first posted in December, 2003

You can send  email to me at projects(at)cappels.org. Replace "(at)" with "@" before mailing.

 Use of information presented on this page is for personal, nonprofit educational and noncommercial use only. This material (including object files) is copyrighted by Richard Cappels and may not be republished or used directly for commercial purposes. For commercial license, click here.

 Liability Disclaimer and intellectual property notice
(Summary: No warranties, use these pages at your own risk. You may use the information provided here for personal and educational purposes but you may not republish or use this information for any commercial purpose without explicit permission.) I neither express nor imply any warranty for the quality, fitness for any particular purpose or  user, or freedom from patents or other restrictions on the rights of use of any software, firmware, hardware, design, service,information, or advice provided, mentioned,or made reference to in these pages. By utilizing or relying on software, firmware, hardware, design, service,information, or advice provided, mentioned, or made reference to in these pages, the user takes responsibility to assume all risk and associated with said activity and hold Richard Cappels harmless in the event of any loss or expense associated with said activity. The contents of this web site, unless otherwise noted, is copyrighted by Richard Cappels. Use of information presented on this site for personal, nonprofit educational and noncommercial use is encouraged, but unless explicitly stated with respect to particular material, the material itself may not be republished or used directly for commercial purposes. For the purposes of this notice, copying binary data resulting from program files, including assembly source code and object (hex) files into semiconductor memories for personal, nonprofit educational or other noncommercial use is not considered republishing. Entities desiring to use any material published in this pages for commercial purposes should contact the respective copyright holder(s).