led_indicating_l-meter

This project started with an interesting L-Meter design by 7N3WVM. He used two oscillators with frequencies that make the dial scale an even multiple/divisor of ten when changing bands. After doing the math I found that by adding a divide-by-ten to each of the two original oscillators I could make the circuit cover a much wider inductance range, thus increasing it's value to me. My unit now covers 0.15 uH to 1500 uH.

After some prototype experimentation I found that you do not need sine waves to measure inductors. Square waves from the oscillator or from decade dividers seem to work just fine. My present prototype version is using 74HC04 as the 32 and 10 MHz oscillators, with a 74HC390 dual decade divider providing 3.2 and 1.0 MHz outputs for measuring larger inductors (schematic and pictures to follow, later).

NOTE: While 7N3WVM used crystal oscillators, my experiments indicate that simple LC type 74HC04 gate based oscillators are stable enough for most inductor measurements. This lets me adjust each VFO for precise dial calibration alignment on all bands.

A simple LED peaking indicator is all that is necessary to find the actual inductance value. Using an indicator diode eliminates need for a meter. This part of the idea came from a similar circuit designed by Alan - VK2ZAY.

Another L-Meter design, this one by N5ESE, is located here.

NOTE: To make overlapping dial scales that use the same numbers but different decimal point placement, the frequency sequence needs to be "F", "F*SQRT(10)", F*10, ((F*10)*(SQRT(10))), and so on. It that the Square Root of 10 is a magic number for making one dial scale work for multiple tuning ranges.

WORK IN PROGRESS

PLEASE BE PATIENT