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# LC meter design question

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#### BobL3394

##### Newbie level 5
Hi,
while working on another project i came into the need of calculating the inductance of a custom made inductor so, for didactical purposes, i started this new project of creating an LC meter using arduino.
The software side is not hard, but problem is that im not able to create a sufficient big oscillation to capture its frequency and so calculate the inductance of the Inductor.
I so far tried to send a square wave through a LC tank provoking it to oscillate but the signal mixes with the circuits noise so that at the end i cant distinguish it.
Could someone explain me how to get a big enough oscillation so that it allows me to clearly distinguish it from the background noise? other types of circuit (op amp oscillators etc) i could try?

There's a classical DIY LCR meter design which has been often discussed or linked at Edaboard, the latest in this thread https://www.edaboard.com/threads/329768/

It uses a LM311 comparator as oscillator, an advantage of the circuit is that it has a direct square wave output. Other oscillator circuits may have advantages in different regards, but they need a comparator as sine to squarewave converter.

You need sufficiently large voltage swings on the L & C. This is directly related to the amount of Amperes going through them. If only small current is available, then you will get small voltage swings.

Small A, large V, are associated with small C, large H.

It usually works well if you select a C value which is between one thousandth and one millionth of your Henry value.

Thank you guys for your answers. I will try the circuit FvM posted, but i already used BradtheRad's suggestion on my current op amp oscillator and it did work! but im still having issues finding the right capacitance value to make the circuit oscillate with lower inductances( 1uH to 100nH) as it apparently looks like as if the "one thousandth to one millionth" rule doesnt apply anymore on those little values.
Side question: does someone know some a good beginner book on the topic?

1 µH and below corresponds to > 5 - 10 MHz oscillation frequency. Standard OPs don't work anymore in this range.

I have used a simple two-transistor ("negative impedance") oscillator circuit for LC measurements for many years, a battery (e.g. 1.5V) and a variable resistor in place of the current source.

_Eduardo_

### _Eduardo_

Points: 2
1 µH and below corresponds to > 5 - 10 MHz oscillation frequency. Standard OPs don't work anymore in this range.
I was so into finding the perfect H,F valuses that i totally forgot about my op amp: it indeed makes sense that a those frequencies the Operational becomes the bottleneck of the system. Ill try out your configuration and let you know if i have other questions.

Do a search for the AADE LC-meter. It's sold in kit form but the schematic is freely available as is the theory of operation. Get some ideas from it and cook up your own. I built mine around a PIC.

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